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

Abstract 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.

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