Wettability and strength of In–Bi–Sn lead-free solder alloy on copper substrate

This work studied the surface and interfacial properties of a proposed lead-free solder material, the In–31.6Bi–19.6Sn system. Surface tension and contact angle of In–31.6Bi–19.6Sn lead-free solder with melting temperature of 61.33 °C was measured on copper substrate at different reflow temperatures. Sessile drop measurements showed that the contact angle depended on reflow temperature. The contact angle gradually decreased from 38.34° to 17.25° as reflow temperature increased from 80 to 140 °C. Energy-dispersive X-ray analysis indicated two layers of intermetallic compound between the solder and the Cu substrate: one of Cu6Sn5 and Cu11In9 (scallop shaped) and the other of Cu11In9 (brightly coloured). As the reflow temperature increased, the shear strength of the In–31.6Bi–19.6Sn/Cu solder joint improved due to reduced contact angle and larger spreading area.

Research highlights▶ In this work In–31.6Bi–19.6Sn being proposed for low temperature lead-free solder alloys. Since this solder alloy has low liquidus temperature, it may be used for low temperature soldering. Since this solder alloy has low liquidus temperature, organic-based transistor such as liquid crystal display (145–178 °C), new generation of nano silicon chips (100–300 °C), polymeric conductive boards (140–400 °C) and outer space nano satellites under cryogenic conditions (−147 to 447 °C) can be manufactured at low temperature. In this paper, microstructural and wettability results of In–31.6Bi–19.6Sn solder on Cu substrate for different reflow temperature are reported. Its wettability and microstructural characteristics on copper substrate were investigated. Surface tension and contact angle of the solder on Cu were evaluated in a reflow temperature range of 80–140 °C. As the temperature increased, the surface tension was reduced. A reduction in contact angle and increment in spreading area, which were the two indicators of wettability of the solder on the substrate, was observed as the temperature was increased. There were two types of IMCs detected between the solder reflow and Cu substrate: mixture of Cu6Sn5 and Cu11In9 (scallop shape) and Cu11In9 (brighter thin flat layer). Shear strength of In–31.6Bi–19.6Sn/Cu solder joint were affected by voids and wettability. Higher wettability, with lower contact angle and larger spreading area give higher shear strength as the reflow temperature is increased.