Comparative study of normal and thin Au/Pd/Ni(P) surface finishes with Sn–3.0Ag–0.5Cu solder joints under isothermal aging

A comparison study of the interfacial reactions and mechanical shear strengths of normal and thin electroless-nickel electroless-palladium immersion gold (ENEPIG) with Sn–3.0Ag–0.5Cu (SAC305) solder during isothermal aging at 150 °C is presented. The thicknesses of the Ni layers of the normal and thin ENEPIG were 6 and 0.1 μm, respectively. In the normal ENEPIG substrate, small and thin needle-shaped (Cu,Ni)₆Sn₅ intermetallic compounds (IMCs) were formed at the interface after reflowing, and a relatively thin IMC layer remained despite a long aging period of 1000 h. On the other hand, in the thin ENEPIG substrate, the thin Ni layer as well as the Au and Pd layers were completely reacted, and a relatively thick (Cu,Ni)₆Sn₅ IMC was formed at the interface after reflowing. In addition, the interfacial IMC layer grew continuously with increasing aging time. During aging at 150 °C, the interfacial IMC layers at the thin ENEPIG joints were consistently thicker than those at the normal ENEPIG joints. During a low-speed shear test, the shear strength did not significantly change depending on the aging time and surface finish, and all the fractures occurred in the ductile mode. On the other hand, in the high-speed shear test, the thick (Cu,Ni)₆Sn₅ IMC layer significantly deteriorated the shear strength of the thin ENEPIG joints. The thin ENEPIG joints showed inferior mechanical reliability (especially, high impact reliability) than the normal ENEPIG joints during solid-state isothermal aging at 150 °C.