A fluxless process of producing tin-rich gold-tin joints in air

A soldering process performed in ambient air without the use of any flux is reported. We believe that this is the first time fluxless soldering process is successfully done in air without prior fluorine treatment. The fluxless process is implemented using Au-Sn binary system. It is based on Au-Sn multilayer design that is substantially tin-rich, namely, with 95 at.% Sn (91.8 wt.% Sn) and 5 at.% Au (8.2 wt.% Au). Over the past 15 years, we have developed numerous fluxless bonding processes. These processes require environments such as H₂ or N₂ during the bonding process to inhibit solder oxidation. This requirement is not compatible with the pick-and-place bonding machines widely employed in the industry. Thus, fluxless processing in air has been our lifelong endeavor. After many attempts, we finally achieved some initial success. The bonding process is carried out at 225°C. The resulting joints are nearly void-free as confirmed by scanning acoustic microscopy (SAM). To study the microstructure and composition of the samples, scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy was performed on the joint cross-section. The results show that the joint is composed of AuSn₄ intermetallic grains embedded in a Sn matrix. Re-melting temperatures of the solder joints were measured to range from 214°C to 220°C, which are consistent with data on the Au-Sn phase diagram.