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Performance Comparison: Liquid and Pad Thermal Interface Material
This study tested the hypothesis that a liquid TIM with the same effective thermal conductivity as a gap pad will have lower thermal resistance in application due to its ability to wet out the interface better than a solid pad. Also hypothesize here, bondline thickness has a significant effect over total thermal resistance.
Technical Paper
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Authored By:
Rita Mohanty, Ph.D., Blake Wageman
Henkel Corporation
MN, USA
Summary
Power density of electronic devices has increased beyond what we thought possible merely a decade back. In addition, the dimension of the CPU/GPU has exceeded 70mmx70mm in size. Today, you find these large, high power processor packages in telecom, datacom, automotive, aviation, consumer electronics and more. The package size, power density and frequency of high-power cycling have created both challenges and opportunities for Thermal Interface Material (TIM) suppliers to address the thermal management needs of these new generation of devices. There are two primary types of TIM used in between the heat generating component and heat spreader. These are known as pad (a solid mat) and liquid (1 or 2 component paste). Standard heat spreader and heat generating component surfaces have very few contact points due to machined surfaces that are rough and wavy. These surface imperfection traps air in between the two surfaces creating a barrier for heat transfer. The objective of the “right” TIM is to replace the air and completely wet out both interfaces to reduce thermal resistance without damaging the delicate large packages.
Pad and liquid TIM use distinctly different assembly processes in applications. Gap pads need a certain amount of compression under pressure to fully wet out the surface and expel any air entrapped on the surfaces. Liquid TIM on the other hand needs little to no external compression force during assembly as it flows easily under stress. Current work is based on the hypothesis that a liquid TIM with the same effective thermal conductivity as a thermal pad will have lower thermal resistance/impedance due to its ability to wet out the interface better than a solid pad. Other factors, such as bondline thickness will also be explored to test the hypothesis that thinner bondline provides lower resistance regardless of TIM type.
Conclusions
This study tested the hypothesis that a liquid TIM with the same effective thermal conductivity as a gap pad will have lower thermal resistance in application due to its ability to wet out the interface better than a solid pad. Also hypothesize here, bondline thickness has a significant effect over total thermal resistance. This comparative study used the ASTM D5470 test method and a custom TTV to characterize TIM thermal impedance and resistance at different thickness. Based on the results from Test 1, we can conclude that liquid TIM’s physical nature and rheological properties do provide a measurable improvement to both thermal impedance and resistance at the same BLT. Based on the same theory, results from Test 2 leads us to the conclusion that regardless of the BLT, liquid TIM outperforms pad at all bondline thickness tested.
A solid pad that requires some pressure to compress causing elastic/plastic deformation has limited capability to fill the uneven surface at both interfaces. Most gap pads cannot be compressed more than 50% of their original thickness as it will have performance loss. This limiting their ability completely wet out the interface. Liquid TIM on the other hand flows with little to no stress or compression force providing excellent wet out of the interfaces.
The performance distinction highlighted here is impactful as next generation electronic device package size, power density and frequency of high-power cycling continue to increase across many markets. Understanding which TIM to select for these devices as well as those with complex design and high number of components is crucial to the thermal management and reliability of these applications.
Initially Published in the SMTA Proceedings
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