Effect of Zn Addition on Interfacial Reactions and Mechanical Properties Between Eutectic Sn58Bi Solder and ENIG Substrate.

A comprehensive study of the interfacial reactions between Zn-added Sn58Bi solder alloys and electroless Ni/immersion Au (ENIG) substrates during reflow and solid-state thermal aging was conducted. The shear strengths of these samples were tested before and after aging. Zn first reacted with Au forming AuZn₃ intermetallic compounds (IMCs) that detached in small pieces from the substrate and immediately moved into the solder bulk. A very thin Au-Zn-Ni-Sn IMC layer was observed attached to the substrate. This layer, with a thickness of 0.25 m, significantly suppressed IMC layer growth during thermal aging, resulting in a thermally stabled joint. By contrast, the thickness of the (Au, Ni)(Sn, Bi)₄ IMC layer on the eutectic Sn58Bi/ENIG interface doubled during aging and, therefore, its shear strength decreased.