978-1-61284-736-8/11/$26.00 ©2011 IEEE 181 27th IEEE SEMI-THERM Symposium

Thermal Performance Of FCMBGA: Exposed Molded Die Compared To Lidded Package

Jesse Galloway, Sasanka Kanuparthi, Qun Wan Amkor Technology Inc.

1900 S. Price Rd., Chandler, AZ 85286 jesse.galloway@amkor.com

Abstract

Thermal resistance data were collected using two different style flip chip ball grid array (FCBGA) packages; one with an exposed molded die and a second with a lid. Eleven different heat sink designs and two different thermal interface materials (TIM) were tested to quantify the thermal interaction between heat sink size, base material and TIM resistance as a function of package style. Package style and TIM material did not appreciably change the total thermal resistance (less than 10%) for small heat sinks 50mm x 50mm smaller. The exposed molded die package thermal resistance was 14% smaller than the lidded package when tested with a heat pipe heat sink. An understanding of the long term performance impact of TIM II degradation was investigated using conduction based models. Lidded style packages may increase safety margin when TIM II materials experience pump-out, dry-out or voiding.

Keywords FCBGA, exposed die, FCMBGA, FCLBGA, finite element

analysis, experimental, thermal test

Nomenclature �ja Junction-to-air thermal resistance �js Junction-to-sink thermal resistance �sa Sink-to-air thermal resistance

1. Introduction Consumer electronics deployed in gaming, notebooks,

desktops and telecommunication equipment require lower thermal resistance solutions to meet future system designs. Additional thermal improvements are difficult to achieve simply by reducing the thermal resistance of TIMs. High performance TIMs have achieved thermal resistance values less than 0.1C/Wcm2 by reducing the bond line thickness (BLT), utilizing higher conductivity filler materials and improving the cohesive strength to avoid delamination during package assembly. Even lower thermal resistance requirements coupled with lower pricing pressures make exposed die packages more attractive; particularly in the graphics card and gaming console space. With lidded style packages, there are two thermal interfaces, TIM I between die and lid and TIM II between lid and external heat as illustrated in Figure 1. An exposed die package (i.e. no lid) eliminates the thermal interface material (TIM I) between the backside of the die and the lid; thereby reducing the junction-to-case thermal resistance, �jc. Whether the elimination of TIM I resistance reduces the overall system level junction-to-ambient thermal resistance, �ja, depends on the selection of the TIM II material and the external heat sink.

TIM ITIM II

Figure 1. Heat flow paths in a FCBGA. The literature indicates that bare die packages can produce

thermal improvements when the external heat sink has a highly conductive base and a low resistance TIM II material. Wakil [1] reported up to a 20% reduction in Theta ja for the bare die case compared to a 0.5mm thick lid mounted to a 50mm x 50mm x 23mm aluminum heat sink. When the 0.5mm lid was replaced with a 2.0mm copper lid, the lidded thermal resistance was lower compared to the bare die case. Kandasamy and Mujumdar [2] investigated the affect of the junction to case resistance as a function of lid configuration. They predicted lower powers were achievable using a bare die compared to a lidded package but it is most likely a result of a lower performing TIM II. It is clear that the junction to ambient thermal resistance is a coupled conduction problem with strong interaction between the TIM II resistance, heat sink design and package design. Thermal design must also take into account the degradation in interfacial resistance as a function of time and temperature. Tonapi et.al. [3] measured the resistance of thermal greases as a function of accelerated thermal and mechanical cycling. Bharatham et.al. [4] studied the impact heat sink clamping pressure has on bare die thermal resistance at the end-of-life conditions using bake-out tests. The tested phase-change TIM II exhibited more pronounced degradation at lower application pressures. Greater degradation was observed at the die corners.

Excessive warpage is a potential problem for exposed die packages, certainly for large body sizes having thin core substrates. Packages having large warpage may create solder ball attach defects (either opens or shorts) and may cause increased die level or bump level stresses. The flip chip molded ball grid array (FCMBGA) style package, see Figure 2(a), offers a warpage reduction solution by encapsulating the die with a rigid mold compound layer while exposing the top surface of the die [5], [6]. Since warpage is controlled without a lid foot, see Figure 2(b), the substrate has additional real estate to mount passive components. Also, since there are no underfill keep-out areas, passive components may be mounted closer to the die edge.

The purpose of this paper is to contrast the thermal performance of an exposed molded die package to a lidded package using experimental tests and finite element analysis (FEA) simulations.

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Figure 2(a) FCMBGA

Lid Foot

Figure 3(b) FCLBGA

Figure 2. Thermal test vehicle top view and cross-section.

2. Experimental Testing A thermal test vehicle (TTV) was developed to compare

the thermal performance of the FCMBGA to the flip chip lidded ball grid array (FCLBGA) package. A description of the package is provided in Table 1. Thermal die equipped with heating elements and diodes, see Figure 3, were used to power the die and sense junction temperatures.

Table 1. TTV design details.

Body (mm) 45 x 45 IO 1935 Die Size (mm) 17.8 x 17.8 x 0.62 Substrate 1-2-1 Cu Lid Thickness (mm) 0.50

(2) Heated Area

(4) Diodes

Figure 3. Thermal test die unit cell (2.54mm x 2.54mm).

A 7x7 array of unit cells were used to fabricate the 17.8mm x 17.8mm die. Power to each cell may be controlled independently to study the impact of hot spot size and location on the peak junction temperature. A total of 196 thermal sensors are available to map out the temperature distribution.

Exposed molded die and lidded package TTVs are shown in Figure 2. The exposed die on the FCMBGA is above the mold compound top surface ensuring that the external heat sink makes contact with the die first producing a thin BLT.

Two different TIM II materials were selected. A higher performance gel, having a rated thermal conductivity of 4.0 W/m/k was selected to provide data on a high performance

TIM II. A gap filling TIM II was selected to provide a lower performance material. The thermal resistance of both materials were measured using a TIM tester following the procedures outlined in ASTM D5470 standard [7]. Heat sinks were clamped to FCMBGA and FCLBGA packages using a set of 4 springs to produce a load of 140N. Based on the area of the lidded package and the die, a load of 140N produces a pressure of 0.12MPa and 0.41MPa respectively. The gel material thermal resistance is not sensitive to loading pressure whereas a 22% reduction in resistance was measured for the gap filler TIM when the loading pressure was increased from 0.12 to 0.41MPa.

Table 2. Thermal Resistance measurements.

TIM II Gel TIM II Gap filler TIM II

Pressure MPA (Psi)

0.12 (17)

0.41 (60)

0.12 (17)

0.41 (60)

Thermal Resistance (C/W*cm

2)

0.15 0.14 2.3 1.8

Eleven different size heat sinks were tested. A description

of their sizes and sink-to-ambient thermal resistance is given in Table 3. A 0.5mm diameter thermistor was flush mounted in a 1.0mm wide by 1.0mm deep groove milled in the base of all heat sinks. The groove was filled with thermally conductive epoxy and lapped smooth with the heat sink base. Thermal data were gathered using a closed-loop wind tunnel, see Figure 4, supplying 2m/s flow at an ambient temperature of 23.0C. The cross-sectional area was 254mm wide by 44mm (1-U) high for all heat sinks except for numbers 2, 3 and 4. The channel height was increased to 88mm (2-U) for heat sink 2 and 3. Heat sink 4 was tested in the ambient using a CPU fan cooler.

An estimate of the case to ambient resistance was made by measuring the heat sink temperature, ambient temperature and the dissipative power from the TTV. The power was adjusted in all tests so that the die temperature was approximately 85 to 90C. A summary of the data for all heat sinks is shown in Figure 5. The sink-to-ambient thermal resistance exhibited only a slight dependency on TIM II material and package style. Theta ja data are plotted for all heat sinks, both TIM II materials and package style in Figure 6. The impact of the TIM II material is pronounced for higher performance heat sinks (i.e. sink-to-ambient thermal resistance less than 1.0C/W). Theta ja is approximately 30% higher for FCLBGA with the gap filler compared to the gel TIM II. The difference is even larger for the FCMBGA package, approximately 45%. The impact of TIM II material in lower performance heat sinks (e.g. numbers 10 and 11) is less than 10%.

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Table 3. Heat sink description (mm). # Size (mm) Metal Base

Thick Fin Thk Fin

Array� ca

(C/W)

1 100x100x40 Al 6.2 1.0 19x10 0.48 2 79x89x52 Cu 7.7 0.3 46 0.52 3 100x100x60 Al 6.4 1.2 9x29 0.39 4 58x105x115 Heat

pipe 4.5 0.4 45 0.25 5 69x70x43 Al /Cu

base 12.2 0.9 30 0.70 6 70x70x14 Al 3.0 0.7 36 1.56 7 53x54x25 Al 3.9 2.5 11x12 1.24 8 92x92x23 Cu 7.5 0.5 51 0.64 9 89x74x11 Cu 3.4 0.4 57 1.70

10 50x50x10 Cu 3.0 0.4 32 2.90 11 44x41x16 Al 2.9 1.9 9x7 2.45

Test Section

Fan/Heat Exchanger Tunnel

TestBoard

Figure 4. Closed-loop wind tunnel.

Theta sa (C/W)

Heat Sink Number Figure 5. Sink to ambient thermal resistance as a function

of heat sink design, TIM II material and style.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1 2 3 4 5 6 7 8 9 10 11

FCLBGA�� Gel FCMBGA�� Gel FCLBGA�� Gap FCMBGA�� Gap

14% reduction

Theta ja (C/W)

Heat Sink Number Figure 6. Theta ja as a function of TIM II material and

package style as a function of heat sink type.

FCMBGA style package offers a significant reduction in package level resistance as noted by the reduction in Theta js compared to the lidded package, see Figure 7. Theta js is the resistance between the junction and the base of the heat sink. The TIM II material is the largest resistor in the path; hence the impact of the 10X higher thermal resistance of the gap fill material is easily detected when observing Theta js trends. Theta js for Gel TIM II is 30 to 50% lower for the FCMBGA compared to the FCLBGA package for all heat sinks except for numbers 7 and 11. Heat sink numbers 7 and 11, made from aluminum with a small base thickness, do not adequately spread heat away from the die. The copper lid in the FCLBGA package allows the heat to be spread with a lower resistance than is possible with the exposed die case.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1 2 3 4 5 6 7 8 9 10 11

FCLBGA�� Gel FCMBGA�� Gel FCLBGA�� Gap FCMBGA�� Gap

Theta js (C/W)

56% reduction

Heat Sink Number Figure 7. Theta js as a function of heat sink design.

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2. FEA simulations FEA simulations were used to predict the affect of lid

thickness, heat sink thickness and heat sink conductivity. Simulations were also used to predict the sensitivity of package styles to TIM II performance degradations. During the life cycle of electronic products, TIM II may experience one or more of the following failures; pump-out, dry-out and voiding. Pump-out is a phenomena whereby low viscosity TIMs are forced out of the gap between the case (bare die or lid) and the external heat sink. At room temperature, the case of the package tends to have concave curvature due to differences in the coefficient of thermal expansion of die, substrate and other packaging materials. The BLT is thinnest in the center and thickest at the corners. At operating temperatures, this curvature is reduced as the package temperature approaches the stress-free temperature. Hence with power cycling, the gaps at the corner of the case open and close pumping out TIM II material. Newer TIM II materials, such as gels or phase-change materials do not suffer pump-out failures as historically observed with low viscosity greases. The volatile components in the TIM II may dry-out with time and cause an increase in the thermal resistance throughout the TIM II layer.

8(a)

Heat Sink

TIM IILidTIM I

Die

Underfill

SubstrateSolder balls

Mother Board 8(b)

8(c)

Figure 8 (a) Model geometry. (b) Close-up view of FEA model of FCLBGA. (c) Heat flux boundary conditions on die matching hot spots shown in Figure 3.

FEA conduction models for heat sink number 1 attached to FCLBGA and FCMBGA packages were developed. Convective heat transfer coefficient boundary conditions for the fins and mother board were extracted using a CFD model and then applied to the conduction only model. Doing so greatly reduced run times and allowed simulations to be run using a FEA code that was more flexible to include details such as die and lid warpage. Curvature of die and lid for the FCMBGA and FCLBGA packages; respectively, were modeled by moving nodes in elements in contact with the TIM II layer based on experimentally measured lid and die curvature. The FEA model geometry for the FCLBGA package is shown in Figure 8.

Predicted temperatures for heat sink number 1 and the FCLBGA package are shown in Figured 9. Isolated hot spots corresponding to heater areas on the die (see Figure 3) are visible in the isotherm plot shown in Figure 9(b).

9(a)

9(b)

Figure 9. FCLBGA mounted to heat sink # 1. (a) Centerline isotherms. (b) Active die side isotherms.

Experimental data measured along the die diagonal are

compared with predictions for FCLBGA and FCMBGA packages in Figure 10. The agreement in data is within ± 2C.

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50

52

54

56

58

60

62

64

66

�15 �10 �5 0 5 10 15

FCMBGA�(Simulation) FCMBGA�(Data)

FCLBGA�(Simulation) FCLBGA�(Data)Te

mpe

ratu

re (C

)

Die Diagonal Distance (mm) Figure 10. Comparison of FEA model die level temperature predictions to experimental data, heat sink #1.

The validated model (based on heat sink number 1) was used to contrast FCMBGA and FCLBGA thermal resistance predictions as a function of lid thickness, heat sink base thickness and heat sink material (copper or aluminum). Simulations indicate that aluminum heat sinks have higher thermal resistance and that thicker lids reduce the thermal resistance by as much as 30% for the 3.0mm base aluminum heat sink. The impact of lid thickness is much less for copper heat sinks, particularly for heat sink bases greater than 3.0mm. When height restrictions are critical, the FCMBGA package with a copper heat sink offers an attractive solution.

Theta ja (C/W)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

3.0mm�/�Al 6.0mm�/�Al 10.0mm�/�Al 3.0mm�/�Cu 6.0mm�/�Cu 10.0mm�/�Cu

Exposed�Die 0.5mm�Lid 1.0mm�Lid 2.0mm�Lid

Figure 11. Affect of heat sink base thickness and material on Theta ja for exposed die compared to lidded FCBGA.

Long term aging may dry-out TIM II causing an increase in thermal resistance. The thermal conductivity of the TIM II layer for both FCMBGA and FCLBGA packages were decreased incrementally from 0 to 85% of its nominal value. Due to the smaller area of the die in contact with the TIM II compared to the lid, the FCMBGA package was more sensitive to increases in resistance compared to the FCLBGA package. A 40% increase in TIM II thermal resistance resulted in a 12% and 40% increase in Theta ja for the FCLBGA and FCMBGA packages, respectively.

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80 100

FCMBGA FCLBGA

TIM II Resistance change (%)

Thet

a ja

(C/W

)

Figure 11. Theta ja as a function of TIM II dry-out. Pump-out presents a more severe challenge for exposed

die packages. If the TIM II pumps out around the edges of the die, small air gaps are left creating local hot spots on the perimeter of the die. This affect was simulated by replacing TIM II material along the edge of the die or lid with an equivalent air gap. Since most of the heat conducted from the 0.5mm thick lidded package occurs over an area slightly larger than the die footprint, gaps along the perimeter of the lid have little influence on junction temperatures. Theta ja predictions as a function of pump-out area are shown in Figure 12. Pump-out area is defined as the area extending from the outer regions of the die or lid to an inner radius. Pump-out issues may be controlled using currently available TIM II (often called TIM 1½) in exposed die packages. Gels or phase change materials have cross-linking properties that resist pump-out phenomena. Nonetheless, pump-out presents a challenge for exposed packages and should be investigated using techniques as reported by [4].

The molded package may offer some of the benefits afforded by lidded packages since greater TIM II coverage is possible by dispensing TIM II not only over the die but also over the mold compound regions surrounding the die. A buffer layer of TIM II between the die and the edge of the package may help to reduce TIM II degradation either by a dry-out or a pump out mechanism.

3. Conclusions A reduction in Theta ja was measured for FCMBGA

packages when highly conductive heat sink base materials and TIM II are selected. The potential for even greater benefits were measured as noted by the 56% lower Theta js compared to an equivalent FCLBGA package. Highly conductive heat sinks provide opportunities to further reduce the system level resistance. More latitude exists for enhancing TIM II (TIM 1½) materials since they have less restricted operation temperature requirements. TIM I materials must survive reflow temperatures in excess of 240C and repeated temperature cycling conditions. For packages having power levels below 30W, the difference between FCMBGA and FCLBGA packages are minimal (for die sizes larger than 100mm2). The FCMBGA package provides a surface to apply

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27th IEEE SEMI-THERM Symposium

TIM II material that extends beyond the die edge. This may help to reduce pump-out and dry-out phenomena.

0.0

0.2

0.4

0.6

0.8

1.0

0 500 1000 1500 2000

FCMBGA FCLBGA

Area Pump out (mm2)

Pump-out TIM II

Thet

a ja

(C/W

)

Figure 12. Theta ja as a function of TIM II pump-out area.

Acknowledgments The authors wish to express their gratitude for the

technical and financial support provided by Cisco Systems Inc. under the direction of Sue Teng.

Experimental data collection and reduction assistance provided by Cameron Nelson is greatly appreciated. Package assembly assistance provided by Amkor's K1 RnD center is also greatly appreciated.

References 1. Wakil J., "Thermal performance impacts of heat spreading

lids on flip chip packages: With and without heat sinks", Microelectronics Reliability, Vol. 46, pp. 380-385, 2006.

2. Kandassamy R. and A.S. Mujumdar, "Interface thermal characteristics of flip chip packages - A numerical study", Applied Thermal Engineering, Vol. 29, pp. 822-829, 2009.

3. Tonapi, S., K. Nagarkar, D. Esler and A. Gowda, "Reliability testing of thermal greases", Electronic s Cooling, November 2007, http://www.electronics-cooling.com/author/sandeep_tonapi/

4. Bharatham L., W.S. Fong, C.J. Leong and C.P. Chiu, "A study of application pressure on thermal interface material performance and reliability on FCBGA package", Thermal and Thermomechanical Phenomena in Electronic Systems, ITHERM 2008, pp. 359-265, May 2008.

5. Islam N., R. Darveaux, M. Jimarez, H. Sy, B.Y. Jung, J.Y. Gim, Y.S. Jung, T.K. Hwang, S.C. Choi and L.Mendoza, "Molded Flip Chip - FCMBGA, IMAPS International Conference and Exhibition on Device Packaging, March 17-20, 2008 Scottsdale, Arizona.

6. Prasad, A., T.G. Kang, Y. Li, D. Robinson, R. Paisa, B. Yoo, "Comparison of lidless and overmold flip chip package with 40nm ultra low-K silicon technology", 2010 Electronic Component and Technology Conference, pp. 31-35.

7. ASTM D5470-06 Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials, www.astm.org.