Formation of intermetallic compounds at eutectic Sn–Zn–Al solder/Cu interface

The eutectic Sn–Zn–Al solder alloy was used [composition: 91Sn–9(5Al–Zn)] to investigate the intermetallic compounds (IMCs) formed between solder and a Cu substrate. Scanning electron microscope, transmission electron microscope, and electron diffraction analysis were used to study the IMCs between solder and a Cu substrate. The γ–Cu₅Zn₈ and γ–Cu₉Al₄ IMC_s were found at the Sn–Zn–Al/Cu interface. Thermodynamic calculation can explain the formation of γ–Cu₅Zn₈ and γ–Cu₉Al₄ IMC_s instead of Cu–Sn compounds. The formation and growth of γ–Cu₉Al₄ IMC at 423 K resulted in the decrease of adhesion strength at the interface of solder and a Cu substrate, where the Kirkendall voids were severely formed. As the heating time increased up to 1000 h at 423 K, the adhesion strength between the eutectic Sn–Zn–Al solder and a Cu substrate decreased from 7.6 ± 0.7 MPa to 4.4 ± 0.8 MPa.