Metallurgical investigation of silver-copper-tin lead-free solder with the addition of gold.

摘要

This thesis details the effect of Au addition on the bulk ACS solder alloy and its interface reaction with Cu. The study was conducted by synthesizing different compositions of Au-ACS alloys by changing the percentage of Au added to the base ACS alloy and subsequent characterization of newly formed alloys with respect to phase studies, hardness and solder-Cu interface investigation. It was found that Au forms quaternary eutectic and reduces the liquidus temperature of the ACS alloy to 204.5 ± 0.3°C. Addition of Au to the bulk ACS solder resulted in the formation of AuSn4 IMCs, which were smaller in size and well distributed as a part of quaternary eutectic, thus enhancing the strength of the ACS alloy as well as lowering the eutectic temperature.; The major observation of this investigation was the effect of Au on changing the solder-Cu interface morphology from scallop to composite type. This composite interface is made of soft β-Sn phases and hard ternary (Au_xCu _1−x)₆Sn₅ IMCs. It was observed that the formation of a composite interface continues during reflow as long as there is sufficient Au supply for the ternary reaction, after which the interface reverts to scallop type. The composite interface effectively prevents the diffusion of Cu into the solder during the solid-state reaction, thereby reducing the chances of dewetting of solder interconnections. Finally, it was observed that the composite interface improves the fatigue and fracture strength of solder joint considerably, as the soft β-Sn phase arrests crack propagation, reducing the chances of brittle failure. With the positive factors such as the enhanced strength, lowered eutectic temperature and the composite interface, the addition of Au in the ACS solder is believed to be beneficial than harmful, and even provide a new class Pb-free Au-Ag-Cu-Sn solder alloy.