03 assembly techniques and packaging

7/27/2019 03 Assembly Techniques and Packaging

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UNIVERSITI TUNKU ABDUL RAHMAN

Assembly Techniquesand

Packaging

Dr. Lim Soo King

05/15/2012



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Chapter 3 Assembly Techniques and Packaging ............................. 4 3.0 Introduction ................................................................................................ 4 3.1 Assembly Technologies .............................................................................. 4

3.2.1 Electrical requirements ....................................................................................... 9 3.2.2 Mechanical and Thermal properties ................................................................ 13 3.2.3 Cost ...................................................................................................................... 16

3.3 Packaging Level Integration ................................................................... 16 3.3.1 Interconnect Levels ............................................................................................ 17 3.3.1 Interconnect Level 1 - Die-to-Package-Substrate............................................ 18 3.3.2 Interconnect Level 2 - Package Substrate to Board........................................ 20 3.3.3 Multi-Chip Modules - Die to Printed Wire Board .......................................... 22

3.4 Assembly Techniques and Processes ...................................................... 24 3.4.1 Wafer Preparation ............................................................................................. 26 3.4.2 Die Attach ........................................................................................................... 26

3.4.2.1 Eutectic Die Attach .......................................................................................................... 27 3.4.2.2 Epoxy Die Attach ............................................................................................................. 28 3.4.3 Wire Bonding...................................................................................................... 28 3.4.4 Molding/Glass Seal............................................................................................. 29 3.4.5 Post Mold Cure/Leak Check ............................................................................. 31 3.4.6 Solder Dip/Tin Plate........................................................................................... 31 3.4.7 Trim/Form .......................................................................................................... 33 3.4.8 Inspection ............................................................................................................ 33

Exercises .......................................................................................................... 33 Answers of Exercises...................................................................................... 34 Bibliography ................................................................................................... 36



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Figure 3.1: Variety of Package types for both plastic and hermetic surface mount and

through-hole mount ............................................................................................ 5 Figure 3.2: A hermetic package showing the integrated circuit is decoupled from external

environment ....................................................................................................... 6 Figure 3.3: A plastic package showing the integrated circuit is not decoupled from

external environment ......................................................................................... 6 Figure 3.4: A plastic package showing the integrated circuit is not decoupled from

external environment ......................................................................................... 7 Figure 3.5: Rent’s constant for varies class of chip and system figure ................................ 8 Figure 3.6: Dielectric constant of common packaging materials ....................................... 10 Figure 3.7: Cross sectional structure for impedance control. (a) Microstrip line, (b) Strip

line, and (c) coplanar structure......................................................................... 11 Figure 3.8: Typial types of noise; (a) cross talk noise and (b) switching noise ................. 13 Figure 3.9: Cross sectional view of multi-layer lead frame package and the heat transfer

mechanism ....................................................................................................... 15 Figure 3.10: Integrated circuit packaging level ................................................................... 17 Figure 3.11: Interconnect hierarchy in traditional integrated circuit packaging .................. 18 Figure 3.12: Wiring bonding connecting pad and lead ........................................................ 18 Figure 3.13: Typical conductance and inductance of package type and wire ...................... 19 Figure 3.14: Automated tap bonding (a) polymer with imprinted wire pattern and (b) die

attach using solder bump ................................................................................. 19 Figure 3.15: Flip-chip bonding ............................................................................................. 20 Figure 3.16: Printed circuit board mounting approach. (a) through-hole mounting and (b)

surface mounting .............................................................................................. 20 Figure 3.16: Commonly use package (1) leadless carrier, (2) DIP, (3) PGA, (4) small

outline IC, (5) quad flatpack, and (6) PLCC .................................................... 21 Figure 3.18: Parameters of various chip carriers .................................................................. 22 Figure 3.19: Ball grid array packaging; (a) cross-section, (b) photo of PGA bottom .......... 22 Figure 3.20: An Avionics processor module. Courtesy of Rockwell International ............. 23 Figure 3.21: Generic electronics packaging assembly sequence for plastic and ceramic

package ............................................................................................................ 26 Figure 3.22: The basic structure of a silicon device die attach with a metal preform .......... 27 Figure 3.23: Structure of ceramic dual inline package (cerdip) showing the base, the lead

frame and a lid with sealing glass .................................................................... 30 Figure 3.34: Schematics of a multi-pot transfer-mold system showing small mold

compound tablets with each large enough to fill a few cavities containing

plastic strips ..................................................................................................... 31 Figure 3.25: Molded plastic package strip showing short between tips of the lead, tight bar

and guide pin hole ............................................................................................ 32 Figure 3.26: The formed molded plastic dip package strip shows that tight bar has not been

removed............................................................................................................ 32



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Chapter 3

Assembly Techniques and Packaging

_____________________________________________

3.0 Introduction

Assembly techniques and packaging involve process of choosing the right type

of package for a particular integrated circuit type and assemble the integrated

circuit in the form of die into package that can be used for application.

3.1 Assembly Technologies

There are many assembly technologies available in today’s assembly of

integrated circuit into package device that can be used to insert into printed wire

board PWB for application. The most common assembly technologies are the

plastics and hermetic assembly technologies. The plastic assembly can be sub-

divided into various package style both for surface mount SM and through-holeTH mount assembly techniques. Plastic package styles can be plastic dual inline

package PDIP, plastic quad flat package PQFP, single outline package SOP,

plastic leadless chip carrier PLCC, small outline integrated circuit SOIC etc.The technology to assemble these package various especially the wire bond,

mold, and plating operation. The overview of the package types is shown in Fig.

3.1. Note that no all available package styles are shown.

Hermetic assembly technology is basically used to assemble high reliability

integrated circuit that are used in industrial, military, and outer space

applications. In this case, the integrated circuit is decoupled from external

environment by a vacuum-tight enclosure. Common packages that are

assembled using this technology are ceramic dual inline package CDIP, pin gridarray PGA, ball grid array BGA etc. An A typical hermetic package with a

silicon chip placed in the cavity of a ceramic-based package and wedge-bonded

to make electrical connections to the terminals on the package is shown in Fig.3.2.



03 Assembly Techniques and Packaging

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Figure 3.1: Variety of Package types for both plastic and hermetic surface mount andthrough-hole mount




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Figure 3.2: A hermetic package showing the integrated circuit is decoupled from external

environment

Plastic assembly technology is usually used to assembly high volume, low cost

integrated circuit. The integrated circuit or die is not decoupled from external

environment. The die is in contact with expoxy resin, whereby in long run

environment contaminant can penetrate the plastic to reach the integrated circuit

causing reliability issue. However, with today’s technology, plastic package

device begins gain acceptable for housing high reliability product. A typicalplastic package structure consisting of a silicon die, a metal lead frame, and aplastic molding compound is shown in Fig. 3.3.

Figure 3.3: A plastic package showing the integrated circuit is not decoupled from external

environment




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Since many of them are used in electronic application, memory device usually is

low cost. The need for low cost memory device has been driving the trend for

cheaper plastic package. A variety of SM plastic packages such as SOJ, SOP,

and thin SOP TSOP has been developed for industrial use, Except for TSOP,these packages have typically 2 mm thick body. TSOP packages have 1-mm-

thick plastic body suitable for compact application. As the chip occupancy

continues to grow and the stringent requirements, this imposes have led to

considerable changes in package structures. The lead-on-chip LOC structure, in

which wire interconnection within the package are made above the die circuitry

surface is notable. In conventional packages for older-generation devices, such

as that these shown in Fig. 3.2 and Fig. 3.3, the interconnection is made only in

the periphery and outside the die area. Exploiting an additional area for the

interconnections and reducing wire length. A typical lead-on-chip LOC packageis shown in Fig. 3.4. Here the tips of the lead frame extend over the chip

surface, and Au wires are stitch-bonded to the lead frame tips to connect them

with the chip bonding pads, which are located in the interior of the chip area.The LOC structure increases the chip occupancy to more than 70% of the

package area. The structure provides die design flexibility because it allows the

pads to be located on the chip in almost any position. It also allows the

placement of substitutes for additional inner lead portions or bus-bars that work

as alternatives to power and ground Al metallizations on the die circuitry. The

bus-bars are numerous in some high-speed memory designs, since they canenhance the electrical performance of electrical performance of the device

without increasing die size.

Figure 3.4: A plastic package showing the integrated circuit is not decoupled from external

environment




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3.2 Packaging Design

Integrated circuit package plays a fundamental role in the operation and

performance of a component. Besides providing a mean of bringing electricalsignal and voltage supply via wires in and out of the silicon die, it also removes

heat generated by the circuit and provides mechanical support. It also protects

the integrated circuit against environmental conditions such as humidity and

heat. Furthermore the package has a major impact on the performance and

power dissipation of the integrated circuit like the microprocessor and signal

processor. This influence is getting more pronounced as technology scaling

down progressed due to reduction of internal signal delays and on-chipcapacitance. Up to 50% of the delay of a high-performance computer is due to

packaging delay caused by inductive and capacitive parasitic from packingmaterial. The increasing complexity of circuit integrated into a single die also

translates into a need for ever more input-output pins. This is because the

number of connections is roughly proportional to the complexity of the circuitry

on the chip. This relationship was first observed by E. Rent of IBM, who

translated it into an empirical formula called Rent’s rule. This formula relates

the number of input/output pins P to the complexity of the circuit as measured

by the number of gates.

P = kGβ

(3.1)

where k is the average number of I/Os per gate, G the number of gates, and β

the Rent exponent. β varies between 0.1 and 0.7. The value is strongly

dependent on the application area, architecture, and organization of the circuit,

as shown in Fig. 3.5.

Chip/System β K

Static memory 0.12 6.00

Microprocessor 0.45 0.82Gate array 0.50 1.90

High speed computer - chip 0.63 1.40

High speed computer -circuit 0.25 82.0

Figure 3.5: Rent’s constant for varies class of chip and system figure

It is clearly shown that microprocessors display a very different input/output

behavior compared to memories. The observed rate of pin-count increase for

integrated circuits varies between 8% to 11% per year and it has been projected

that packages with more than 2,000 pins will be required by the year 2010. For




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all these reasons, traditional dual-in-line, through-hole mounted packages have

been replaced by other approaches such as surface-mount, ball grid array, and

multichip module techniques. It is useful for the circuit designer to aware of the

available options, and their advantages and disadvantages.

Owing to its multi-functionality, a good package must comply with a large

variety of requirements namely the electrical, mechanical, thermal, and costrequirements.

3.2.1 Electrical requirements

As the speed of integrated circuit increases, their noise margin decreases. Thus,

electricl design for package must be carefully considered. The pins shouldexhibit low capacitance - both inter-wire and to the substrate, resistance, and

inductance. Large characteristic impedance should be tuned to optimize

transmission line behavior and observe that intrinsic integrated circuit

impedances are high. Packages with a design geometry larger than the siliconcan significantly affect the electrical performance of the packaged chips.

Several electrical performance criteria are important such as minimum signal

delay, signal-reflection control, and noise reduction, including simultaneous

switching noise and cross talk. These criteria, often discussed in PWB design,

which is also applied to the package. They are mutually dependent and requiretrade off.

Signal Delay

The signal delay time td is defined by equation (3.2).

td =r / c

ll

ε=

ν(3.2)

where l is the length of signal line, v is the velocity of sigal, c is the velocity of

light, and εr is the dielectric constant of the surrounding material.

High-speed operation requires smaller td. The ratio of td to the cycle timeusually dominates the system performance. In package construction, a short

signal line including bonding wire length and lead length in small dielectric

material typically polyimide resin is preferred. Table 3 lists the dielectric

constants of common packaging materials. An excessively small dielectric-

constant of the surrounding material induces signal reflections that degrade




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operating speed. Hence, an optimum dielectric value is required. The dielectric

constant of the common packaging material is shown in Fig. 3.6.

Materials Dielectric Constant at 1.0MHzAl2O3 9.6 – 10.2AlN 8.7

Mold compound 3.9- 4.3

Polyamide 3.5

Si 11.7

GaAs 12.9

SiO2 3.9

Glass ceramic 3.9- 7.8

Glass epoxy 4.2

Figure 3.6: Dielectric constant of common packaging materials

Signal Reflection

A mismatched impedance causes signal reflections when a signal is transmitted

from a driver to a receiver through a transmission line. In CMOS VLSI/ULSI

devices, multiple reflections occur at the driver and receiver ends when the

output impedance of the output buffer is smaller than that of the transmissionline. These reflections cause a ringing phenomenon that may slow down

operation or cause the circuit to malfunction. These reflections cannot be treated

lightly when the relationship exists as shown in equation (3.3).

r

r

35.0

ctl

ε> (3.3)

where l is the signal line length, c is the velocity of the light, εr is the dielectric

constant of the surrounding material, v is the critical frequency, and tr is thesignal fall) time. The equations show that a shorter signal line l is required for

high-speed operation that needs a smaller package size. Larger packageconstructions have longer signal lines that cannot be dealt with as lumped-

element circuits but it must consider distributed-element circuits. Longer wires,

longer via hole connects that have larger impedance-mismatch potentials,

should be avoided if possible or matched-impedance designs should be used

instead. Leading edge package such as themulti-lead frame plastic packages,

and some other multi-layered packages containing strip, microstrip, or coplanar

constructions provides better impedance matching. Figure 3.7 illustrates the

strip, microstrip, and coplanar structures used for impedance control.




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(a) (b) (c)

Figure 3.7: Cross sectional structure for impedance control. (a) Microstrip line, (b) Strip line,

and (c) coplanar structure

The characteristic impedances Zo of strip, microstrip, and coplanar structures

are expressed by equation (3.4), (3.5), and (3.6) respectively.

+πε=

)W / t8.0(W67.0

b4ln

60Z

r

o strip (3.4)

++ε=

tW8.0

h98.5ln

41.1

87Z

r

o microstrip (3.5)

+−

++⋅

+επ

η=

)SW2 /(S1

)SW2 /(S12ln

2 / )1(

Zr

oo coplanar (3.6)

where εr is the dielectric constant of the dielectric material, t and W are the

thickness and width of the conductor, and h and b are the thicknesses of the

dielectric material beneath and surrounding the conductor respectively.

For a typical coplanar structure such as a symmetrical double-strip, where

ηo (equal to 120π) is the characteristic impedance of free space. S is the gap

between the two lines and W is the width of the line. Assumptions are that the

conductor lines are infinitely thin (t = 0), the dielectric material is infinitelythick (h = ∞), and the gap widths range is 0.173 < S/(2W + S) < 1.

Noise

Two typical types of noise are cross-talk noise and simultaneous switching

noise (∆I noise) are found in integrated circuit. Cross-talk noise as shown in

Fig. 3.8(a) occurs when a line is undesirably affected by another line that is

placed very close to it because of the electromagnetic coupling between the two

lines. The noise, coupled by Cm, Lm, or both (m: mutual) between the two lines,




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increases in proportion to the signal-voltage or current gradient and the ac-

coupling strength. Cross-talk noise is a more serious problem in VLSI/ULSI

packaging intended to handle higher speeds, larger signal counts, and the

resulting narrow signal-line spacings. Major counter measures in packagedesign are shorter parallel signal runs, closer ground or power planes, and lower

dielectric-constant materials.

(a)

(b)




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Figure 3.8: Typial types of noise; (a) cross talk noise and (b) switching noise

Simultaneous switching noise, one of the most practical electrical design

problems, particularly in CMOS ASIC devices, occurs when many output

buffers switch simultaneously. Figure 3.8(b) illustrates the mechanism. Whenan output buffer switches from high to low, transition current i flows from thepower line VCC into the load capacitance Cl inducing the noise voltage given by

equation (3.8).

dt

diLV gn = (3.8)

where Vn is the induced voltage, Lg is the inductance of the power lead, and

di/dt is the derivative of current with respect to time.

In addition, when a line switches from low to high, an electric charge

stored in the load capacitance flows into the ground line through the

transmission line, inducing the same noise voltage as shown in equation (3.9). If j line is switching simultaneouslythen Vn is given by

∑= j

gndt

diLV (3.9)

3.2.2 Mechanical and Thermal properties

Package should be designed in such the heat removal rate should be as high as

possible. Mechanical reliability requires a good matching between the thermal

properties like the coefficient of thermal expansion CTE of the integrated circuit

and the chip carrier. Long term reliability requires a strong connection from die

to package as well as from package to printed circuit board.

As the power consumption of integrated circuits rises, it becomes

increasingly important to efficiently remove the heat generated by the die. A

large number of failure mechanisms in integrated circuit are accentuated by

increase of temperature. Examples are leakage in reverse biased diodes, electro-migration, and hot electron trapping. To prevent failure, temperature of the die

must be kept within certain ranges. The temperature range for commercial

graded devices is between 0° and 70°C. Military parts are more demanding and

require a temperature range varying from -55° to 125°C.

The cooling effectiveness of a package depends upon the thermal

conduction of the package material, which consists of the package substrate and




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body, the package composition, and the effectiveness of the heat transfer

between package and cooling medium.

As for the microprocessor device and other high performance device suchvideo device used in graphic card of a computer, thermal interface material TIM

is used to reduce thermal impedance between the device and heat sink. TIM is a

type of conductive paste used to fill any void or irregularity between the device

and heat sink.

Standard packaging approaches use still or circulating air as the cooling

medium. The transfer efficiency can be improved by adding finned metal heat

sinks to the package. More expensive packaging approaches, such as those used

in mainframes or super computers, force air, liquids, or inert gases through tinyducts in the package to achieve even greater cooling efficiencies. As an

example, a 40-pin DIP has a thermal resistance of 38°C/W and 25°C/W for

natural and forced convection air. This means that a DIP can dissipate 2 watts (3

watts) of power with natural (forced) air convection, and still keep the

temperature difference between the die and the environment below 75°C. For

comparison, the thermal resistance of a ceramic PGA ranges from 15° to

30°C/W. Since packaging approaches with decreased thermal resistance are

prohibitively expensive, keeping the power dissipation of an integrated circuit

within bounds is an economic necessity. The increasing integration levels and

circuit performance make this task nontrivial. An interesting relationship shown

in equation (3.9) has been derived by Nagata. It provides a bound on the

integration complexity and performance as a function of the thermal parameters.

E

T

t

N

p

G

θ

∆≤ (3.9)

where NG is the number of gates on the chip, tp the propagation delay, ∆T the

maximum temperature difference between chip and ambient environment, θ the

thermal resistance between them, and E the switching energy of each gate.

Fortunately, not all gates are operating simultaneously in real systems. The

maximum number of gates can be substantially larger, based on the activity

coefficient in the circuit. For instance, it was experimentally derived that theratio between the average switching period and the propagation delay ranges

from 20 to 200 in mini- and large-scale computers.

Nevertheless, equation (3.9) demonstrates that heat dissipation and thermal

concern present an important limitation on circuit integration. Design




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approaches for low power that reduce either E or the activity coefficient are

rapidly gaining importance.

Let’s consider from the aspect of the thermal resistance θ ja of a package asshown in Fig. 3.9.

Figure 3.9: Cross sectional view of multi-layer lead frame package and the heat transfer

mechanism

The thermal resistance of a package is defined

P

TT a j

ja

−=θ (3.10)

where θ ja in0C/W is the junction to ambient thermal resistance, T j is the

average chip or junction temperature, Ta is the ambient temperature, and P is the

power dissipation.

Figure 3.9 is also a simplified heat-transfer model of a packaged die, where

heat is transferred from the die to the surface of the package by conduction andfrom the package surface to the ambient by convection and radiation. In most

applications, the temperature difference between the case or the package surface

and the ambient is small. Thus, radiation can be neglected. Conduction heat

transfer through the package terminals can be significant, particularly in high-

I/O VLSI/ULSI packages. However, if one neglects it for simplification, the

overall thermal resistance in this model can be considered as the sum of two

thermal components, which are θ jc and θca and is defined




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P

TT

P

TTacc j

ca jc ja

−+

−=θ+θ=θ (3.11)

where θ jc is the junction to case thermal resistance, θca is the case to ambient

thermal resistance, and TC is the average case temperature. θ jC is relativelyinsensitive to the ambient and is mainly a function of package materials and

geometry. θca depends on the package geometry, the package orientation in the

application, and the conditions of the ambient in the operating environment

whether heat transfer is free or by forced-convection. Heat transfer is classified

into three categories conduction, convection, and radiation. We shall not discuss

here.

3.2.3 Cost

Cost is always one of the most important requirements. Ceramic package has a

superior performance over plastic package but it is also substantially more

expensive. Increasing the heat removal capacity of a package also tends to raise

the package cost. The least expensive plastic package can dissipate up to 1.0W

of heat. More expensive but still cheap plastic package can dissipate up to

2.0W. Higher heat dissipation requires more expensive ceramic package. Chips

dissipating over 50.0W require special heat sink attachment. Extreme

techniques such as fans and blowers, liquid cooling hardware, or heat pipes, are

needed for higher dissipation levels.

Packing density is a major factor in reducing the cost of the printed circuit

board. The increasing pin count either requires an increase in the package size

or a reduction in the pitch between the pins. Both have a profound effect on thecost of package.

3.3 Packaging Level Integration

As circuit integration proceeds on the die, it also proceeds in the packagingthrough interactions among several levels of packaging. Generally, packaging

exclusive of the final system construction is classified into three levels, as

shown in Fig. 3.10. Final system requirements determine a specific selection of

the packaging method or how to combine the levels. Types 1 through 4 show

the major methods that have been used in the industry. Type 1 is the most

common choice. In type 1, the die is first packaged as a single chip and then

packaged at the third level, typically at the PWB level. Types 2 and 3, usuallycalled multi-chip-module MCM technologies, are used in high performance

systems, typically in mainframe computer. In type 2, the die is single-die-

packaged as in type 1, and the packaged dices are then packaged at the second




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level onto a smaller substrate; this forms a functionally larger and

geometrically smaller unit and utilizes the finer multilayer wiring of the

substrate. The substrate is attached to a larger mother board in the third-level

packaging. Type 3 is similar to type 2 in that it uses a smaller substrate as anintermediary stage. However, bare chips are attached directly to the substrate,

usually providing a superior electrical and geometrical performance but with

some disadvantages, such as more difficult testability, increased cost, and loweryield. Type 4 is the simplest way of packaging, where bare dices are attached

directly to the system board. The brief descriptions of the various type

packaging level integrations are mentioned in the sub-sections below.

Figure 3.10: Integrated circuit packaging level

3.3.1 Interconnect Levels

The traditional packaging approach uses a two-level interconnection strategy.

The die is first attached to an individual chip carrier or package. The package

body contains an internal cavity where the chip is mounted. These cavities

provide ample room for many connections between chip and leads. The leads

compose the second interconnect level and connect the chip to the external

interconnect medium, which is normally the printed circuit board. Complexsystems contain even more interconnect levels, since boards are connected

together using backplanes or ribbon cables. The first two layers of the

interconnect hierarchy are illustrated in the drawing of Fig. 3.11.




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Figure 3.11: Interconnect hierarchy in traditional integrated circuit packaging

The interconnect techniques used at levels one and two of the interconnect

hierarchy are shown here.

3.3.1 Interconnect Level 1 - Die-to-Package-Substrate

Traditionally wire bonding is the technique of choice to provide an electrical

connection between die and package. In this approach, the backside of the die is

attached to the substrate using glue with a good thermal conductance. Next, the

chip pads are individually connected to the lead frame with aluminum or goldwire. An example of wire bonding is shown in Fig. 3.12. Although the wire-

bonding process is automated, it has some major disadvantages.

Figure 3.12: Wiring bonding connecting pad and lead

Wire must be attached serially one after the other. The lead time is longer with

increasing pin counts. Larger pin counts make it substantially more challenging

to find bonding patterns that avoid shorts between the wires. Bonding wire has

inferior electrical properties such as a high individual inductance (5nH or more)

and mutual inductance with neighboring signals. The inductance of a bonding




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wire is typically about 1.0nH/mm, while the inductance per package pin ranges

between 7.0 and 40.0nH per pin depending on the type of package as well as the

positioning of the pin on the package boundary. Typical values of the parasitic

inductances and capacitances for a number of commonly used packages aresummarized in Fig. 3.13.

Package Type Capacitance pF Inductance nH

68 pin plastic DIP 4 35

68 pin ceramic DIP 7 20

256 pin grid array 1-5 2-15Wire bond 0.5-1 1-2

Solder bump 0.1-0.5 0.01-0.1

Figure 3.13: Typical conductance and inductance of package type and wire

The exact value of the parasitic component is hard to predict because of themanufacturing approach and irregular outlay. New attachment techniques are

being used as a result of these deficiencies. In one approach is called Tape

Automated Bonding TAB. The die is attached to a metal lead frame that is

printed on a polymer film typically polyimide as shown in Fig. 3.14(a). The

connection between chip pad and polymer film is wired by solder bump as

shown in Fig. 3.14(b). The tape can then be connected to the package bodyusing a number of techniques. One approach is using pressure connection.

(a) (b)

Figure 3.14: Automated tap bonding (a) polymer with imprinted wire pattern and (b) die

attach using solder bump

There are many advantages of the TAB process. Besides the process is highly

automated, the sprockets in the film are used for automatic transport, and all

wire connections are made simultaneously. The printed approach helps to




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reduce the wiring pitch, which results in higher lead counts. Elimination of the

long bonding wires improves the electrical performance.

Another approach is to flip the die upside down and attach it directly to thesubstrate using solder bumps. This technique, called flip-chip mounting, has the

advantage of a superior electrical performance as shown in Fig. 3.15. Instead of

making all the I/O connections on the die boundary, pads can be placed at any

position on the chip. This can help to address the power and clock distribution

problems, since the interconnect materials on the substrate are typically copper

Cu or gold Au that have better quality than the aluminum (Al) on the chip.

Figure 3.15: Flip-chip bonding

3.3.2 Interconnect Level 2 - Package Substrate to Board

When connecting the package to the printed circuit board PCB, through-hole

mounting has been the packaging style of choice. A PCB is manufactured bystacking layers of copper and insulating epoxy glass. In the through-hole

mounting approach, holes are drilled through the board and plated with copper.The package pins are inserted and electrical connection is made with solder as

shown in Fig. 3.16(a). The traditional package in this class is the dual-in-line

package DIP. The packaging density of the DIP degrades rapidly when the

number of pins exceeds 64. This problem can be alleviated by using the pin-

grid-array PGA package that has leads on the entire bottom surface instead of

only on the periphery. PGA package can extend to large pin counts over 400

pins.

(a) (b)

Figure 3.16: Printed circuit board mounting approach. (a) through-hole mounting and (b)

surface mounting




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The through-hole mounting approach offers a mechanically reliable and sturdy

connection. However, the setback is the packaging density. For mechanical and

sturdy reasons, a minimum pitch of 2.54mm between the through-holes is

required. Even with this pitch, PGAs with large numbers of pins would alsosubstantially weaken the printed circuit board. In addition, through-holes limit

the board packing density by blocking lines that might otherwise have been

routed below them, which results in longer interconnections. PGAs with largepin counts hence require extra routing layers to connect to the huge number of

pins. Although the parasitic capacitance and inductance of the PGA are slightly

lower than DIP, their values are still substantially large. These shortcomings of

the through-hole mounting can be solved by using the surface mount technique.

A chip is attached to the surface of the board with a solder connection without

requiring any through-holes as shown in Fig. 3.16(b). Consequently, surfacemount increases packing density due to elimination of through-holes mounting

that provides more wiring space and mechanically strengthens the PCB. Thelead pitch is reduced and chips can be mounted on both sides of the board.

The negative effects of the surface mount are many. The connection makes

the connection of printed circuit board weak. It is also cumbersome to mount a

component on a board that requires expensive mounting equipment, difficult for

board repair, and finally testing of board is more complex because the package

pins are no longer accessible at the backside of the board. Signal probing harderor even impossible.

A variety of surface mount packages is currently in use with different pitch

and pin count parameters. Four of these packages are shown in Fig. 3.16: the

small-outline package SOIC with gull wings, the plastic leaded package PLCC

with J-shaped leads, the leadless chip carrier LCC, and quad flat pack QFP.

Figure 3.16: Commonly use package (1) leadless carrier, (2) DIP, (3) PGA, (4) small outlineIC, (5) quad flatpack, and (6) PLCC




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An overview of the most important parameters for a number of packages is

shown in Fig. 3.18.

Package Type Typical Lead Spacing Maximum LeadCount

Dual in Line DIP 2.54mm 64

Pin Grid Array PGA 2.54mm >300

Small Outline IC SOIC 1.27mm 28

Plastic Leadless Chip

Carrier PLCC1.27mm 124

Leadless Chip Carrier LCC 0.75mm 124

Figure 3.18: Parameters of various chip carriers

Even surface mount packaging is unable to satisfy the quest for evermore higher

pin counts. This is worsened by the demand for power connections in high

performance chips, operating at low supply voltages, require as many power and

ground pins as signal I/O. When more than 300 I/O connections are needed,

solder balls replace pins as the preferred interconnect medium between package

and board. An example of such a packaging approach, called ceramic ball grid

array (BGA) is shown in Fig. 3.19. Solder bumps are used to connect both the

die to the package substrate and the package to the board. The area arrayinterconnect of the BGA provides constant input/output density regardless of

the number of total package I/O pins. A minimum pitch between solder balls as

low as 0.8mm can be obtained, and packages with multiple 1000’s of I/O

signals are feasible.

(a) (b)

Figure 3.19: Ball grid array packaging; (a) cross-section, (b) photo of PGA bottom

3.3.3 Multi-Chip Modules - Die to Printed Wire Board

The deep hierarchy of interconnect levels in the package is becoming

unacceptable in modern complex circuit designs with higher levels of




- 23 -

integration, large number of signals, and performance requirements. There is a

need to reduce the number of levels.

At the meantime, attention is focused on the elimination of the first level inthe packaging hierarchy, which is eliminating die to package level by mounting

the die directly on the wiring backplanes board or substrate. It offers a

substantial benefit when performance or density is a major concern. Thispackaging approach is called multichip module MCM. As the result, there is a

substantial increase in packing density as well as improved performance. A

number of the previously mentioned die mounting techniques can be adapted to

mount dice directly on the substrate. This includes wire bonding, Tape

Automated Bonding TAB, and flip chip, although the later two are preferable.

The substrate itself can be varying over a wide range of materials dependingupon the required mechanical, electrical, thermal, and cost requirements.

Materials of choice are epoxy substrates similar to printed circuit boards, metal,ceramics, and silicon. Silicon has the advantage of presenting a perfect match in

mechanical and thermal properties with respect to the die material.

The main advantages of the MCM approach are; it increases packaging

density and and device’s performance. An example of an MCM module

implemented using a silicon substrate; commonly dubbed silicon-on-silicon is

shown in Fig. 3.20. The module that implements an avionics processor moduleand is fabricated by Rockwell International contains 53 ICs and 40 discrete

devices on a 2.2″x2.2″ substrate with aluminum polyimide interconnect.

Figure 3.20: An Avionics processor module. Courtesy of Rockwell International




- 24 -

The interconnect wires of the module are only an order of magnitude wider than

what is typical for on-chip wires because similar patterning approaches are

used. The module itself has 180 I/O pins. Performance is improved by the

elimination of the chip to carrier layer with variety of parasitic components, andthrough a reduction of the global wiring lengths on the die, a result of the

increased packaging density. For instance, a solder bump has an assorted

capacitance and inductance of only 0.1pF and 0.01nH respectively. The MCM

technology can also reduce power consumption significantly, since large output

drivers and associated dissipation become redundant due to the reduced load

capacitance of the output pads.

While MCM technology offers some clear benefits, its main disadvantage

is economic. This technology requires some advanced manufacturing steps thatmake the process expensive. The approach is only justifiable when either dense

housing or extreme performance is essential. In the near future, this argument

might become obsolete as MCM approaches proliferate.

3.4 Assembly Techniques and Processes

This section describes the basic assembly processes and techniques used for

assembly ceramic and plastic packaged VLSI device. The processes cover a

number of assembly steps from wafer preparation through die attach, includingwire bonding, encapsulation/molding, stabilization bake/post mold bake,temperature cycle to tin plating/solder plating, trim/form, and inspection.

Student will learn these process steps based on the generic assembly sequence

shown in Fig. 3.21 and at the same time understanding the physics of technique

for process step.

The generic assembly process steps are mainly for ceramic and plastic

packaged integrated circuit or die. The first step is the preparation of wafer step,

which basically cuts and separates the die from the wafer. The second processstep is the die attach. It is a process step that bonding the die to the paddle of the

package. The third step is the wire bonding, which is the process of connecting

the bond pad on the die to the lead of the package. This step allows the die to be

connected to external world. Encapsulation is the step involves closing the die

from the interference of external contaminant and protecting from damage etc.

The encapsulation can be done by mean of molding for the plastic package or

glass seal the lid of the package to the hermetic ceramic package. Post mold

cure or baking process is a necessary step to cross linked the plastic resinmaterial to provide hardening effect. Owing to high temperature process, lead of

package would be tarnished due to heat. The next process step is either tin plate




- 25 -

or solders plate, which involves removing the oxide and plating the leads for

preventing oxidation and providing good solderability connecting contact to the

circuit board. The second last process step involves removing the shorting bar

from the hermetic package, or trims and forms the plastic package. The lastprocess step is an inspection step that involves removing of the non-compliance

device such as lead problem, package crack etc from the from the production

batch.




- 26 -

Figure 3.21: Generic electronics packaging assembly sequence for plastic and ceramic

package

3.4.1 Wafer Preparation

Thickness of the fabricated wafer is normally around 650µm. It needs to be

thinned before the assembly begins. Depending on the package style that the

integrated circuit to be housed, it can be thinned to approximately 400µm. The

thinning is necessary to reduce thermal stress due to mismatch of the coefficient

of thermal expansion CTE between the silicon die and the packaging material.

Again depending on the type of packaging material, the back-side of the wafer

may require metallization deposition consisting of multilayer metallic elements

like gold-nickel-silver Au-Ni-Ag or titanium-nickel-silver Ti-Ni-Au in order

from silicon side. This helps in thermal conduction between the silicon die andthe package, and provides the superior adhesion strength and electrical

connection.

Every integrated circuit in the wafer is probed electrically to check its

functionality. The malfunction dice are marked with a drop of red ink that they

can be sorted out during die attach or die bonding process. Different color

inking schemes may be adopted to distinguish between commercial/industrial

compliance die and military compliance die. One of the schemes is to ink thedie with green color for commercial/industrial compliance die, no color is to be

used for military compliance die, and red color for fail die.

The probed wafer is then adhesively mounted to a tape that has been pre-

assembled to a frame using a wafer dispenser. The frame is then mounted on the

dicing machine with a diamond blade to cut the scribe line for separating the

dice. The thickness of diamond blade is typically 25µm thick rotating at a speed

of 20,000rpm cuts the wafer from 90.0% to 100.0% saw-through allowing

dicing street as narrow as 60µm, which is closed to the width of scribe line of

between dice. 100.0% saw-though is necessary for VLSI devices, which have

large area because it reduces the chance of chipping at the edge during dieseparation breaking process. In the modern VLSI assembly, the 100% saw-

through dice allow automatic picking of good dice with the aid of optical visual

system during die attach.

3.4.2 Die Attach

It is a process of attaching the die permanently to the paddle of lead frame or

ceramic package. One of the important conditions of the die attachment process

and several other processes are the requirements of high temperature and




- 27 -

cooling down to room temperature. This would cause thermal stress due to the

difference of coefficient of thermal expansion CTE of the silicon die and

material of package. The results are crack on the die and metallization peeling

off. Material that has CTE close to that of silicon crystal is preferable inpackage construction. Choosing the packaging materials that have CTE the

same as that of silicon would be ideal reducing thermal stress to zero. In real

situation, there is no such material that can provide perfect match with the

silicon. However, in the industrial ceramic Al2O3 substrate and Alloy 42 (42%

nickel-58% iron alloy) lead frame have been used for many years due to close

TCE match for hermetic-ceramic package and plastic package. However, Alloy

42 is no longer the only choice in today’s assembly. Copper alloy lead frame is

preferred today for logic and microprocessor devices because copper alloy has

approximately ten times the thermal conductivity of Alloy 42. It allows betterheat transfer from the die to package via die attach material.

3.4.2.1 Eutectic Die Attach

Figure 3.22 illustrates the fundamental aspect of a die attach. Eutectic chip die

attach is metallurgically attached from the die to substrate material typically

made from Alloy 42 or attached to a ceramic substrate (90-99.5% Al2O3).

Metallization is often required on the back of the chip so that it is wettable by

die attach perform. The perform is a thin sheet of thickness less than 0.05µmconsisting of solder-bonding alloy such as 98% gold-2% silicon. The paddle and

lead of the ceramic package is usually plated with gold, while the paddle and

lead of the alloy 42 lead frame or copper alloys lead frame is plated with silver.

Figure 3.22: The basic structure of a silicon device die attach with a metal preform

During the die attach, the preform is placed on the paddle heated to about

3700C. Mechanical scrubbing is done so that the preform melts and reacts with

silicon to form an Au-Si composition bond between the backside of the die and

the substrate of the ceramic. The bonding is considered complete when the Au-Si composition structure becomes rich in silicon.




- 28 -

3.4.2.2 Epoxy Die Attach

Silver filled epoxy adhesive is the choice of polymer based material for die

attach. The silver filler usually would flake that makes epoxy electrically

conductive and thermally conductive to allow good thermal path between die

and the rest of the package. In VLSI assembly, epoxy is fed onto the substrate

material through a multi-nozzle or single nozzle dispenser to ensure the required

bond line thickness is created avoiding void. The back side of the die often does

not require metallization because epoxy provides better adhesion with bare

silicon or silicon dioxide. The process time of die attach usually is 1 to 2

seconds at room temperature. The epoxy is thermosetting polymers, after the dieattach, it must be cured at elevated temperature to complete the die bond. The

cure conditions range from 1250C to 175

0C and require 1 to 2 hours.

3.4.3 Wire Bonding

Wire bonding is the most common method for connecting the bond pads on the

die to the leads of the package. Aluminum or gold wires are usually the choice

because they bond well to the bond pads on the die and to the metallized part of

the package forming Au-Al and Al-Ag metallurgical diffused materials. Gold oraluminum wire of diameter 25 to 30µm is balled and wedge bonded by

thermosonic or thermocompression, where the ball is bonded to the bond pad

made of aluminum and wedge-bonded at the lead plated with either gold or

silver. The temperature of wire bonding for thermosonic ranges from 1500C to

2500C, while for thermocompression process, the temperature ranges from

3000C to 350

0C.

The metallurgical diffusion primary follows equation

X2 = Dt (3.12)

( )RT / QexpDD 0 −= (3.13)

where X is the diffusion thickness, D is the diffusion constant, t is the storage

time, D0 is the frequency factor, Q is the activation energy, R is the gas

constant, and T is the storage absolute temperature.

Gold and aluminum form a variety of inter-metallic with Au-Al first

formed and gradually change to Au-Al4 that will degrade the bond strength.




- 29 -

Following equation (3.12) and (3.13), Au-Al inter-metallic growth would be

severed at elevated temperature especially the temperature of molding and

hermetic glass seal. Thus, aluminum-aluminum wiring bonding at bond pad is

preferred for high reliable products.

Besides, using gold and aluminum wires to interconnect bond pads of the

die to lead of package, copper bond wire is also used in today’s modern VLSI

assembly due to a few obvious reasons. They are cost, electrical and thermal

conductivity, less inter-metallic growths, and better reliability of bond at

elevated temperature.

Copper costs 90% less than gold. It is obvious in terms of cost of assembly.

It costs less. Copper wire has electrical resistivity of 0.017x10-4

Ω-cm, which isabout 30% better than the resistivity of gold, which is 0.022x10

-4Ω-cm. The low

electrical resistivity of copper results in better electrical performance in

particular for bonding high current device. Copper has thermal conductivity of

395m-1

K-1

, which 25% better than the thermal conductivity of gold, which is

316Wm-1

K-1

. Thus, copper wire dissipates heat within the package faster than

gold wire. Thus, it minimizes the thermal stress.

Copper has a lower tendency to form inter-metallic compound with

aluminum. Unlike gold, it forms inter-metallic compound with aluminumespecially at elevated temperature due to inter-diffusivity of gold and aluminum

(bond pad). It can create voids at the bond interface that would weaken the bond

and can lead to bond lifting as well as other wire bond problems.

3.4.4 Molding/Glass Seal

Upon completion of wire bonding, the next operation is either molding or glass

seal process, which depending on the package style used. Glass seal refractory

technology relies on glass sealing a lead between two pressed ceramics asillustrated in Fig. 3.23 using low temperature glass. The glass used for glass

sealing is PbO-ZnO-B2O3 type. Sealing is usually done at temperature above

4000C in an oxidizing ambient to avoid deoxidizing the metallic components of

the glass that would degrade the electrical insulation. For VLSI device, sealing

at temperature greater than 4000C would cause additional thermal diffusion at

the junction of transistor that would shift slightly the electrical characteristics of the die. Thus, choosing glass-sealing technology for sealing must be carefully.




- 30 -

Figure 3.23: Structure of ceramic dual inline package (cerdip) showing the base, the lead

frame and a lid with sealing glass

Plastic encapsulation involves a number of techniques. For an example, in glob-top-coating, the post wire bonded die is coated with liquid plastic resin and theplastic are cured for the cross liking.

In VLSI plastic packaging, a pre-molding technique is sometimes used.

This technology is the plastic equivalent of the refractory ceramic cavity

package. The package is molded together with a lead frame forming a plastic

body and cavity, whereby the die is attached and bond pads to lead are wire

bonded.

The post molding technology is a transfer molding method using

thermosetting epoxy resins to mold around the frame-die assembly to form

package body. The molding process has to be controlled carefully because this

process is relatively harsh that the die and bond wire are exposed to viscous

molding material that may cause wire sweeping or lifted wire.

The molding material that is epoxy resin is made by condensing

epichlorohydrin with bisphenol-A to produce a material called Epoxy-A. Anexcess of epichlorohydrin was used to leave epoxy groups on each end of low

molecular weight polymer. Today NOVOLAC epoxy is general preferred due toits higher functionality that makes heat resist.

The molding compound is usually pre-heated and transferred into pots of

large multi-cavity mold. After it enters the pot, the molding compound melts

under pressure and heat, flows to fill the mold cavities containing frame strips

with their attached dice. For molding of VLSI die, which has large area, longer

bond wire, the multi-pot molding as shown in Fig. 3.24 is preferred aiming toreduce damage of wire due to viscosity of the mold compound and incomplete

mold due to partial cross linking.




- 31 -

Figure 3.34: Schematics of a multi-pot transfer-mold system showing small mold compound

tablets with each large enough to fill a few cavities containing plastic strips

3.4.5 Post Mold Cure/Leak Check

For plastic packaged device, post mold cure or baking process is a necessarystep to cross linked the plastic resin material to provide hardening effect. The

curing is normally done in an oven set at temperature 1500C for three hours of

curing time. As for the hermetic package, cure is not necessary for the package.

For ceramic packaged device, leak test is usually done to check if there is any

glass seal problem or micro-crack of the ceramic package. The leak tests are

divided into gross leak and fine leak tests. The gross leak test is easy. It is done

by immersing the ceramic package into water like the way we check the leak of

a car wheel. Fine leakage is done by placing the ceramic devices in the pressure

compressed chamber containing radioactive source for 1 to 2 hours. Thedevices are then checked for fine leak with a α particle counter. If the counter

shows count result, it means that there is fine leak due to micro-crack whereby itcannot be visually detected by naked eyes.

3.4.6 Solder Dip/Tin Plate

Owing to high temperature process, lead of package would be tarnished due to

heat. Thus, it is necessary to remove the oxide before either solder dip or tin

plate is done depending on package type. The leads of plastic package are




- 32 -

normally solder dipped, while the leads of hermetic package is normally tin

plated or solder plated. Solder dip or tin plate is necessary to protect the base

metal of the package from oxidation in order to preserve its solderability.

For plastic device, upon removing the oxide and extra mold fresh, the short

between the tips of the lead between adjacent packages is removed as shown in

Fig. 3.25 and the package is formed to the desire shape as shown in Fig. 3.26 for

plastic dual in-line DIP package. The device strips are dipped with solder flux

and then into solder bath so that solder will cover the non-oxidized bare-leads of

the plastic device.

As for the ceramic package, it is normally tin plated or solder plated via

electrolysis. It is normal electrolysis process, whereby the hermetic devices arehung in a bracket at the negative electrode of the plating bath filled with

electrolyte.

Figure 3.25: Molded plastic package strip showing short between tips of the lead, tight bar

and guide pin hole

Figure 3.26: The formed molded plastic dip package strip shows that tight bar has not beenremoved




- 33 -

3.4.7 Trim/Form

One already knows that generally the device is assembled either in plastic

package style or hermetic package style. The leads of the device are shortedwith a tight bar with the aims to prevent lead bending and also to protect the die

from damage due to electrostatic discharge ESD so that every lead is at same

potential with no potential difference for electron discharge that would damage

the die. The short between the tips of the leads is trimmed off using puncher.

The device is then formed according to the package style such the dual inline

package DIP as shown in Fig. 3.26. Take for an example, the lead of thepackage is formed into gull-wing style for single outline package SOP.

For plastic package, trim and form are performed before solder dip process.As for the hermetic package, form is normally not necessary, while trim is done

after tin plating or solder plating process to remove the shorting bar connecting

the tips of all leads.

3.4.8 Inspection

The tight bar of the plastic device is removed to singulate device. Inspection is

done to sort out non-conformance device such as lead defect, package crack,

package chip, insufficient solder coverage etc. before loaded into tube. The tubeor tray loaded devices is then transferred to test operation for initial and final

tests.

Exercises

3.1. Why is it necessary to measure the physical parameter of a fabricated

integrated circuit?

3.2. Calculate the current transfer distance (li) if the contact resistivity is2.0x10

-7Ωcm2

and the resistivity of silicon is 100Ω / .

3.3. If the physical contact area of the n+

diffusion region is 1.0µmx0.5µm,

using the result of question 9.2, calculate the contact resistance.

3.4. Using a cross-bridge Kelvin structure with a 1.0µm x 1.0µm contact, the

current is found to be 10.0µA through the contact and the voltage

difference across the contact is 320µV, find the contact resistivity of this

contact.




3.5. State the reason necessary to have coefficient of thermal expansion

matching materials for package and silicon die.

3.6. Why aluminum wire bonding is preferred than gold wire bonding?

3.7. If the Au-Al bond diffusion thickness is increased by 1.0µm during 1500C

time-temperature storage operation, calculate the period of time. You

may use D0 = 2.2x10-4

m2 /s, Q = 134kJ/mol and R= 8.31J/mol-K to help

you in the calculation.

3.8. For VLSI device plastic molding, state the reason why multi-pot molding

is necessary.

3.9. State the reasons why the leads of the package are normally shorted

together in a assembly operation.

3.10. Calculate the number of gates that can be included on a logic-array chip

which is to be assembled in 120 I/O package assuming that k = 4.5 and β

= 0.5.

3.11. An integrated circuit has 800 gates, its nominal propagation delay for a

transistor is 5.0x10-15s, its junction to ambient maximum temperature

difference is 550C, and junction to ambient thermal resistance is

1000C/W. Calculate the activation energy of each gate of this circuit.

3.12. State the reason why it is necessary to have heat sink for conducting away

extra heat from the package of integrated circuit?

Answers of Exercises

3.1. It is necessary because the manufacturer would like to integrated circuit is

fabricated in accordance to the specifications.

3.2. The current transfer length (li) is equal toS

Ci

σ

ρ=l =

100

10x0.2 7−

= 0.447µm.

3.3. The area of current flow is 0.447µmx0.5µm = 2.235x10-9

cm2. Given that

the contact resistivity ρC = 2.0x10-7Ωcm

2, the contact resistance RC is

2.0x10-7

Ωcm2

/2.235x10-9

cm2

= 89.48Ω.

03 assembly techniques and packaging

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