Roles of phosphorous in Sn4Ag0.5Cu solder reaction with electrolytic Ni–Au

In this study, the interfacial reactions of different P-doped and undoped Sn4Ag0.5Cu solders with electrolytic Ni–Au were investigated. The solder joints were reflowed once and then subjected to two more reflow cycles. The thickness of the intermetallic compound (IMC) layer and the IMC composition were analyzed by scanning electron microscopy and transmission electron microscopy (TEM). The tensile properties of the solder joint were measured using a Dage 4000 instrument. The results showed that the P content of the solder was inversely proportional to the IMC layer thickness and maximum tensile strain. TEM observations showed that a nanocrystallite layer existed between (Ni,Cu)6Sn5 and (Cu,Ni)3Sn4. This nanocrystallite layer was responsible for the abovementioned relationship. This layer not only suppressed the growth of the IMC layer effectively but also decreased the pull strength of the solder joint after three reflow cycles. Energy-dispersive X-ray spectra of this interfacial layer were recorded to determine the P, Ni, Cu, and Sn contents. As the P content of the P-doped Sn4Ag0.5Cu solder was increased, the IMC layer, which originally had a chunky morphology, became thin and flat. In this paper, the role of P in the Sn4Ag0.5Cu solder reactions and the potential applications of P-doped solders are discussed.