Minor Au element effects on phase transformation and tensile strength

Abstract

Background

SnBi is an attractive solder owing to its low cost and step-soldering capability. However, its ductility is lower than that of conventional solder such as SnAgCu. Addition of elements to SnBi can help improve its properties. Au is considered an effective way to improve the undercooling and tensile strength of SnBi.

Methods

In this study, a solder joint of Sn56Bi2Au/Cu is reflowed at 160 °C and then subjected to solid–solid reactions from 80 to 130 °C. Fracture morphologies indicate the Au addition increases the joint ductility and reliability.

Significant Findings

The addition of minor Au causes needle-type AuSn₄ to disperse in the solder. The (Au,Cu)Sn formed at the interface during the 80 °C solid–solid reaction transforms to (Cu,Au)₆Sn₅ and Cu₃Sn as the temperature increases to 100–130 °C. The formation of intermetallic compounds has a significant effect on the reliability. Au–Sn compounds are extremely important in light-emitting diodes, while Cu–Sn compounds are commonly used as connections for die-attached devices. The aim of this study is to investigate the phase transformations among AuSn, AuSn₄, Cu₆Sn₅, and Cu₃Sn. The effects of Au addition on the microstructure and mechanical properties are also investigated.