Influence of Thermal Cycling on Degradation Behavior of Thermal Greases

Thermal management of electronic devices has been an ongoing challenge in the semiconductor industry owing to the increasing power density. The bottleneck of thermal resistance between the device and the heat sink solution is often interfaces between two different substrates. These substrates also often have different coefficient of thermal expansion (CTE). Therefore, a thermal grease is generally employed to reduce the overall thermal resistance. But the performance of thermal grease often worsens with time due to the thermomechanical cycling driven by the CTE mismatch between the substrates. This leads to degradation of thermal greases, which increases the interface resistance and the junction temperatures. In this study, we isolate the effect of thermal cycling (from the mechanical cycling) on degradation of two thermal greases: DOWSIL 340 heat sink compound and DOWSIL TC-5622 thermally conductive compound, by subjecting them to temperature cycling between 20°C to 100 °C with a cycling frequency of 30 minutes per cycle while holding the bond line thickness constant. We perform a parametric study at different bond line thicknesses (BLTs) of 23 µm, 59 µm, and 84 µm, Before testing the grease, a set of heat loss calibration experiments are performed to enable quantitative evaluation of the grease thermal performance. In addition to ther-mocouples in the system, we leverage high resolution temperature mapping of the thermal grease using an infrared (IR) microscope for evaluation of local degradation. Specifically, we sandwich the thermal grease (dot dispense) between a ceramic heater and calcium fluoride wafer (due to its IR transparency) to resolve the temperature maps at the top surface of thermal grease. Further, the thermal resistance, void fraction, and area of thermal grease are measured at 24 hour intervals to monitor the degradation performance in situ. We observe outward radial movement in DOWSIL-340 (i.e., increase in thermal grease area) and crack formations at thick BLT in early stages of thermal cycling, while at the thin BLT, the degradation is primarily observed at the circumference of the grease. Further, no degradation is observed in DOWSIL-5622 at all three BLTs. We hypothesize this is due to its stable and repeatable viscosity variation with temperature cycling. These results demonstrate a pathway for evaluating thermal grease performance by showcasing importance on the viscosity-temperature hysteresis. This in-turn forms a stepping stone to characterize the coupled thermo-mechanical degradation of thermal greases.