Solid state interaction between a Sn–Ag–Cu–In solder alloy and Cu substrate

The solid-state reactions between the Sn-3.0Ag-0.4Cu-7.0In (wt%) solder and Cu substrate, at temperatures 100, 125, 150 and 180 °C and for aging times up to 1506 h, are studied. The interfacial layers formed between solder and substrates are characterized using optical and scanning electron microscopy and electron microprobe analysis. Two ternary intermetallic layers Cu6(Sn,In)5 (η-phase) and Cu3(Sn,In) (ɛ-phase) are formed at the interface between the solder and Cu substrate. Kirkendall voids were observed at Cu/ɛ interface as well as inside the ɛ layer. The growth kinetics of η and ɛ phases and the activation energies for their growth are calculated and results are compared with growth kinetics of these phases in the case of Sn–Ag–Cu solders/Cu systems.

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► The role of In on solid-state reaction between a lead-free solder and Cu is studied.
► Two layers of η-Cu6(Sn,In)5 and ɛ-Cu3(Sn,In) phases are formed at the interface.
► The maximum solubility of In in the η phase is about 6 at% at 150–180 °C.
► The growth coefficient of ɛ layer is lower than that of Cu3Sn in Sn–Ag–Cu/Cu systems.
► The activation energy for growth of Cu3(Sn,In) is higher than that for Cu3Sn.