Effect of Surface Finishes on the Welding of Sn58Bi Solder

This paper investigated the melting point and surface oxide thickness of Sn58Bi clean solder (non-flux-coated solder with a clean surface). The surface morphology, spreadability, voidage, and interfacial reaction of Sn58Bi clean solder and flux-coated solder welded with three different surface finishes were studied during vacuum welding and atmospheric reflow welding. The surface finishes consisted of pure copper, matte tin, and gold plating. The results indicated that the optimal welding surface finish of Sn58Bi clean solder was gold plating, and the interfacial reaction was sufficient and the average voidage was 7.69%. The interfacial intermetallic compound (IMC) was approximately 0.6-1 um thick, uneven and discontinuous. As for flux-coated solder, the preferable surface finish was matte tin, which had good welding surface morphology and formed a continuous smooth IMC layer with a thickness of about 0.4-0.8 um; spreading ratio and average voidage were 101.64% and 4.03%, respectively. An unknown particle-shaped Au-Sn-Bi phase ɑ with Au:Sn:Bi (wt.%) ≈ 18.5:56.2:24.2 (Au15Sn68Bi17 (at.%)) was discovered in the welding interface of Sn58Bi solder and gold-plated substrate. In addition, another Au-Sn-Bi ternary compound β with Au:Sn:Bi (wt.%) ≈ 27.8:65.5:6.7 (Au19Sn76Bi5 (at.%)) was found in Au18.5Sn56.2Bi24.2 (wt.%) alloy which prepared by smelting. (Au15Sn68Bi17 (at.%) named ɑ and Au19Sn76Bi5 (at.%) named β in the present work).