Effects of a phosphorous-containing Pd layer in a thin-ENEPIG surface finish on the interfacial reactions and mechanical strength of a Sn–58Bi solder joint

Abstract

We analyzed the effects of phosphorous (P) in a palladium (Pd) layer on the interfacial reactions and mechanical properties of Sn–58Bi solder with thin electroless-nickel electroless-palladium immersion gold (ENEPIG) joints (pure Pd and Pd(P) joints) after a high-temperature storage test. Flake-type (Pd,Au)Sn4 intermetallic compounds (IMCs) mainly formed in the pure Pd and Pd(P) joints. After being aged at 85–115 °C for 300 h, top- and bottom-side (Cu,Ni)6Sn5 IMCs formed at the interface between the (Pd,Au)Sn4 IMC and partially destroyed P-rich Ni layer of the pure Pd joints. In contrast, a Ni3Sn4 IMC formed at the interface between the (Pd,Au)Sn4 IMC and P-rich Ni layer of the Pd(P) joints after most aging temperatures and durations. This is because the reaction rate of the Pd(P) joint was lower than that of the pure Pd joints. The unreacted Ni layer remained at the Pd(P) joints after aging. In a high-speed shear test, the shear strength of the pure Pd joints rapidly decreased after being aged at 115 °C for 100 h. In contrast, the shear strength of the Pd(P) joints slightly decreased after being aged at 115 °C for 1000 h. The shear strengths of the pure Pd and Pd(P) joints were quite different under same aging conditions because of the different diffusion rates and IMC compositions of the pure Pd and Pd(P) joints. Fracture surfaces of the pure Pd joints after being aged at 115 °C were observed on the Cu substrate, but those of the Pd(P) joints were on the Ni3Sn4 IMC surface. Therefore, the Sn–58Bi solder with a Pd(P) layer in thin-ENEPIG is expected to be more reliable than that with pure Pd joints under aging treatment.