Microstructure of Sn-20In-2.8Ag solder and mechanical properties of joint with Cu

Abstract

Purpose

To explore substitutes for traditional Sn-Pb solder, Sn-20In-2.8Ag was considered because of its appropriate melting temperature, good reliability and high ductility at less than 100°C. However, the mechanical properties of Sn-20In-2.8Ag were not satisfactory. The reason for the poor mechanical properties of the Sn-20In-2.8Ag/Cu joint was revealed, and a way to solve the problem was found.

Design/methodology/approach

The microstructure evolution, characteristics of melting and solidification and joining performance with Cu were investigated using scanning electron microscopy (SEM), electron probe microanalysis, differential scanning calorimetry (DSC) and mechanical testing.

Findings

SEM results showed that the microstructure of Sn-20In-2.8Ag was composed of coarse dendritic Ag₂In and γ phases, with Ag₂In distributed at the grain boundaries. DSC measurements revealed that small amount of low temperature eutectic reaction, L → Ag₂In + β + γ, occurred at 112.9°C. This reaction was caused by the segregation of indium, which is a process that has a strong driving force. In the lap-shear testing, a crack propagated along the grain boundary of the solder, and failure showed an intergranular fracture. This failure was connected with the three-phase eutectic and coarse Ag₂In. Thus, to improve the mechanical properties, segregation of indium should be reduced and coarsening of Ag₂In should be prevented.

Originality/value

The reason for the unsatisfactory mechanical properties of Sn-20In-2.8Ag was revealed via microstructural observations and solidification analysis, and the way to solve this problem was found.

Citation

Peng, F., Liu, W., Ma, Y., Liang, C., Huang, Y. and Tang, S. (2019), "Microstructure of Sn-20In-2.8Ag solder and mechanical properties of joint with Cu", Soldering & Surface Mount Technology, Vol. 31 No. 1, pp. 1-5. https://doi.org/10.1108/SSMT-12-2017-0044