Effect of Aluminum Clad Cu Wire Bonds on Power Cycle Lifetime for High Current Density Power Module Packages

Abstract

Significant growth of the Electric Vehicle （EV） market is making a valuable contribution to the efficient reduction of CO₂ emissions. Application of power semiconductors in EVs is attracting a lot of attention, since many silicon （Si） power semiconductor chips, packages and modules are implemented in the main traction inverter, electric power steering system （EPS）, battery management system, and other motor drivers that are key components for EVs. Recently, silicon Carbide （SiC） power devices are promising alternatives, and with a view to achieving the superior semiconductor performance, the transition from Si power devices to SiC devices is underway. The maximum operating temperature of Si power devices is around 175℃, that of SiC devices is expected to be as high as 250℃, taking advantage of their wide bandgap characteristics, which induces high thermal stress around a SiC chip owing to a Coefficient of Thermal Expansion（ CTE） mismatch and deteriorates the power module reliability. Al wire bond interconnections damage rapidly due to a thermal strain between the SiC chip and the wire material, which accelerates failures near the interface of the wire bonds in the power module, due to the T_j changes caused by higher current loads. In this study, Al-Clad Cu （AlCu） wire bonding structure on a thick ion-plated Cu over-pad metallization （OPM） layer is proposed. To demonstrate the power cycling durability, SiC-Schottky barrier diodes （SBD） with AlCu wire bonds on a 25 μm-thick Cu-OPM layer were assembled into the test vehicles. Applying finite element method （FEM） analysis, the reliability of AlCu wire bonds on Cu-OPM was demonstrated. The lifetime during power cycling tests of AlCu wire bonds on a 25 μm-thick Cu-OPM layer was 14-times longer than that of Al wire bonds on an Al pad at ΔT_j = 75℃. In light of inelastic stress-strain analysis, the lifetimes of the AlCu wire bonds on a 100 μmthick and a 300 μm-thick Cu-OPM layer during power cycling tests were predicted to be 42-times and 1,600 times longer than that of Al wire bonds on an Al pad, respectively.