Influence of Dopant on Growth of Intermetallic Layers in Sn-Ag-Cu Solder Joints

Abstract

The interfacial interaction between Cu substrates and Sn-3.5Ag-0.7Cu-xSb (x = 0, 0.2, 0.5, 0.8, 1.0, 1.5, and 2.0) solder alloys has been investigated under different isothermal aging temperatures of 100°C, 150°C, and 190°C. Scanning electron microscopy (SEM) was used to measure the thickness of the intermetallic compound (IMC) layer and observe the microstructural evolution of the solder joints. The IMC phases were identified by energy-dispersive x-ray spectroscopy (EDX) and x-ray diffractometry (XRD). The growth of both the Cu 6 Sn 5 and Cu 3 Sn IMC layers at the interface between the Cu substrate and the solder fits a power-law relationship with the exponent ranging from 0.42 to 0.83, which suggests that the IMC growth is primarily controlled by diffusion but may also be influenced by interface reactions. The activation energies and interdiffusion coefficients of the IMC formation of seven solder alloys were determined. The addition of Sb has a strong influence on the growth of the Cu 6 Sn 5 layer, but very little influence on the formation of the Cu 3 Sn IMC phase. The thickness of the Cu 3 Sn layer rapidly increases with aging time and temperature, whereas the thickness of the Cu 6 Sn 5 layer increases slowly. This is probably due to the formation of Cu 3 Sn at the interface between two IMC phases, which occurs with consumption of Cu 6 Sn 5 . Adding antimony to Sn-3.5Ag-0.7Cu solder can evidently increase the activation energy of Cu 6 Sn 5 IMC formation, reduce the atomic diffusion rate, and thus inhibit excessive growth of Cu 6 Sn 5 IMCs. This study suggests that grain boundary pinning is one of the most important mechanisms for inhibiting the growth of Cu 6 Sn 5 IMCs in such solder joints when Sb is added.