thermally conductive polymer composites for … thermally conductive polymer composites for...

ThermallyConductivePolymerCompositesforElectronicPackagingApplications

by

Muhammad Omer Khan

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Mechanical and Industrial Engineering University of Toronto

Copyright by Muhammad Omer Khan 2012

ii

ThermallyConductivePolymerCompositesforElectronicPackaging

Applications

Muhammad Omer Khan

Master of Applied Science

Mechanical and Industrial Engineering University of Toronto

2012

Abstract

Advancements in the semiconductor industry have lead to the miniaturization of components and

increased power densities, resulting in thermal management issues. In response to this shift,

finding multifunctional materials with excellent thermal conductivity and tailored electrical

properties are becoming increasingly important. For this research thesis, three different studies

were conducted to develop and characterize thermally conductive polymer composites. In the

first study, a PPS matrix was combined with different types of carbon-based fillers to determine

the effects of fillers size, shape, and orientation on thermal conductivity. In the second study,

effects of adding ceramic- and carbon- based fillers on the tailored thermal and electrical

properties of composites were investigated. Lastly, the possibility of improving the thermal

conductivity by introducing and aligning polymer fibers in the composites was investigated. The

composites were characterized with respect to their physical, thermal, and electrical properties to

propose possibilities of application in the electronic packaging industries.

iii

Acknowledgments

I would like to thank my supervisors Prof. Hani Naguib and Prof. Francis Dawson for giving the

opportunity to conduct my MASc research in their laboratories. I really appreciated their

guidance and mentorship throughout the duration of the project.

I would like to thank my fellow colleagues in SAPL for helping me with my research whenever I

needed. I would like to especially thank Sunny Leung and Reza Rizvi for always being there

when I seemed to have roadblocks in my research. They were always very helpful with their

insightful input about my results. My other SAPL colleagues: Shahrzad Ghaffari, Eunji nuna

In, Janice Song, Aaron Guan, and Terence Lee and my undergraduate students: Ahmed

Mahmoud and Harvey Shi.

I would like to thank and dedicate this thesis to my parents; abbu and ammi. Without their

continuous love, encouragement, and advices, I would not have completed this thesis project. I

would like to thank my brothers for keeping me entertained with their company.

Last but not least, I thank Allah SWT for His countless blessings, guidance, and help.

iv

Table of Contents

Acknowledgments .......................................................................................................................... iii

Table of Contents ........................................................................................................................... iv

List of Tables ............................................................................................................................... viii

List of Figures ................................................................................................................................ ix

Chapter 1 Introduction .................................................................................................................... 1

1 Preamble ..................................................................................................................................... 1

1.1 Multifunctional Composite Materials for Electronic Packaging Applications ................... 2

1.2 Thesis Objectives ................................................................................................................ 3

1.3 Thesis Organization ............................................................................................................ 3

Chapter 2 Literature Survey and Theoretical Background ............................................................. 5

2 Introduction ................................................................................................................................ 5

2.1 Thermal Management and Electronic Packaging ............................................................... 5

2.2 Materials for Electronic Packaging ..................................................................................... 8

2.3 Thermally Conductive Polymeric Composites ................................................................... 9

2.3.1 Polymer Composites Embedded with Carbon-Based Particles ............................ 10

2.3.2 Polymer Composites Embedded with Hybrid Fillers ........................................... 16

2.3.3 Composites with Aligned Polymer Fibers ............................................................ 18

2.4 Theoretical Predictions for Thermal Conductivity of Composite Materials .................... 22

2.5 Interfacial Thermal Resistance ......................................................................................... 24

2.6 Summary of Guidelines .................................................................................................... 25

2.6.1 Material Selection ................................................................................................. 25

2.6.2 Matrix Materials .................................................................................................... 25

v

2.6.3 Filler Materials: ..................................................................................................... 26

2.6.4 Micromechanical Models Selection ...................................................................... 26

Chapter 3 Effects of Size and Structure of Micro- and Nano- Sized Carbon Fillers on Composites Properties ............................................................................................................ 28

3 Introduction .............................................................................................................................. 28

3.1 Experimental ..................................................................................................................... 28

3.1.1 Materials ............................................................................................................... 28

3.1.2 Sample Preparation ............................................................................................... 30

3.2 Sample Characterization ................................................................................................... 31

3.3 Results and Discussion ..................................................................................................... 32

3.3.1 Effects of Carbon Fillers on the Effective Thermal Conductivity ........................ 32

3.3.2 Effect of Carbon- Based Fillers on the Coefficient of Thermal Expansion .......... 35

3.3.3 Effect of Carbon- Based Fillers on Glass Transition Temperature ....................... 38

3.3.4 Effects of Carbon- Based Fillers on the Electrical Properties by Measuring Electrical Conductivity ......................................................................................... 40

3.3.5 Effect of Carbon- Based Fillers on the Dielectric Constant ................................. 42

3.3.6 Effect of Carbon- Based Fillers on the Mechanical Properties ............................. 44

3.4 Micromechanical Modeling of Thermal Conductivity ..................................................... 46

3.5 Summary ........................................................................................................................... 49

Chapter 4 Composites Embedded with Hybrid Fillers to Tailor Thermal and Electrical Properties .................................................................................................................................. 51

4 Introduction .............................................................................................................................. 51

4.1 Experimental ..................................................................................................................... 52

4.1.1 Materials ............................................................................................................... 52

4.1.2 Surface Modification ............................................................................................ 53

4.1.3 Composite Preparation .......................................................................................... 54

4.1.4 Test Specimen Preparation ................................................................................... 55

vi

4.2 Characterization ................................................................................................................ 55

4.2.1 Surface Modifications of hBN .............................................................................. 55

4.2.2 Composite Morphologies ...................................................................................... 56

4.2.3 Effective Thermal Conductivity ........................................................................... 56

4.2.4 Coefficient of Thermal Expansion ........................................................................ 56

4.2.5 Impedance ............................................................................................................. 56


4.3.1 Surface Modification of hBN ................................................................................ 57

4.3.2 Effect of Hybrid Fillers on the PMCs Effective Thermal Conductivity .............. 58

4.3.3 Effect of Hybrid Fillers on the PMCs Coefficient of Thermal Expansion .......... 62

4.3.4 Effect of Hybrid Fillers on the PMCs Impedance ............................................... 64

4.3.5 Dielectric Constant ................................................................................................ 68

4.4 Micromechanical Modeling of Thermal Conductivity ..................................................... 70

4.5 Possibility of Replacing Kapton with the Polymer Composites ....................................... 73

4.6 Summary ........................................................................................................................... 74

Chapter 5 Composites with Aligned Polymer Fibers ................................................................... 76

5 Introduction .............................................................................................................................. 76

5.1 Experimental ..................................................................................................................... 76

5.1.1 Materials ............................................................................................................... 76

5.1.2 Sample Preparation ............................................................................................... 77

5.1.3 Characterization .................................................................................................... 79


5.2.1 Effect of Temperature and Orientation on the Alignment of Polymer Fibers ...... 80

5.2.2 Effect of GNP Content on the Effective Thermal Conductivity of the LCP/GNP Composites Fabricated by Aligning Polymer Fibers ........................... 84

5.2.3 Effect of Alignment of Polymer Fibers on the Impedance of LCP/GNP Composites ............................................................................................................ 87

vii

5.2.4 Effect of Alignment of Polymer Fibers on the Dielectric Constant of LCP/GNP Composites .......................................................................................... 88

5.3 Summary ........................................................................................................................... 89

Chapter 6 Conclusions and Recommendations ............................................................................. 91

6 Summary .................................................................................................................................. 91

6.1 Recommendations ............................................................................................................. 94

Bibliography ................................................................................................................................. 96

viii

List of Tables Table 2-1: General Characteristics of Polymers used in Electronic Packaging .............................. 9

Table 3-1: Physical Properties of polyphenylene sulfide ............................................................. 29

Table 3-2: Physical Properties of carbon fiber, multi-wall nanotubes, and graphene nanoplatelets

............................................................................................................................................... 29

Table 3-3: A summary of the compositions of PPS filled with micro- and nano- sized carbon

fillers ..................................................................................................................................... 31

Table 3-4: Schematics of the samples used for characterization of PPS embedded with micro-

and nano- sized carbon fillers ............................................................................................... 31

Table 3-5: Percolation threshold calculated by fitting a curve to the experimental data .............. 42

Table 4-1: Physical properties of hBNPTX60 .............................................................................. 53

Table 4-2: Compositions of PPS-hBN-CF composites ................................................................. 54

Table 4-3: Compositions of PPS-hBN-MWNT composites ......................................................... 54

Table 4-4: Compositions of PPS-hBN-GNP composites .............................................................. 55

Table 4-5: Fractional loss in mass of hBN particles upon heating to 900 C ............................... 58

Table 5-1: Physical Properties of LCP .......................................................................................... 77

ix

List of Figures Figure 2-1: Examples of packaging of various electronic devices [12] .......................................... 7

Figure 2-2: Schematic of fillers forming conductive pathway in polymer matrix ....................... 12

Figure 2-3: Schematic for SWNT and MWNT [19] ..................................................................... 13

Figure 2-4: Discontinuous Carbon Fibers [30] ............................................................................. 14

Figure 2-5: Graphene Nanoplatelets [36] ..................................................................................... 15

Figure 2-6: Hexagonal Boron Nitride Spheres [64] ...................................................................... 18

Figure 2-7: Representation of structural differences between liquid crystal polymers and

conventional semi-crystalline polymers [69] ........................................................................ 21

Figure 3-1: Thermal Conductivity of Carbon-Based Filler reinforced PPS composites .............. 34

Figure 3-2: SEM micrographs of PPS/CF composite with 20 wt.% CF at (a) 100X; (b) 2500X;

and (c) 10000X ..................................................................................................................... 35

Figure 3-3: Coefficient of thermal expansion of composites at 50C .......................................... 37

Figure 3-4: Coefficient of thermal expansion of composites at 150C ........................................ 38

Figure 3-5: Tg of PPS filled with micro- and nano- sized carbon fillers ...................................... 39

Figure 3-6: SEM micrographs of PPS/MWNT composite with 20 wt.% MWNT at (a) 5000X (b)

20000X and (c) 50000X ........................................................................................................ 41

Figure 3-7: Change in electrical conductivity as the filler conent increases for PPS embedded

with carbon based fillers ....................................................................................................... 41

Figure 3-8: Effect of carbon-based fillers on the dielectric constant at 3105 Hz ........................ 43

Figure 3-9: SEM micrograph of PPS/GNP nanocomposite with 20 wt% GNP at 50X, 2000X, and

13,000X ................................................................................................................................. 45

Figure 3-10: Compressive Modulus of PPS filled with micro- and nano- sized carbon fillers .... 45

Figure 3-11: Theoretical predictions for thermal conductivity compared to experimental results

for PPS/CF composites ......................................................................................................... 47

Figure 3-12: Theoretical Predictions for thermal conductivity compared to experimental results

for PPS/MWNT composites ................................................................................................. 48


for PPS/GNP composites ...................................................................................................... 49

Figure 4-1: Hybrid fillers in polymer matrix ................................................................................ 52

Figure 4-2: Effect of using a silane-based coupling agent on PPS-hBN composites ................... 58

x

Figure 4-3: Effects of hybrid filler compositions on PMC's keff ................................................... 60

Figure 4-4: SEM Micrographs of (a) PPS-PTX60 with 33.3 vol.% hBN ..................................... 61

Figure 4-5: SEM Micrographs of PPS-hBN-CF composites with 33.3 vol.% hBNPTX60-CF

hybrid fillers with hBNPTX60-to-CF ratios of (a) 27:6.7; (b) 25:8.3; and (c) 22:11 ........... 61

Figure 4-6: SEM Micrographs of PPS-hBN-MWNT Composites with 33.3 vol.% hBNPTX60-

MWNT Hybrid Fillers with hBNPTX60-to-MWNT ratios of (a) 27:6.7 and (b) 25:8.3 ..... 62

Figure 4-7: Total surface area of secondary fillers (filler content = 33.3 vol.%) ......................... 62

Figure 4-8: Effects of hybrid filler composition on PMC's CTE (below the glass transition

temperature of PPS) .............................................................................................................. 63

Figure 4-9: SEM Micrographs of PPS-hBN-GNP Composites with 33.3 vol.% hBNPTX60-GNP

Hybrid Fillers with hBNPTX60-to-GNP ratios of (a) 27:6.7; (b) 25:8.3; and (c) 22:11 ...... 64

Figure 4-10: Effects of hybrid filler compositions on PMC's Impedance for PPS-hBNPTX60-CF

composites ............................................................................................................................. 66

Figure 4-11: Effects of hybrid filler compositions on PMC's Impedance for PPS-hBNPTX60-

MWNT composites ............................................................................................................... 67

Figure 4-12: Effects of hybrid filler compositions on PMC's Impedance for PPS-hBNPTX60-GNP

composites ............................................................................................................................. 68

Figure 4-13: Dielectric constant of PPS-based hybrid fillers ....................................................... 69


for PPS-hBN-CF composites ................................................................................................ 71


for PPS-hBN-MWNT composites ........................................................................................ 72


for PPS-hBN-GNP composites ............................................................................................. 73

Figure 4-17: The maximum temperature of the heat source measured for Aluminum heat sink

with a. no base plate b. with Kapton base plate and c) with PPS-33vol.% hBN .................. 74

Figure 4-18: The maximum temperature of the heat source measured for Aluminum heat sink

with a. PPS-33 vol.% (hBN:MWNT=3:1), b. PPS-33 vol.% (hBN:CF = 2:1), and c. PPS-33

vol.% (hBN:CF = 3:1) base plates ........................................................................................ 74

Figure 5-1: Fiber drawing system ................................................................................................. 78

Figure 5-2: Effect of Compression Sintering and Compression Moldingon the Thermal

Conductivity .......................................................................................................................... 81

xi

Figure 5-3: SEMs of 1 wt.% GNP/LCP composites filled with aligned fibers that were molten-

compression molded; (a) 100X and (b) 1000X ..................................................................... 82

Figure 5-4: SEMs of 1 wt.% GNP/LCP composites filled with randomly oriented fibers that were

molten-compression molded; (a) 100X and (b) 1000X ........................................................ 82

Figure 5-5: SEMs of 1 wt.% GNP/LCP composites filled with aligned fibers that were sintered-

compression molded; (a) 100X and (b) 1000X ..................................................................... 83

Figure 5-6: SEMs of 1 wt.% GNP/LCP composites filled with randomly oriented fibers that were

sintered-compression molded; (a) 100X and (b) 1000X ....................................................... 83

Figure 5-7: Comparison of thermal conductivity of LCP/GNP composites fabricated by

conventional method and by the polymer fiber alignment method ...................................... 85

Figure 5-8: SEM micrographs of neat LCP with aligned polymer fibers fabricated by

compression molding method; (a) 100X and (b) 1000X ...................................................... 86

Figure 5-9: SEMs of 1 wt.% GNP/LCP composites fabricated using the conventional method; (a)

100X and (b) 1000X ............................................................................................................. 86

Figure 5-10: SEM micrographs of 5 wt.% GNP/LCP composites with aligned polymer fibers

fabricated by compression molding method; (a) 100X and (b) 1000X ................................ 86

Figure 5-11: Impedance of LCP/GNP Composites Fabricated by Alignment of Polymer Fibers 88

Figure 5-12: The change in dielectric constant of LCP/GNP composites as a function of filler

content ................................................................................................................................... 89

1

Chapter 1 Introduction

1 Preamble The continuous increase in power outputs along with the reduction of overall size of

components in the computer, microelectronics, automotive and aerospace industries have

resulted in the need for materials with multifunctional properties. These multifunctional

materials are desired to have thermal, electrical, and mechanical properties that are suitable

for the application-specific electronic components to provide higher efficiency without

compromising their performance.

The high power density of electronic components results in rapid heat generation. Therefore,

it is necessary to dissipate heat away from the components sufficiently fast enough in order

to keep their temperatures below critical values and prevent the device from permanent

damage. Metal-based heat sinks and heat spreaders have conventionally been used to

dissipate the excess heat and maintain the device's temperature to an acceptable value. Due

to metals' high mass- density, their uses are limited to heat sink applications for electronic

components. Many portable systems such as laptop computers, handheld telephones, and

avionics require low mass-density to minimize the damaging stresses resulting from shock

loads during shipping and handling. In addition, materials with their coefficients of thermal

expansion similar to those of ceramic substrates and semiconductors are favorable to

minimize the thermo-mechanical stress. In order to reduce the weight of the high power

density electronic devices, it is desired that the entire packaging of the device be made out of

heat dissipating material eliminating the need for heat sinks. Conventional metal-based

materials such as aluminum and copper alloys do not possess all the optimal properties

required for electronic packaging application [1].

In this context, the design and fabrication of multifunctional materials with 1) relatively low

mass-density; 2) coefficient of thermal expansion similar to metals; 3) lower electrical

conductivity than most metals; 4) higher thermal conductivity than polymers; and 5) high

compressive moduli are needed. These materials can be used to fabricate not only heat sinks

2

but also the entire casing and packaging of the high power density electronic devices. Such

materials would serve as a mean to promote heat dissipation as well as to reduce the weight

of the electronic devices.

1.1 Multifunctional Composite Materials for Electronic Packaging Applications

In order to tailor the aforementioned multifunctional properties of the material, one potential

direction is to develop composite materials that possess combined properties of two or more

materials. Composite materials have traditionally been used for structural applications.

Currently, ongoing research is being conducted on multifunctional composites that can be

used in thermal management and electronic packaging applications. For these applications,

high thermal conductivity is the most important objective when selecting materials. Most

metal matrix composites (MMCs) such as carbon fiber-reinforced aluminum or copper,

silicon carbide particle-reinforced aluminum, and diamond particle-reinforced aluminum do

not show substantial improvement in thermal conductivity. There are exceptions, such as,

vapor grown carbon fiber (VGCF) reinforced aluminum matrix composite exhibited a

thermal conductivity as high as 642 W/mK with a density of 2.44 g/cc [2]. However, a very

common disadvantage of all MMCs, including VGCF reinforced aluminum, is that they are

electrically conductive, which may not be suitable for many electronic packaging

applications due to the possibility of having short-circuits. Research has also been conducted

on a second type of composites, which are polymer matrix composites (PMCs), where

polymers are embedded with fillers. PMCs have a number of advantages, including their low

mass-density and the ability to be injection molded into complex shapes with tight

tolerances, eliminating the need for further machining that are often required with metal-cast

parts. Due to their multifunctionality, which includes enhanced thermal conductivity and

mechanical properties, these composites are gaining traction in a number of markets such as

aerospace, automotive, microelectronics, and thermal management industries [3-6]. These

multifunctional polymeric composites may result in materials for electronic packaging

having high thermal conductivities, coefficient of thermal expansion similar to that of

metals, light weight, high strength, increased reliability, and good manufacturability.

3

1.2 Thesis Objectives The goals for multifunctional electronic packaging materials are that they should be light-

weight, injection moldable into complex shapes, and should have tailored electrical

conductivity. Polymers have properties that may help in achieving these goals; however,

they are poor thermal conductors, which is the required fundamental property for electronic

packaging application. One of the common methods to fabricate materials with

multifunctional properties is by having a composite of two or more materials. For the

electronic packaging application, a polymer matrix may be filled with conductive fillers to

obtain all the above mentioned multifunctional properties. A continuous conductive network

of particles throughout the matrix material can improve the effective thermal conductivity of

a polymer composite while keeping the intrinsic properties of a polymer.

For this research thesis, a number of different hypotheses were evaluated with the goal to

improve the effective thermal conductivity of the multifunctional composite material while

keeping the intrinsic properties of the polymer matrix. The main objectives of this research

thesis are summarized as follows:

1. To fabricate and characterize thermally conductive polymer composites with the

maximum possible thermal conductivity and minimum coefficient of thermal

expansion while keeping all the other intrinsic properties of polymers.

2. To fabricate and characterize thermally conductive polymer composites with tailored

electrical conductivity for application-specific electronic components.

3. To investigate the possibility of fabricating thermally conductive polymer

composites by introducing polymer fibers with no or minimum filler content.

1.3 Thesis Organization The body of this thesis is separated into five chapters. A background and literature review is

presented in Chapter 2 on the following topics: thermal management and electronic

packaging, materials for electronic packaging, thermally conductive polymer composite and

4

common models to predict the thermal conductivity of composites. Chapters 3 presents the

study on fabrication and characterization of polymers embedded with different size and

structure of carbon-based fillers to improve thermal properties. Chapter 4 presents the study

on fabrication and characterization of polymer embedded with carbon- and ceramic- based

hybrid fillers in order to improve thermal as well as tailor electrical properties for specific

applications. Chapter 5 discusses the possibility of improving thermal properties of

composites by aligning polymer fibers embedded with minimum or no filler content. This

chapter also includes a method for fabricating LCP based composites by compounding fibers

with a draw ratio of 50 and an analysis of the properties of the resulting material. Finally, the

thesis concludes with an overview of this research and future recommendations.

5

Chapter 2 Literature Survey and Theoretical Background

2 Introduction Since 1930s, polymers have made significant advances in the markets of metals, wood,

glass, paper, leather, and vulcanized rubber that were conventionally used in most household

goods and industrial components as well as creating new markets of their own. The main

reason behind the widespread use of polymers is their unique set of properties such as

toughness, light weight, low cost, and ease of processing and fabrication. Even though

polymers are not the panacea of industrys material problems, their unique set of properties

have made them one of the important classes of materials finding their way into widespread

use in the electronic industries [7].

The most common application of polymers in electrical and electronic devices is for

electrical insulation, which prevents the loss of signal currents and confines them to the

desired paths. These electrical insulation systems may exist in liquid, solid or gas form, and

the type of material used determines the life span of the device. Polymeric materials have

started to also perform structural roles by supporting the circuit physically, and providing

environmental protection from moisture, heat, and radiation to sensitive electronic devices.

The continuing improvements in the multifunctional properties of polymers over the years

have made them even more important to the electrical industry by extending their useful

range [7].

2.1 Thermal Management and Electronic Packaging Moores Law, the famous prediction that states that the number of transistors on a chip will

double about every two years, has held for four decades. The emerging three-dimensional

chip (3D) architecture [8] has offered a new opportunity to continue the CMOS

(complementary metal-oxide-semiconductor) performance trends for the next decade. It not

only serves as a promising solution to mitigate the interconnect problem in modern

6

microprocessor designs, but also enable much higher memory bandwidth for future 3D

microprocessors. Recent progress in the fabrication of high-aspect-ratio vertical

interconnects, called through-silicon vias (TSVs) [9-11], have opened new avenues for high

density area array interconnects between stacked processor and memory chips. Through the

integration of a very large System on a Chip (SoC) in multiple tiers, the average distance

between system components is reduced, leading to improved performance. However, the

surging performance and functions on a chip has come with a hidden cost heat. The

continuous increase in component densities being packed onto a chip requires more power to

run it. A large portion of that power turns into heat, leading to the fast heat up of the chips, if

not properly cooled. Overheating results in hardware failure reducing chips reliability,

potentially leading to computer crashes, mangled files, graphical glitches, and even

permanent damage. In other words, the future of 3D Integrated Circuits crucially hinges on

the development of practical solutions for heat removal.

As it is known, the trend in packaging electronic systems and subsystems has been to reduce

size and increase performance. Higher levels of integration in semi-conductors and the

increased usage of hybrids and multichip modules (MCMs) are some examples. System-in-

Package (SiP) and few chip package (FCP) are also examples of high-density multichip

packaging. Few examples of packaging of electronic devices, components, and systems are

shown in Figure 2-1. The placement of more functions in a smaller package has an inevitable

result of higher heat densities, which requires that high priority be given to thermal

management in their design to maintain system performance and reliability. As clock rates

increase every year, the power dissipated in the semiconductors during switching is

increasing at a linear rate proportional to frequency.

.......................... 2-1

where,

C = input capacitance in farads

V = peak-to-peak voltage swing of signal in volts

f = switching frequency in hertz

7

If the values of input capacitance and voltage swing had remained the same while the clock

frequency increased, the amount of power dissipated would have grown in a linear rate and

would be unmanageable. Fortunately, reduced input capacitance for each new generation of

semiconductors and continuous reduction of voltage swings, which are directly tied to logic

power supply voltages, from the classic 5.0 V to a forecasted value of less than 1 V have

aided the thermal management of electronic devices. An ASIC process such as IBMs 0.10

m Cu-11 using a 1.8 V supply has a heat dissipation of 0.009 W/MHz/gate. An older 0.55

m process using a 5.0 V supply from Texas Instruments has a dissipation of 0.42

W/MHz/gate.

(a) Multichip Module, (b) Integrated Circuit, and (c) System in Package

Figure 2-1: Examples of packaging of various electronic devices [12]

Electrical parameters such as gain, leakages, offset, or forward drop of an active device are

changed if the temperature is increased. Most circuit simulations incorporate the variations

in electrical parameters due to temperature change. It is estimated that leakage currents in

circuits often double every 10 C. If the temperature of an active device increases too much,

it will exceed the manufacturers specifications and usually fail [7]. Therefore, it is desired

to have the temperature of electronic components below their critical value to avoid any

permanent damages. Since there is a need of high performance and small size of electronic

components, installation of separate heat sinks is not an option anymore. It is desired that the

entire packaging of electronic devices be made out of composite materials that can

simultaneously provide signal distribution, heat dissipation, package protection, and power

distribution [13]. In other words, this prompts the need to develop advanced monolithic and

composite materials that are tailored to meet the specific requirements of the electronic

packaging or other heat management solutions.

8

2.2 Materials for Electronic Packaging The continuing increase in electronic packaging density has resulted in a need for materials

with high thermal conductivities. Furthermore, low mass-density is desirable in many

applications, including portable systems such as laptop computers, hand-held telephones,

and avionics. Reducing weight also minimizes potentially damaging stresses resulting from

shock loads that can occur during shipping and from other causes. Low cost is also a key

consideration. As traditional materials used in electronic packaging do not meet all of these

requirements, new multifunctional materials have been and are continuing to be developed

[1].

The multifunctional materials are desired to have thermal, electrical, and mechanical

properties that are suitable for the application-specific components. The high power density

of electronic components results in rapid heat generation. Metal-based heat sinks and heat

spreaders have conventionally been used to dissipate the excess heat and maintain the

devices temperature to an acceptable value. Conventional materials such as aluminum and

copper alloys do not possess all the optimal properties such as low electrical conductivity,

ease of fabrication, and low mass-density required for electronic packaging [1]. In this

context, the design and fabrication of multifunctional materials with relatively low mass-

density, coefficient of thermal expansion similar to metals, lower electrical conductivity than

most metals, and higher thermal conductivity than polymers. These materials can be used to

fabricate not only heat sinks but also the entire electronic casing and packaging. They can

promote heat dissipation as well as reduce the weight of the electronic device. In order to

tailor the aforementioned multifunctional properties of the material, one potential way is to

develop polymer-matrix composites with functional fillers that will help improve polymers

properties.

The electronic packaging industry has been making use of composite material, defined as

two or more materials bonded together. These materials have traditionally been used for

structural applications such as printed circuit boards (PCBs) and encapsulants. However,

there is a potential of creating novel materials with unique set of desired functional

properties by mixing two or more constituents. Polymers filled with thermally conductive

particles that provide aforementioned multifunctional properties have been used in electronic

9

packaging for device encapsulation. Encapsulation of electronic devices protects them from

adverse environment and increases their long-term reliability. For example, polymer-matrix

composites (PMCs) in the form of E-glass fiber-reinforced polymer printed circuit board

(PCBs) are well-established packaging materials [1]. Traditionally, epoxy-based

encapsulants are filled with silica. Silica has a low thermal conductivity of 1.5 W/mK,

hence, these encapsulants show a very poor thermal performance. As the heat dissipation

requirements increase, improved thermally conducting packaging composite materials are

required [14].

2.3 Thermally Conductive Polymeric Composites Polymers have found their usage in almost every application and industry ranging from

automotive to medical. Table 2-1 summarizes a list of polymers along with their usage in

electrical and electronic industry to show how different polymers are used for different

application [7].

Table 2-1: General Characteristics of Polymers used in Electronic Packaging

Materials Processing Electrical/Electronic Application

Fluoroplastics Power metallurgy, sintering,

dispersion coating, compression molding, electrostatic coating

Wire and cable insulation, electrical components

Liquid Crystal Polymers Injection molding

Chip carriers, sockets, connectors, relay cases

Nylon Injection molding, extrusion,

thermoforming, blow molding, casting, rotational molding

Connectors, wire jackets, wire ties, coil bobbins

Polycarbonate Injection molding, extrusion, thermoforming, blow molding Connectors, terminal boards,

bobbins

Polyesters (PBT, PCT, PET) Injection molding, extrusion

Connectors, sockets, chip carriers, switches, coil

bobbins, relays

10

Polyimides Injection molding, casting, compression molding

Insulation for electric motors, magnet wire, flat cable, integrated-circuit

applications

Polyphenylene sulfide Injection molding Connectors

Even though the above mentioned polymers are being used for electrical and electronic

devices, they all lack in their ability to conduct heat, a property fundamentally required for

electronic packaging application. Polymers often lack in areas where materials superior

mechanical, thermal and electrical properties are desired. To overcome these obstacles,

polymers are filled with particles that enable them to fulfill the application-specific

requirements while keeping their low density, easy manufacturability, and low cost.

In this context, thermally conductive but electrically insulating polymer composites have

become increasingly important for the industry. Various kinds of fillers, such as metal, metal

oxide, carbon black, carbon fiber, carbon nanotube, ceramic, etc. have been studied

extensively to prepare thermally conductive polymer composites. Previous studies revealed

that the effective thermal conductivities (keff) possible with polymer matrix composites

(PMCs) are not as high as those of the other advanced materials, such as metal matrix

composites or ceramic matrix composites [15]. However, they have a number of advantages,

including ease of fabrication, low density, and, for some materials, relatively low material

costs. Therefore, a technological breakthrough that allows the fabrication of PMCs with

dramatic improvement in keff, without the compromise of the electrical insulating property of

polymers, will be vital to the technological advancement in the field of heat management for

electrical and electronic devices.

2.3.1 Polymer Composites Embedded with Carbon-Based Particles

Metal matrix composites (MMC) such as carbon fiber-reinforced aluminum or copper,

silicon carbide particle-reinforced aluminum, and diamond particle-reinforced aluminum do

not show substantial improvement in thermal conductivity. However, vapor grown carbon

fiber (VGCF) reinforced aluminum matrix composite exhibited a thermal conductivity as

high as 642 W/mK with a density of 2.44 g/cc [2]. A common disadvantage of all MMCs is

11

that they are electrically conductive, which may not be suitable for many electronic

packaging applications. Research has also been conducted on a second type of composites

i.e. polymer matrix composite (PMC) where polymers are embedded with fillers. Micro- and

Nano- sized carbon fillers, including multi-walled carbon nanotubes (MWNTs), carbon

fibers (CFs), and graphene nanoplatelets (GNPs) have been embedded in polymers to

develop multifunctional polymer composites [3]. Carbon filler-reinforced polymer

composites have a number of advantages, including their low density and the ability to be

injection molded into complex shapes with tight tolerances, eliminating the need for further

machining that are often required with metal-cast parts. Together with the added

multifunctionality, such as enhanced thermal conductivity as well as mechanical properties,

these composites are gaining traction in a number of markets such as aerospace, automotive,

and microelectronics industries for thermal management and electronic packaging

applications [3-5]. These multifunctional polymeric composites may result in materials for

electronic packaging having high thermal conductivities, coefficient of thermal expansion

similar to that of metals, light weight, high strength, increased reliability, and, above all,

good manufacturability. The commonly used reinforcing carbon-based fillers in these

composites are Multi Walled Carbon Nanotubes (MWNTs), Carbon Fibers (CF) and

Graphene Nanoplatelets (GNPs) because of their extremely good mechanical, thermal and

electrical properties. Figure 2-2 shows a schematic of how thermally conductive fillers may

create conductive pathways in the polymer matrix.

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13

Figure 2-3: Schematic for SWNT and MWNT [19]

The structure of CNTs, i.e. the helical arrangement of the carbon atoms in hexagonal arrays

on their surface honeycomb lattices, gives rise to their unique properties. For example, the

topology, i.e. the closed nature of individual nanotube shells, determines the unique physical

properties of CNTs. Their structure is remarkably different from graphite because when

individual layers are closed on to themselves, certain aspects of the anisotropic properties of

graphite disappear [20]. Carbon nanotubes also have very high stiffness values as well as

very high thermal and electrical conductivities only along the axial direction. Therefore, the

introduction of a network of these nanofillers in polymers can convert the thermally and

electrically insulating materials into conductive materials [21]. For example, with the

extremely high thermal conductivity (k) of MWNTs (i.e., 3000 W/mK) [22], an earlier

theoretical work showed MWNT-reinforced polymer composites exhibited significant

improvement (i.e. about 5000% increase) in k over the neat polymers [23]. However, a

number of experimental studies reported that the promotion in k was less than a factor of two

for low loading (e.g., 1 wt.%) of CNTs; some other studies revealed no enhancement or even

a reduction in k relative to the neat polymer [24, 25]. In contrast, the dramatic improvements

(i.e. by 10 orders of magnitude) in electrical conductivity () by the formation of conductive

networks of CNTs at even low loadings (e.g. < 1 wt. %) were routinely observed among

different researchers [26-27]. The significantly different trends for electrical and thermal

conductivity of CNT based composites suggest that two different phenomena are responsible

for the change in k and in CNT based composites.

14

Theoretical and experimental results have shown that the MWNTs have a high elastic

modulus of about 1TP; thermal conductivity of 3000 W/mK and an electrical conductivity

greater than 100 S/cm. Researchers have shown that the addition of MWNTs in composites

assists the transfer of heat by phonon carriers, which are heat follow through lattice

vibration, leading to a significant improvement in the value of k [28]. However, due to a

relatively high thermal interfacial resistance, the thermal conductivity (k) of the composite is

usually much lower than the theoretical value of the bulk material because k is also governed

by the interfacial thermal resistance. On the contrary, the electrical conductivity of the

composites reinforced with MWNTs increased by 10 orders of magnitude by forming

conductive networks of MWNTs [27], as reported by other researchers.

2.3.1.2 Carbon Fibers

Most carbon fibers have a k value close to that of copper i.e. ~ 400 W/mK. However, there

are commercially available carbon fibers that have a k of more than twice that of copper. In

addition to their high k, CFs have a relatively low mass density of 2100 kg/m3. They are also

potentially useful for packaging since they have elastic moduli as high as 965 GPa which is

more than an order of magnitude greater than that of aluminum. CFs having such prospective

properties are currently being used with polymers to form composites that can be injection

molded and have extremely high thermal conductivities [1]. Previous studies had shown

significant increase of k in polymer composites reinforced with CFs. For example, Kim et al.

[29] reported an increase from 0.29 W/mK to 3.1 W/mK in k for a Polyetheretherketone

(PEEK)-CF composite with the addition of 40 vol.% of CFs due to the increased contact

among microfibers in the presence of nanofibers.

Figure 2-4: Discontinuous Carbon Fibers [30]

15

2.3.1.3 Graphene Nanoplatelets

A monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice is

referred to as Graphene. Graphene is a basic building block for graphitic materials of all

other dimensionalities; for example, they can be wrapped up into 0D buckyballs, rolled into

a 1D nanotube or stacked into 3D graphite [31]. Ghosh et al. [32] suggested that if equal

phonon velocities in CNTs and graphene are assumed, the k of graphene should be larger

than that in CNTs. The reported k of CNTs by different researchers would give the

graphenes lower bound k estimate of ~4100-4800 W/mK. Based on the theoretical

calculations, the phonon group velocity in graphene is higher than that in CNTs, which leads

to a larger k. The extremely high thermal conductivity of graphene makes it a potential

candidate for thermal management applications by using it as fillers in polymer composites

that can be used as heat sinks. Multilayers of graphene sheets are much cheaper and are

expected to retain their heat conducting property. There have been studies conducted to

explore the behavior of k in polymer composites by adding graphene. The k of

Polypropylene (PP) showed an increase of 480% i.e. from 0.25 W/mK to 1.2 W/mK [33] by

adding up to 25 vol.% graphene. Reinforcing Nylon 6 with 20 vol.% graphene increased the

k from 0.3 W/mK to 4.1 W/mK [34]. Similar trends showing a dramatic increase in k for

polymer composites due to the addition of graphene were observed by Kim et. al. [35].

Figure 2-5: Graphene Nanoplatelets [36]

16

2.3.2 Polymer Composites Embedded with Hybrid Fillers

PMCs with high keff can be obtained by dispersing highly thermally conductive fillers in the

polymer matrix. Bigg et. al. [37] indicated that there is no additional improvement when the

ratio of fillers thermal conductivity to matrixs thermal conductivity is over 100 times.

Therefore, for fillers with thermal conductivities that are equal to or greater than 100 times

of the polymeric matrices thermal conductivities, the fillers volume fraction may become a

critical factor governing the composites thermal conductivity. In general, composites with

low loadings of filler particles would allow the heat to flow uniformly through them since all

particles are well dispersed, but such low filler content limits the interconnection among the

filler particles. Increasing the filler content promotes the formation of preferential heat

conducting paths for dissipating heat through the composite [38-39].

Extensive studies have been conducted to promote PMCs keff by different kinds of fillers.

For example, metal- [40-43], ceramics- [44-49], and carbon-based fillers [33, 50-52] have

been applied to fabricate PMCs. keff of PMCs fabricated in these studies were in the range of

0.5 W/mK to 2.0 W/mK when the filler content was lower than 33.3 vol.%. Although PMCs

with keff (i.e., 5.0 W/mK to 32.5 W/mK) were reported in literatures [43-45], it required

either the addition of 60-90 vol.% of thermally conductive fillers or the uses of electrically

conductive metal fillers. These would either compromise the processibility of the composites

or the electrical insulating property of PMCs, which are two key benefits, offered by PMCs

in heat management applications. In addition to filler contents, previous studies also

suggested that the development of thermally conductive networks hinges on the sizes and

shapes [39, 53], the aspect ratios [53-54], as well as the spatial and geometric arrangements

[55] of the fillers. Experimental studies revealed that smaller filler particles would result in

lower keff because of a larger polymer-filler interfacial area, causing phonon scattering and

hindering phonon transport [56]. Furthermore, fillers with high aspect ratio would enhance

the formation of heat conductive networks [40-41], promoting the composites thermal

conductivities.

Various studies on thermally conductive PMCs filled with ceramic fillers had incorporated

silicon carbide (SiC) [29], aluminum nitride (AlN) [46, 57], or hexagonal boron nitride

(hBN) [45-49] in polymer matrices. The relatively high hardness of SiC and AlN are

17

detrimental to the manufacturability of the composites. In contrast, hBN, which resembles

the two-dimensional covalent structure of graphite, has a low dielectric constant (i.e., ~4)

and is lubricious, which helps efficient wetting. These intrinsic properties are consistent with

the objectives of developing thermally conductive PMCs, which includes high thermal

conductivity, high electrical resistivity, and good processibility. While there are practical

limits on the maximum amount of thermally conductive fillers that can be used without

compromising the good processibility and mechanical performance of PMCs [58], strategies

that can promote the formation of random bridges or networks would be beneficial to

promote PMCs keff. Few researchers have investigated whether the addition of hybrid fillers

of different shapes and/or sizes would lead to higher PMCs keff due to the formation of

effective thermal conductive path [54, 58-60]. However, these studies focused solely on the

effect of hybrid fillers on the PMCs keff. In fact, the addition of secondary filler not only can

promote the keff of the PMCs but also can tailor the electrical resistivity of the PMCs if the

secondary filler is electrically conductive. This will be discussed in chapter 5. Thermally

conductive PMCs with tailored electrical resistivity can be used for ULSI, capacitors, and

electronic packaging materials.

2.3.2.1 Hexagonal Boron Nitride

Boron Nitride (BN), a wide band gap semiconductor, is known to have a high thermal

conductivity and better performance at high temperatures. It is also lightweight and cost

effective, which are both important qualities for electronic packaging materials [61].

Hexagonal boron nitride is comprised of planar sheets of covalently bonded boron and

nitrogen atoms, which make up the in-plane structure of the crystal [62]. The different layers

are held together in the through-plane direction through Van der Waals forces. Since the

crystal structure is anisotropic, the thermal conductivity varies based on the direction. The

in-plane thermal conductivity (k) has been estimated to be greater than 300 W/m-K, while

the through-plane k is much lower, around 3 W/m-K [62]. Hexagonal boron nitrides

graphite-like structure also provides a lubricative effect which is desirable for processing.

Minimal tangential resistance, seen with this type of structure that results in minimum

18

coefficient of friction, is attributed to shearing of the weak bonding between planes, in turn

creating a low coefficient of friction and a high carrying load capacity [63].

Figure 2-6: Hexagonal Boron Nitride Spheres [64]

2.3.3 Composites with Aligned Polymer Fibers

Heat transfer by conduction involves transfer of energy within a material without any motion

of the material as a whole. Polymers transfer heat by lattice vibrations so that there is no net

motion of the media as the energy propagates through polymer matrix. Such heat transfer is

often described in terms of phonons, which are quanta of lattice vibrations. Metals are

much better thermal conductors than polymers because the same mobile electrons that

participate in electrical conduction also take part in the transfer of heat. Therefore, the one

way to increase electrical conductivity of a polymer matrix composite (PMC) is by the

addition of electrically conductive filler particles i.e. the higher the filler content the higher

the electrical conductivity. Furthermore, electrical conductivity of a PMC is not significantly

affected by the addition of conductive filler content beyond the percolation threshold. On the

other hand, the thermal conductivity in PMC is determined by the phonon transfer within the

matrix and the filler. The thermal conductivity of a material is temperature dependant and

the effect of temperature on polymers is of practical importance because electronic

19

packaging applications have relatively higher service temperatures. Generally, with increase

in temperature, thermal conductivity for amorphous polymers increases gradually in the

glassy region and decreases slowly or remains constant in the rubbery region. For crystalline

polymers, thermal conductivity decreases steadily with the increase in temperature below the

melting point. At temperature above the melting point, it behaves in a similar way as

amorphous polymers [65].

Thermal conductivity of polymers is also highly dependent on polymer chain segment

orientation. This is because thermal energy transports more efficiently along the polymer

chain. Crystalline polymers have highly ordered chain segments, and therefore have higher

thermal conductivity than amorphous polymers. Amorphous polymers may exhibit

anisotropic thermal transport properties if polymer chains are partially oriented, with thermal

conductivity along the chains higher than that perpendicular to the chains [65]. Thermal

conductivity of polymers may be increased by addition of fillers. However, addition of

conductive filler in PMC increases its thermal and electrical conductivities simultaneously.

The value of thermal conductivity substantially depends on the nature of filler distribution,

its structure, orientation relative to heat flux, and the ability to form chains in the matrix of

polymer [66]; whereas, the value of electrical conductivity only depends on the mere

presence of conductive fillers.

Kline et. al. concluded that the thermal conductivity level of the polymers depends largely

on whether they were amorphous or partially crystalline in nature in the range of 0 to 100C

[67]. In amorphous regions the thermal conductivity increases as the chain segments become

more mobile. At the glass transition temperature, the mobility and thus the conductivity

should increase rapidly. In partially-crystalline polymers, the conductivity in the crystalline

regions was evidently somewhat higher than that in the amorphous regions because of the

cooperation between movements of neighboring molecules in the crystallites. The crystalline

regions could thus strongly influence the composite conductivity of the polymer. As the

temperature rose in the crystallites toward the melting temperature, the overall conductivity

decreased rather sharply, although that of the amorphous regions was gradually increasing

[67]. Nysten et. al. reported that the thermal conductivity of stretched film with a draw ratio

of 40 of polyethylene (PE) along longitudinal direction was significantly higher than in the

transverse direction [68]. The increase of the thermal conductivity in stretched polymers was

20

generally attributed to the alignment of polymer chains in the crystalline phase. However,

the thermal conductivity of drawn polymers was strongly anisotropic. Hence, thermally

conductive and electrically insulative PMCs may be fabricated by introducing polymer fibers

that will transfer heat more efficiently and adding fillers below their percolation threshold to

keep the PMC electrically insulative. Liquid crystal polymer (LCP) shows evidence of

polymer fiber formation. This property of LCP may be used to fabricate composites with

aligned LCP polymer fibers.

2.3.3.1 Liquid Crystal Polymer (LCP)

Liquid crystal polymers (LCPs), a family of high-performance polymers, are distinguished

from semi-crystalline polymers by their special molecular structure that consists of rigid,

rod-like macromolecules ordered in the melt phase to form liquid crystal structures, as seen

in Figure 2-7.

21

Figure 2-7: Representation of structural differences between liquid crystal polymers and

conventional semi-crystalline polymers [69]

They were first discovered in 1950 [70]. They are formed as a consequence of molecular

asymmetry. Since two molecules cannot occupy the same space simultaneously [70], it

results in long, rigid, rod-like molecules. LCPs possess the properties of polymers and

liquids by being in a state of highly ordered condensed fluid that exists between crystalline

solids and isotropic liquids. They acquire the properties of solids and liquids via the

realignment of LCP molecules at elevated temperature. The properties of liquid crystal phase

combined with the specific properties of polymer have given rise to new potentials that are

impossible with conventional materials. Due to aromatic ring-structured compounds, LCP is

a highly crystalline material that is very stable after polymerization. Fibrous nature of liquid

crystal morphology results in excellent thermal stability, chemical and flame resistance, high

modulus, high melting point, and good impact strength. Therefore, LCP is suitable for and

widely used in electrical and electronic components, cables, and connectors for fiber optics,

apparatus for chemical processes, medical equipment, automotive and mechanical

22

engineering, and the aerospace industry [71]. It is believed that ceramics, metals, and other

polymers used in packaging applications may be replaced by LCP based composites.

Many researchers have conducted studies in order to improve composites properties by

reinforcing polymer matrix with LCP fibers. For example, Chinsirikul et. al. reported ~400%

increase in elastic modulus over that of neat PE matrix by blending PE with LCP. It was

reported that this property improvement appeared to be the result of self-reinforcement by

the LCP fibrillar morphology. In that study, LCP fibrils with high aspect ratios of ~ 100 or

higher were developed in the presence of a strong elongation flow field [72]. Another study

conducted by Kim et. al. reported significant increase in the tensile strength and modulus of

LCP/PEN/PET composite fibers. The increase was attributed to both the reinforcement

effect of polymer matrix by LCP and the development of more ordered and oriented

structures with well-distributed LCP [73]. Furthermore, Wong et. al. reported that the

difference in the thermal conductivities of LCP/CB composites in the three orthogonal

directions was the result of the molecular alignment within the LCP matrix. They also

reported that CB aggregates did not make dominant contributions to the thermal conductivity

as they did in electrical conductivity [74]. Hence, in light of these studies conducted it is

proposed that LCP fibers may be introduced and aligned in order to promote thermal

conductivity of PMC while keeping the filler content below the percolation threshold so that

PMC remain electrically insulative.

2.4 Theoretical Predictions for Thermal Conductivity of Composite Materials

In a solid material, there are two main mechanism of heat conduction: through lattice

vibration waves, which are called phonons, and through free electrons. The movement of

phonons is associated with a certain thermal energy, while free or conducting electrons

participate in electronic thermal conduction [75]. Transport of heat within a non-metallic

material occurs predominantly by phonon or lattice vibrations [76]. The fundamental models

to predict the thermal conductivity of polymer composites are Series, Parallel, and

Geometric Mean models, which are defined below.

23

Series Model: 1 .......................... 2-2

Parallel Model: .......................... 2-3

Geometric Mean Model: .......................... 2-4

where,

kc = thermal conductivity of continuous phase

kd = thermal conductivity of discrete phase

= volume fraction of discrete phase

The series and parallel models are used to represent the upper and lower bounds of the

effective thermal conductivity, respectively. The series model considers a case where the

fillers are fully interconnected forming an internal network structure, generally

overestimating the keff-value. For the parallel model, each particle is assumed to be an

isolated entity; therefore, minimizing the effect of the dispersed phase and underestimating

keff. The geometric mean model is a combination of the parallel and series models, assuming

an equal proportion of each arrangement. This model is dependent on the weighted average

of the filler and matrix materials, and has no physical basis [77].

Maxwell Theoretical: The Maxwell model uses potential theory to obtain an exact

solution for the conductivity of randomly distributed and non-interacting homogeneous

spheres in a homogeneous continuous medium, as shown below

.......................... 2-5

This theory describing electrical conductivity was then adapted by Eucken for thermal

conductivity. Predictions obtained using this model are generally more accurate for low filler

concentrations, for scenarios where the particles do not network [78].

Lewis and Nielson Semi-Theoretical Model: Many studies have found the Lewis and

Nielson semi-theoretical model to be the best fit for solid-solid compositions. Lewis and

24

Nielsen modified the Halpin-Tsai [77] equation to include the effect of the shape of the

particles and the orientation or type of packing for a two-phase system [79].

.......................... 2-6

where

, 1

The values of A and for many geometric shapes and orientation can be found in the literature.

2.5 Interfacial Thermal Resistance Earlier, it was thought that the only parameters affecting the effective thermal conductivity

of the composite were the thermal conductivity, the geometric distribution and the volume

fraction of each component but independent of their dimensions. Starting in 1980s, it was

found out that poor mechanical or chemical adherence at the interface and a mismatch in the

coefficients of thermal expansion, such as that on cooling of the composite from the

temperature at which it was manufactured, an interfacial gap developed between the

dispersed and matrix phases [80]. It was reported that as the interfacial gap decreased the

interfacial thermal resistance decreased due to the re-establishment of interfacial contact

with increasing temperature. The most commonly used fillers can be classified widely into

three categories i.e. spherical, cylindrical, and flat-plate fillers. The effect of interfacial

thermal resistance on the effective thermal conductivity of composites with spherical or

cylindrical dispersed phase geometry could be modelled by minor modification of the

original Rayleigh and Maxwell theories. On the other hand, the series-circuit approach could

be used to model the effect of flat plate geometry fillers with heat flow perpendicular to the

interface [81].

25

2.6 Summary of Guidelines There are many approaches associated with electronic packaging and the materials used for

its components. Each of these elements was discussed within this chapter including: a

general overview of thermal management and electronic packaging, materials for electronic

packaging, and thermally conductive polymer composites. Documentation on basic

principles as well as a comprehensive review of previous research has been included as a

guide for the selection of the best materials, fabrication techniques, and processing

parameters for this research thesis. Polymer matrix composites were selected for the basis of

this work. Carbon-based materials were decidedly the best filler material due to their high

thermal conductivity and ability to tailor electrical conductivity. Use of hybrid materials was

also suggested to introduce a positive synergistic effect. Introduction and alignment of

polymer fibers in LCP was also proposed to be a method to improve thermal conductivity.

2.6.1 Material Selection

Two types of polymer matrix materials (i) polyphenylene sulfide (PPS), and (ii) liquid

crystal polymer (LCP) and four types of thermally conductive fillers (i) hexagonal boron

nitride (hBN) (ii) carbon fiber (CF); (iii) multi-walled carbon nanotube (MWNT); and (iv)

graphene nanoplatelets (GNP) had been selected for this research. In addition, various novel

fabrication strategies were explored to stretch the value of composite keff.

2.6.2 Matrix Materials

For the electronic packaging applications being considered in this research thesis, it is

necessary for the polymer to have a high service temperature (> 180C). In this context,

thermosetting plastics such as epoxy may be a natural choice because of the good thermal

stability and low cost. However, if taking the processibility and the recyclability of the

materials as other key criteria in choosing the base materials, engineering thermoplastics,

which exhibit superior mechanical and thermal properties over a wide range of conditions,

seems to be appropriate choices. After comparing the cost, thermal conductivity, and service

26

temperature of potential engineering thermoplastics, PPS and LCP were selected as the

matrix materials. The rationales behind the selection of these materials were that they are

thermoplastics that have sufficiently high service temperatures (i.e., about 180C or above),

and can easily crystallize or align their molecular chains. In particular, PPS has high service

temperature and good ability to crystallize. Fortron 0203 and 0214 PPS, supplied by Ticona,

were used in experiments related to PPS-based composites discussed in this thesis report.

The particular grades of PPS were chosen because of their abilities of rapid crystallization

and their low viscosity, which allow a high loading of fillers. LCP (Ticona, Vectra A950)

was chosen of its ability to align its molecular chain in the liquid state. Both of them have a

service temperature of 200C, melting point of 280C and thermal conductivity of 0.22

W/mK.

2.6.3 Filler Materials:

The filler materials being selected in this research can be subdivided into ceramic fillers and

carbon fillers. The chosen ceramic filler is hexagonal boron nitride (hBN). It was chosen

because of its high thermal conductivity and low electrical conductivity. Furthermore, hBN

was chosen because its molecular structure resembles that of graphite. Therefore, it can act

as a solid lubricant and facilitate the melt compounding process of composites with high

filler contents. For carbon fillers, three types of fillers with different sizes or different

geometries were investigated. These fillers include carbon fibers (CFs), multi-walled carbon

nanotubes (MWNTs), and graphene nanoplates (GNPs). All of these three types of carbon

fillers have high thermal and electrical conductivities. GNPs are composed of the same

material as carbon nanotubes, and they share many of MWNTs properties. It can be

described as an unrolled carbon nanotube. Its two-dimensional structure is expected to

enhance the interconnection of the fillers dispersed in the polymer matrix.

2.6.4 Micromechanical Models Selection

Intuitively, adding filler with a very high thermal conductivity should increase the thermal

conductivity of the composite significantly. However, experimental results by other

27

researchers showed lower than expected thermal conductivity of the composites embedded

with conductive fillers, some of which have about 4 orders of magnitude higher thermal

conductivity than the polymer. Research is being conducted to predict this loss in the

thermal conductivity at the interface. Models are needed to predict the thermal conductivity

of the composites for high filler content where there is filler-filler and polymer-filler

interfacial thermal resistance present. These types of models will allow us to predict the

thermal conductivity of composites embedded with filler content as high as 50 wt.%.The

series, the parallel, and the geometric-mean models are the simplest models that provide

upper, lower, and middle values for effective thermal conductivities of composites. On the

other hand, Maxwell theoretical model uses potential theory to obtain an exact solution for

the conductivity of randomly distributed and non-interacting homogeneous spheres in a

homogeneous continuous medium. A more realistic model is the Nielsen Semi-Theoretical

model that takes into account the effect of the shape of the particles and the orientation or

type of packing for a two-phase system. These are the most commonly used models in

literature and therefore, will be used to compare with the experimental data in this report.

28

Chapter 3 Effects of Size and Structure of Micro- and Nano- Sized

Carbon Fillers on Composites Properties

3 Introduction Although extensive research efforts were made by various researchers [3, 82-86] to

investigate the roles of micro- and nano- sized carbon fillers in promoting the thermal,

electrical, or mechanical properties individually, the interrelations among mechanisms

through which the fillers enhance these multifunctional characteristics are scarce. In light of

this, this chapter systematically studies the multifunctional characteristics, including thermal,

electrical, and mechanical properties, of polyphenylene sulfide-based micro- and nano-

composites filled with CFs, MWNTs, and GNPs. Analyses were done to explore the

different requirements to develop carbon filler network in order to promote different

functional performances (e.g., thermal, electrical and mechanical) of the composites.

3.1 Experimental

3.1.1 Materials

For electronic packaging and thermal management applications, desirable materials should

have service temperature (Tserv) as high as 200C [87]. Since a wide spectrum of emerging

applications of polymer composites demand the stability of the materials at high

temperature, polyphenylene sulphide (Fortron 0214, Ticona), which has a melting

temperature (Tmelt) of 280C and a service temperature of 200C, was chosen as the matrix

material. The thermally conductive fillers used were CFs (Cytec Industries, Thermalgraph

Pitch-based DKD X), MWNTs (CheapTubes Inc., Industrial Grade), and GNPs

(CheapTubes Inc., Grade 2). All materials were used as received without any further

modification. The physical properties of the polymer and fillers are summarized in Table 3-1

and Table 3-2, respectively.

29

Table 3-1: Physical Properties of polyphenylene sulfide

Physical Properties Values Units

Density () 1350 Kg/m3

Melting Temperature (Tm) 280 C

Maximum Service Temperature (Tmax)

200 C

Thermal Conductivity (k) 0.22 W/m-K

Dielectric Strength 18 kV/mm

Dielectric Constant 10kHz 3.2 -

Coefficient of linear thermal expansion

52 m/m/C

Table 3-2: Physical Properties of carbon fiber, multi-wall nanotubes, and graphene

nanoplatelets

Physical Properties CF MWNT GNP

Density () 2200 kg/m3 2100 kg/m3 2000 kg/m3

Thermal Conductivity (k)

400-650 W/m-K 3000 W/m-K 3000 W/m-K

Electrical Conductivity

>3300 S/cm >100 S/cm ~20,000 S/cm

Shape Fibrous Hollow tube Platelet

Length 200 m 10-30 m 10 nm

Diameter 10 m Inner: 5-10 nm

Outer: 20-40 nm

25 m

30

3.1.2 Sample Preparation Uniform dispersion of the filler particles in the polymer matrix was desired in order to form

a 3-D conductive network in the composites with high filler content, i.e., above the

percolation threshold. A twin-screw DSM Xplore 15 micro-compounder was used to

uniformly disperse the CFs, MWNTs, and GNPs in PPS. The filler contents were varied

from 1 wt.% to 20 wt.% for MWNTs and from 1 wt.% to 30 wt.% for CFs and GNPs. The

highest loadings of CFs, MWCNTs, and GNPs were chosen based on the maximum amounts

of fillers that could be processed by the micro-compounder. Table 3-3 summarizes the

compositions of the PPS-based composites with different fillers. The fillers and the polymer

powders were first dry-blended at room temperature. The mixture was then compounded by

the microcompounder at 300C and 50 rpm for 6 minutes. The extrudates were cooled in a

water bath at room temperature. Consequently, they were pelletized and ground into fine

powders using a pelletizer and a freeze mill, respectively. The fine powders were

compressed at room temperature for 1 minute to ensure the samples were compact. The mold

was then heated to 310C and the composite melt was compressed at 1500 psi for 20 minutes

into disk-like samples for thermal, mechanical, and electrical analyses.

Table 3-4 summarizes the dimensions of the samples that were used for various tests. The

mass fraction of the composites may be converted to volume fraction by using the densities

of the matrix and the filler materials. All samples prepared for thermal conductivity

measurements were machined and ground to improve the surface smoothness to improve the

accuracy of the measurements. Three 2 mm deep radial holes were drilled at 1 mm below the

top surface, in the middle, as well as at 1 mm above the bottom surface of the samples for

thermal conductivity measurements. Three thermocouples, with diameters of 0.076 mm,

were inserted into the holes for these samples for the measurements.

31

Table 3-3: A summary of the compositions of PPS filled with micro- and nano- sized carbon

fillers

Mass Fraction of PPS Mass Fraction of Filler Fillers

100% 0% -

99% 1% CF, MWNT, GNP



85% 15% MWNT, GNP


70% 30% CF, GNP

Table 3-4: Schematics of the samples used for characterization of PPS embedded with micro- and nano- sized carbon fillers

Thermal Conductivity Coefficient of Thermal Expansion & Mechanical

Test

Electrical Conductivity

3.2 Sample Characterization Effective thermal conductivity (keff), coefficient of thermal expansion (CTE), glass transition

temperature (Tg), impedance (Z), dielectric constant (), compressive modulus (Ecomp) and

morphology were measured and studied to investigate the multifunctional properties of PPS-

based micro- and nano- composites. Since electronic packaging application requires high

thermal conductivity, improvement in keff of the composites by at least 10 times over neat

PPS was sought. Composites CTE was desired to be similar to the devices CTE on which

20 mm 6 mm

20 mm10 mm 8 mm

2.5 mm

32

they will be attached to reduce the occurrence of thermal stresses. In order for the composite

materials to be used for different electronic components such as ULSI, IC and capacitors,

composites were needed to be slightly electrically conductive with higher i.e. 3 < < 100

for charge accumulation. It should be noted that for electronic packaging materials, low

electrical conductivity is sought; however, dielectric measurement can be used to provide

material selection criteria for various electronic components. Improved tensile strength and

elastic modulus were also required to use the composites as a support structure for the

electronic components. The keff of the composites was measured by a thermal conductivity

analyzer (ASTM E1225-04) [88] at 150C. CTE was measured by analyzing the dimension

change of the composite materials as they were heated from room temperature to 200C at a

rate of 10C /min by the Q400-series thermomechanical analyzer (TMA) from TA

Instruments. CTEs were determined by determining the instantaneous rate of change of

samples dimension with respect to temperature at 50C and 150C. These temperatures

were selected to provide a range of CTE for the potential range of in-service temperatures of

composites. Tg was measured by determining the inflection point of the CTE versus

temperature graph. Z and were measured using a dielectric/impedance analyzer (Alpha-N-

Novocontrol Technologies) over a frequency range of 10-2 to 105 Hz with an applied AC

voltage of 1 V. The morphology of the composites was investigated by scanning electronic

microscopy (SEM, JEOL, model JSM6060) on the cross-sections of the samples that were

fractured in liquid nitrogen.

3.3 Results and Discussion

3.3.1 Effects of Carbon Fillers on the Effective Thermal Conductivity

The results obtained from effective thermal conductivity (keff) measurement of PPS based

composites filled with CF, MWNT, and GNP are depicted in Figure 3-1. The data for PPS

composites filled with 30 wt.% MWNT (i.e., 21.63 vol.% MWNT) is not included because

this composition was too viscous for the compounder to process. It was observed that the

addition of CF, MWNT, and GNP increased keff by 5 to 10 times at high filler loadings.

33

Comparing the keff of PPS-based composites with different carbon fillers, it can be observed

that GNPs prom

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