Novel graphite-based TIM for high performance computing

A new class of compressible graphite thermal interface material (TIM) was installed, tested, and qualified for use in a new series of IBM high performance computing (HPC) systems. Each system incorporated several high power graphics processing unit (300W GPU) assemblies in which a graphite TIM provided direct contact between bare die GPU devices and either air cooled heatsinks or water cooled cold plates. GPU hardware was removed from the system and exposed to a battery of thermal and mechanical stress tests, and reinstalled for in-system power age and power cycle tests, to quantify TIM reliability within a 3-year service life. After the GPUs were subjected to thermal-mechanical tests, which spanned accelerated thermal cycling (ATC), deep thermal cycling (DTC), thermal chip shock, temperature/humidity exposures, and system shock/vibration tests, the components were periodically reinstalled into systems to monitor power stability and to assess thermal reliability. For in-system tests, continuous power and thermal monitors were incorporated for all power cycle/power age regimens. Control groups of GPUs mounted with conventional grease-based TIMs were exposed to the same battery of thermal-mechanical and in-system server tests. All GPU hardware used for testing shared a common mounting design for TIM and cooling hardware attachments that provided a constant spring force clamping mechanism over the GPU bare die device area while enabling module flexure under load throughout stress test and system operating temperature ranges. Thermal effectiveness was measured by periodically monitoring the power draw of each GPU module and an internal device (junction) temperature over the course of the simulated life cycle. At the end of each evaluation, all GPU assemblies were then disassembled and assessed for TIM condition, which was then correlated with the final thermal resistance and power measurements. Based on comparison of both initial build performance and final test results, an optimized mounting construction was developed that incorporates the compressible graphite TIM.