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)] t o investigate the intermetallic compounds (IMCs) formed between solder and a Cu substrate. Scanning electron microscope, transmission electron microsc ope, and electron diffraction analysis were used to study the IMCs between solder and a Cu substrate. The gamma -Cu5Zn8 and gamma'-Cu9Al4 IMCs were fo und at the Sn-Zn-Al/Cu interface. Thermodynamic calculation can explain the formation of gamma -Cu5Zn8 and gamma'-Cu9Al4 IMCs instead of CuSn compound s. The formation and growth of gamma'-Cu9Al4 IMC at 423 K resulted in the d ecrease of adhesion strength at the interface of solder and a Cu substrate, where the Kirkendall voids were severely formed. As the heating time incre ased up to 1000 h at 423 K, the adhesion strength between the eutectic Sn-Z n-Al solder and a Cu substrate decreased from 7.6 +/- 0.7 MPa to 4.4 +/- 0. 8 MPa.