A fluxless fabrication process producing Sn-rich Bi–Sn joints with high melting temperature

A fluxless bonding process to produce tin-rich bismuth–tin solder joints is presented. The process was designed using Sn–Bi multilayer structure deposited in high vacuum to inhibit oxidation and to achieve the fluxless feature. The composition was chosen to be tin-rich, 95.0 wt.% Sn and 5.0 wt.% Bi, to achieve a solidus temperature significantly higher the 139 °C eutectic temperature of the 57Bi–43Sn eutectuic alloy. The resulting joints were imaged with a scanning acoustic microscope (SAM) and shown to be nearly void-free. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy were used to study the microstructure and compositions of the joints. The measured joint thickness was 4.1 μm. SAM and EDX results indicate that the solder joint consists of (β-Sn) matrix embedded with small grains of (Bi) solid solution phase. The melting temperature of the joint was measured to be 210–215 °C, as expected from the designed composition. The fluxless feature of this process gives the packaging community another process to consider for use in devices and packages where the use of flux is prohibited, such as some biomedical, MEMs, fiber optical and photonic devices.