Investigation of Interfacial Reactions and Sn Whisker Formation in the Matte Sn Layer With NiP/Ni/Cu and Ni/Cu Multilayer Systems

This paper investigates the relationship between the interfacial reactions and Sn whisker formation in the matte Sn layer in NiP/Ni/Cu and Ni/Cu multilayer systems. All specimens were first reflowed at 250°C for 10 min and then aged at 150°C for 500 h. Sn whiskers were found only in the Sn/Ni/Cu multilayer specimen. Ni from the NiP layer was consumed to form the Ni₃Sn₄ phase in the Sn/NiP/Ni/Cu specimen. Thus, the phosphorous (P) atoms segregated at the interface and reacted with Ni and Sn atoms to form the Ni-P-Sn ternary phase between the phase and NiP layer. This Ni-P-Sn ternary phase acted as a diffusion barrier and decreased the diffusion rate and diffusion flux of the Ni atoms. The compressive stress induced by the flux of the Ni atoms was the driving force resulting in Sn whisker growth. The Ni-P-Sn ternary can inhibit this compressive stress. That is why no Sn whiskers were found in the Sn/NiP/Ni/Cu multilayer specimen.