Reliability Study of High-Temperature-Resistant Mounting Structure Considering Shear Behavior of Joint Layer

Wide band gap semiconductor devices such as Silicon Carbide (SiC) or Gallium Nitride (GaN) capable of operation at high temperature over 300 degrees offer the potential of higher performance with reduced size, weight and eventually cost in power electronics equipment. For these high-temperature-resistant devices, the authors have proposed a new chip mounting structure that the stress relaxation function rests with the circuit metal on the substrate rather than the joint layer. In this study, to evaluate thermal fatigue of the new structure precisely, material properties of the high-temperature-resistant joint layer were measured by a new method and this method enabled analysis using measured properties. The joint layer which became very thin was formed by a low temperature sinter technology using silver (Ag) nano particles. The shear test to obtain the nonlinear properties of the joint layer was carried out by a proposed new method using bimetal fixtures which are composed of two materials whose Coefficient of Thermal Expansion (CTE) is different. Displacement of shear direction on the joint layer formed between the fixtures can be generated by heating the fixtures. Micro displacement was measured by Digital Image Correlation Method (DICM) using optical microscope and minute load on the joint layer was detected by strain gauge attached at fixture. Using the properties of the joint layer obtained as the test results, shear behavior on the chip joint was evaluated by Finite Element Analysis (FEA). Furthermore, harsh Thermal Cycle Test between −50 and 300 degree Celsius was carried out in the samples as the same structure as FEA model. From these results, the fatigue mechanism became clear and an improvement of the thermal cycle life was discussed.