Coupling effects at Cu(Ni)–SnAgCu–Cu(Ni) sandwich solder joint during isothermal aging

Interfacial reactions and consumption of metallization during liquid reaction and isothermal aging at 175 °C for Cu–SnAgCu–Cu, Ni–SnAgCu–Ni and Cu–SnAgCu–Ni solder joints were investigated. Interfacial microstructures were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). Optical microscopy (OM) was employed to measure the consumed thickness of metallization layer. A coupling effect occurs between Cu/SnAgCu and Ni/SnAgCu interfaces at Cu–SnAgCu–Ni solder joint. The inter-diffusion of Cu and Ni atoms contributes to the formation of ternary (Cu,Ni)6Sn5 IMCs at both interfaces. The Ni(P) layer in Ni–Ni solder joint is fully depleted and changes to Ni3P after aging 500 h. However, due to the coupling effect, the consumption of Ni(P) layer is limited and the Cu consumption is accelerated compared to Cu–Cu and Ni–Ni solder joint, which prevents the Ni(P) layer from being depleted and improves the reliability.