Fracture behavior and mechanical strength of sandwich structure solder joints with Cu–Ni(P) coating during thermal aging

Coating at the interface of solder joint has a great impact on the growth behavior of intermetallic compounds (IMCs) and the mechanical properties. In this study, the sandwich structure samples of Cu/Sn3.0Ag0.5Cu/Cu solder joints with coatings of Ni(P) monolayer and dual Cu–Ni(P) layers on Cu substrate were designed to conduct the shear test. The mechanical strength, fracture modes, and fractographic morphologies of Sn3.0Ag0.5Cu/Cu, Sn3.0Ag0.5Cu/Ni(P)–Cu, and Sn3.0Ag0.5Cu/Cu–Ni(P)–Cu solder joints reflowed at 280 °C for 10 min and then aged at 150 °C for up to 360 h were investigated. Experimental results show that the mechanical strength of Sn3.0Ag0.5Cu/Cu solder joints firstly increased because of the pinning effect of IMC, and then decreased due to the brittleness of excessive growth of IMC during solid-state aging processes. The Sn3.0Ag0.5Cu/Cu solder joints presented ductile fracture, mixed fracture, and brittle fracture modes during shear testing processes. The shear strength of Sn3.0Ag0.5Cu/Ni(P)–Cu solder joints decreased to about 17 MPa after 120 h thermal aging. At the same time, the shear failure mode of Sn3.0Ag0.5Cu/Ni(P)–Cu solder joints changed from the ductile dominant fracture to the brittle dominant fracture. The shear strength of Sn3.0Ag0.5Cu/Cu–Ni(P)–Cu solder joints remained approximate 21 MPa and the fractured position always occurred inside the IMC layer regardless of aging time.