Fluxless bonding process in air using Sn–Bi–Au design

A new fluxless bonding technique using Sn–Bi–Au design is presented. Microstructure and phase growth mechanism of as deposited Au/Sn/Bi films were first studied by scanning electron microscopy (SEM), X-ray diffraction method (XRD) and X-ray photoelectron spectroscopy (XPS). It is found that the crystallites of Au–Sn intermetallics are randomly distributed in Bi-rich matrix that forms a top-capping layer. The average size of AuSn2 and AuSn4 crystallite are 756 and 667 Å, respectively. The bonding process was performed at 250 °C in air without the use of flux. The resulting joints exhibit high shear strength of 4.9 kg and show only a few voids. Melting temperature of 225 °C was measured. The joint is composed of near eutectic Sn–Bi alloy with ribbon shaped AuSn2+AuSn4 intermetallic compound layer embedded inside the solder joint. The fracture of the Sn–Bi–Au solder joint mainly occurs along the interface between the Au–Sn intermetallic layer and the Sn-rich regions. This facture mode is ductile. The flux-free process is compatible with many pick-and-place machines that operate in ambient air.