Effects of Pd addition on the interfacial reactions between Cu and Al during ultrasonic welding

The interfacial reactions between Cu and Al during ultrasonic welding were investigated, focusing on the effect of Pd in Pd-coated Cu and Pd-alloyed Cu (0.3 wt% and 2 wt% PdCu). For Pd-coated Cu–Al bonding, it is found that Pd layer would act as diffusion barrier layer at the interface, and intermetallic compounds (IMCs) formed at Pd-free location for as-welded condition. As annealing time increases, Cu atoms and Al atoms bypassed the fractured Pd layer to form IMCs at the interface. Combining experimental observations, a model of Pd diffusion barrier effect was first proposed at the Pd-coated Cu–Al interface. For Pd-alloyed Cu–Al bonding, Pd can promote the atomic diffusion of active elements and accelerate the formation of IMCs, and excessive 2 wt% Pd addition can lead to the appearance of voids and cracks at interface, which is destructive for reliable interconnection structure. The thickness of the interfacial IMCs for pure Cu and 0.3 wt% PdCu at different heat treatment conditions has a linear relationship with the square root of time, which means that the growth of IMCs is controlled by volume diffusion. The diffusion activation energy of IMCs for 0.3 wt% PdCu (46.08 kJ/mol, 52.88 kJ/mol, and 65.35 kJ/mol for Cu₉Al₄, CuAl and CuAl₂ respectively) was smaller than that for pure Cu (60.24 kJ/mol, 58.12 kJ/mol, and 82.94 kJ/mol for Cu₉Al₄, CuAl and CuAl₂ respectively). Furthermore, the percentage of Cu₉Al₄ in the whole IMC decreased for 0.3 wt% PdCu, which can reduce the halogen corrosion from epoxy molding compound (EMC) during the reliability test of highly accelerated temperature and humidity stress testing (HAST).