Liquid–solid interfacial reactions of Sn–Ag and Sn–Ag–In solders with Cu under bump metallization

Abstract

Intermetallic compounds formed during the liquid–solid interfacial reaction of Sn–Ag and Sn–Ag–In solder bumps on Cu under bump metallization at temperatures ranging from 240 to 300 °C were investigated. Two types of intermetallic compounds layer, η Cu6Sn5 type and e Cu3Sn type, were formed between solder and Cu. It was found that indium addition was effective in suppressing the formation of large Ag3Sn plate in Sn–Ag solder. During interfacial reaction, Cu consumption rate was mainly influenced by superheat of solder, contact area between solder and Cu and morphology of intermetallic compounds. The growth of η intermetallic compounds was governed by a kinetic relation: ΔX = tn, where the exponent n values for Sn–Ag/Cu and Sn–Ag–In/Cu samples at 240 °C were 0.35 ± 0.01 and 0.34 ± 0.02, respectively. The n values increased with reaction temperature, and it was higher for Sn–Ag/Cu than that for Sn–Ag–In/Cu sample at the same temperature. After Cu was exhausted, e intermetallic compound was converted to η intermetallic compound. The mechanisms for such growth of interfacial intermetallic compounds during the liquid–solid reaction were investigated.