Effect of alloying Cu substrate on microstructure and coarsening behavior of Cu₆Sn₅ grains of soldered joints

Abstract

In order to investigate the effects of alloying Cu substrate on microstructure and coarsening behavior of Cu₆Sn₅ grains of soldered joints, in the paper, four types of wetting layers (Cu, Cu–8 wt% Zn, Cu–15 wt% Zn and Cu–30 wt% Zn) reaction with Sn–3.0Ag–0.5Cu (SAC305) solder were fabricated. The effects of reflowing and isothermal aging on the interface of soldered joints and the morphology of Cu₆Sn₅ grains were studied. Experimental results show that the scallop-shaped Cu₆Sn₅ formed at the interface of SAC/Cu after reflowing. After isothermal aging for 120 h, a bi-layer structure of Cu₆Sn₅ and Cu₃Sn formed at the interface of SAC/Cu and some Ag₃Sn grains decorated in Cu₆Sn₅, and Kirkendall voids were observed inside the Cu₃Sn layer after aging for 240 h. However, Cu₆Sn₅ was the predominant reaction product and Cu₃Sn and Kirkendall voids were not observed at the interface of SAC/Cu–Zn irrespective of aging time. The thickness of IMC layer increases with increasing aging time. Compared to SAC/Cu, the additions of Zn can effectively suppress the growth of IMC. During the aging process, the growth rate constants of IMC layers of SAC/Cu, SAC/Cu–8Zn, SAC/Cu–15Zn and SAC/Cu–30Zn were obtained by using fitting method from the experimental data, which were 7.78 × 10⁻¹⁷, 5.42 × 10⁻¹⁷, 2.51 × 10⁻¹⁷ and 1.46 × 10⁻¹⁷ m² s⁻¹, respectively. The mean diameter of IMC grains increases with the increasing aging time. The relationships of the mean diameter of Cu₆Sn₅ grains of SAC/Cu, SAC/Cu–8Zn, SAC/Cu–15Zn, SAC/Cu–30Zn interface joints and aging time were given, i.e., d = 1.49t ^0.237, d = 1.46t ^0.204, d = 1.43t ^0.203 and d = 1.71t ^0.197, respectively.