Morphological Evolution and Growth Kinetics of Interfacial Cu₆Sn₅ and Cu₃Sn Layers in Low-Ag Sn-0.3Ag-0.7Cu-xMn/Cu Solder Joints During Isothermal Ageing

Abstract

The morphological evolution and growth kinetics of interfacial Cu₆Sn₅ and Cu₃Sn intermetallic compound (IMC) layers between Cu substrates and Sn-0.3Ag-0.7Cu-xMn (x = 0 wt.%, 0.02 wt.%, 0.05 wt.%, 0.1 wt.%, and 0.15 wt.%) (SAC0307-xMn) solders were investigated. After ageing, the uneven scallop-like morphology of Cu₆Sn₅ transforms into a layer-like morphology, and the Cu₃Sn morphology remains layer-like. Kirkendall voids at the Cu/Cu₃Sn interface and in the Cu₃Sn layer are observed at high ageing temperatures. The Cu₆Sn₅ layer predominantly governs the growth of the total IMC layer at low ageing temperatures, whereas the Cu₃Sn layer primarily influences the total IMC layer at high ageing temperatures. The growth of the Cu₆Sn₅ and Cu₃Sn layers fits a power–law relationship with an exponent between 0.44 and 0.82, indicating that IMC growth is primarily controlled by diffusion but may also be affected by interfacial reactions. The activation energies and interdiffusion coefficients of the Cu₆Sn₅ and Cu₃Sn layers were determined. The addition of Mn nanoparticles strongly affected the growth of the Cu₆Sn₅ layer but weakly impacted the growth of the Cu₃Sn layer, particularly at low ageing temperatures. Adding Mn nanoparticles to the SAC0307 solder can evidently increase the activation energy of the Cu₆Sn₅ layer, reduce the atomic diffusion rate, and inhibit the excessive growth of the Cu₆Sn₅ IMC.