Thermo-Mechanical Analysis of a 650 V/150 A e-GaN HEMT Sandwiched Between a PCB and DBC Substrate

Following successful packaging of a 650 V/150 A e-GaN HEMT between a printed-circuit board and a direct-bond-copper substrate, this study evaluates the thermo-mechanical reliability of the sandwich-style package. Thermo-mechanical fatigue of both soldered and silver-sintered bonds in the package under temperature cycling was studied using ANSYS Mechanical. Deformation properties of eutectic lead-tin solder and sintered silver were modeled after Anand viscoplastic behavior. The inelastic volume-averaged strain energy density accumulated per thermal cycle at each bond was used as a metric of bond fatigue. Simulation results showed that the silver-sintered bond at the gate pad and the solder bond closest to the package center have the highest per-cycle strain energy density. Simulations also evaluated the effect of encapsulation materials on bond fatigue. It was found that using rigid encapsulants, such as one with a coefficient of thermal expansion of 14.9 ppm/°C and an elastic modulus of 17.5 GPa, instead of a commonly used silicone gel reduced the per-cycle bond fatigue by 97%. Findings of this study would help guide the development of high performing and high reliability packaging technology for the 650V/150A GaN HEMT.