EVALUATION OF THERMAL INTERFACE MATERIAL PERFORMANCE IN A CYCLIC STRAIN ENVIRONMENT
Authors: Joe Doman, Eddie Kobeda, Ph.D., Jim Bielick, Joe Kuczynski, Tim Tofil, and Mike Vaughn Company: IBM Corporation Date Published: 10/24/2010
Abstract: Today’s electronic technologies require improvements in circuit and packaging density to meet the future needs of state-of-the-art high performance systems. These server technologies are dependent upon scalable processors meeting multi-gigahertz frequencies in smaller footprints, requiring new innovative design methods. We have developed a decentralized power distribution scheme utilizing voltage transformation modules to improve power distribution efficiency. The voltage transformation module (VTM, registered trademark of Vicor Corporation) is an isolated DC-DC converter in a compact over-molded package operating as a fixed-ratio step-down DC transformer, with high power density (103 A/in2), rated at up to 185W per module. Our work here focuses on thermal interface materials (TIMs) that provide a medium between heat-generating devices and heat sinking elements. A common heat spreader/cold plate is an efficient way to dissipate the power generated by arrays of VTM modules, which can number in the hundreds per assembly. This presents a challenging mechanical tolerance stack, with significant TIM gap variation across the array. TIM compression up to 70% is required to accommodate the gap variation. CTE mismatch between the card assembly and cold plate drive significant strains to the TIM gap during power on/off cycling. Mechanical cycling of a power card assembly was performed using an Instron electromechanical test system to simulate long term product conditions. Maximum compressive/tensile loads were monitored along with creep during dwells. The device was powered with a static DC voltage input and loaded with a constant current load, while a data acquisition system monitored long term electrical and thermal performance. Continuous sampling was used to detect high speed glitching due to any potential thermo-mechanical solder joint fractures. Long term TIM reliability under varying compression factors and cyclic loading was evaluated to ensure acceptable field life in the customer environment.