Calculated Shear Stress Produced by Silicone and Epoxy Thermal Interface Materials (TIMs) During Thermal Cycling

Choosing a Thermal Interface Material adhesive can have an impact on the reliability of the microelectronic package in harsh thermal environments where thermal cycling temperature ranges are more extreme. This can occur during assembly with lead free solders, packages that generate heat due to their small size and processing power and applications where the package is in proximity to high temperatures. Due to the differing Coefficients of Thermal Expansion and Elastic Modulus of the materials used in hybrid electronic packages, the heating and cooling causes these materials to expand and contract, creating stress on parts of assembly where failure modes can be from warping, cracking, and delamination. Epoxy TIM adhesives have been used traditionally but silicones are becoming more popular due to their inherently low elastic modulus. A simplified mathematical model was evaluated that calculates relative inherent stress based on CTE of substrate and TIM adhesive, temperature range, and Elastic modulus of TIM. The purpose was to evaluate if equation could be used to aid the engineer in a first order material selection based on desired relative inherent stress using literature values for properties above versus expensive empirical testing. Three ceramic filled TIM adhesives were evaluated; an epoxy, and two Silicones (40 Type A versus 30 ´00´ Durometer) using the equation and then recalculated using values from empirically obtained Elastic Modulus. The substrates considered were silicon, gold, copper and aluminum. The evaluation demonstrated that the large difference in Elastic Modulus of epoxy versus silicone did show an overall lower relative inherent stress in the package assembly.