Thermal Conductivity and Interface Strength Evolution of TIM-Copper with Temperature and Humidity Conditioning

A thermal interface material (TIM) with high bulk thermal conductivity is inserted between the FCBGA package and the heatsink to reduce the thermal resistance of the heat transfer path. Exposure to high humidity, temperature storage, and wide temperature extremes expected in automotive environments may result in interfacial damage, resulting in the initiation of cracks at the TIM-to-Copper interface, increased thermal resistance and thermal runaway of the device. The research objectives include the study of the effect of hygrothermal exposure and thermal cycling on the TIM-Cu interfaces, the effect of environmental exposure on the thermal conductivity of the interface, and the evolution of the fracture toughness of the interface with the time in the use-case scenario. Presently, there is lack of damage models for the prediction of TIM interface survivability and assessment of performance degradation. In this study, the bi-material samples are stored in a Hygrothermal test chamber for moisture absorption at three conditions - 60°C/C/60% RH and 85°C/85% RH. In addition, a separate set of samples has been kept in a thermal cycling chamber with a profile ranging from -40°C to 125°C for 50, 100, 200, 400, and 1000 automotive-grade thermal cycles. Both monotonic and Fatigue testing are conducted on all bi-material samples for the measurement of the mode-1 and mode-2 critical stress intensity factors and Paris’s Power Law Exponent. The thermal conductivity has been measured using a thermal interface tester. Moisture diffusivity, activation energy, and acceleration factors are investigated for hygrothermal exposure.