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Special Section Articles

Mechanical Characterization of Thermal Interface Materials and Its Challenges

[+] Author and Article Information
Vijay Subramanian

Intel Corporation,
5000 W Chandler Blvd,
Chandler, AZ 85226
e-mail: vijay.subramanian@intel.com

Jorge Sanchez, Joseph Bautista, Yi He, Jinlin Wang, Jesus Gerardo Reyes Schuldes, Hemanth K. Dhavaleswarapu

Intel Corporation,
5000 W Chandler Blvd,
Chandler, AZ 85226

Abhishek Das

Intel Corporation,
5200 NE Elam Young Parkway,
Hillsboro, OR 97124

Kyle Yazzie

Intel Corporation,
5000 W Chandler Blvd,
Chandler, AZ 85226

Pramod Malatkar

Intel Corporation,
5000 W Chandler Blvd,
Chandler, AZ 85226
e-mail: pramod.malatkar@intel.com

1Corresponding author.

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received October 15, 2018; final manuscript received February 7, 2019; published online March 13, 2019. Assoc. Editor: Jin Yang.

J. Electron. Packag 141(1), 010804 (Mar 13, 2019) (10 pages) Paper No: EP-18-1086; doi: 10.1115/1.4042805 History: Received October 15, 2018; Revised February 07, 2019

Thermal interface materials (TIMs) play a vital role in the performance of electronic packages by enabling improved heat dissipation. These materials typically have high thermal conductivity and are designed to offer a lower thermal resistance path for efficient heat transfer. For some semiconductor components, thermal solutions are attached directly to the bare silicon die using TIM materials, while other components use an integrated heat spreader (IHS) attached on top of the die(s) and the thermal solution attached on top of the IHS. For cases with an IHS, two TIM materials are used—TIM1 is applied between the silicon die and IHS and TIM2 is used between IHS and thermal solution. TIM materials are usually comprised of a polymer matrix with thermally conductive fillers such as silica, aluminum, alumina, boron nitride, zinc oxide, etc. The polymer matrix wets the contact surface to lower the contact resistance, while the fillers help reduce the bulk resistance by increasing the bulk thermal conductivity. TIM thickness varies by application but is typically between 25 μm and around 250 μm. Selection of appropriate TIM1 and TIM2 materials is necessary for the reliable thermal performance of a product over its life and end-use conditions. It has been observed that during reliability testing, TIM materials are prone to degradation which in turn leads to a reduction in the thermal performance of the product. Typical material degradation is in the form of hardening, compression set, interfacial delamination, voiding, or excessive bleed-out. Therefore, in order to identify viable TIM materials, characterization of the thermomechanical behavior of these materials becomes important. However, developing effective metrologies for TIM characterization is difficult for two reasons: TIM materials are very soft, and the sample thickness is very small. Therefore, a well-designed test setup and a repeatable sample preparation and test procedure are needed to overcome these challenges and to obtain reliable data. In this paper, we will share some of the TIM characterization techniques developed for TIM material down-selection. The focus will be on mechanical characterization of TIM materials—including modulus, compression set, coefficient of thermal expansion (CTE), adhesion strength, and pump-out/bleed-out measurement techniques. Also, results from several TIM formulations, such as polymer TIMs and thermal gap pads, will be shared.

Copyright © 2019 by ASME
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Figures

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Fig. 1

Schematics of typical flip chip package cooling solutions for (a) high thermal dissipation packages such as servers and high-performance desktops and (b) low thermal dissipation packages such as laptops and hand-held devices. The schematic highlights the various thermal interfaces.

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Fig. 2

Schematic of (a) typical IHS package under use condition at the beginning of life depicting good contact of TIM between IHS and die and (b) schematic of typical end of life IHS package depicting TIM pump out and air gaps that get created

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Fig. 3

Stress contour of a typical TIM material exhibiting tensile and compressive behavior

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Fig. 4

Schematic of the TIM uniaxial compression tester

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Fig. 5

Schematic of compression set test for TIM samples: (a) compressed (loaded) state of TIM materials under evaluation and (b) recovered state after the compressive load is removed

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Fig. 6

Schematic of shear and pull (tensile) test setups

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Fig. 7

Schematic of asymmetric DCB setup for measuring the adhesive strength of TIM attached to silicon die and IHS lid

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Fig. 8

Schematic of a 3D-DIC based CTE measurement system. PTIM sample sitting on top graphite plate with graphite powder used as antistick agent.

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Fig. 9

Dynamic mechanical analyzer storage modulus as a function of temperature for PTIM materials A and C

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Fig. 10

Dynamic strain sweep test results showing the crossover points for the two PTIM candidates: (a) material A and (b) material B

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Fig. 11

Normalized stress versus strain curves for two different thermal gap pad materials D and E. Each sample is loaded to 10%, 20%, and 40% strains.

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Fig. 12

Normalized stress versus strain curves for PTIM A, B, and C shown in (a), (b), and (c), respectively. Three samples were tested for each material type to show the repeatability of this methodology.

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Fig. 13

Normalized compression set results for materials B and C. Lower values indicate more recovery.

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Fig. 14

Plot showing normalized peak load to failure in pull tests for materials A, B, and C under different processing conditions such as postcuring (time 0) and 168 h of bake

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Fig. 15

Normalized adhesion strength of PTIM attached to silicon die and IHS lid as measured using two different techniques: (a) shear test and (b) pull test

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Fig. 16

Normalized adhesion strength of PTIM to silicon and IHS lid as measured using a DCB test. Plot also shows the effects of bake on interfacial adhesion strength.

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Fig. 17

Typical TMA results showing the relative sample length as a function of temperature for PTIM material A

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Fig. 18

Normalized values of CTE for the three PTIM materials evaluated (materials A, B, and C)

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