Liquid structure of Sn-Ag-xCu solders and its effect on the formation and growth of interfacial Cu6Sn5

The liquid structure of Sn-3.5Ag and Sn-3.5Ag-0.7Cu (wt%) solders was first studied using high-temperature X-ray diffraction method. Only short-range order (SRO) was detected in the liquid Sn-3.5Ag melts. However, besides SRO, medium-range order (MRO) was also detected in the liquid Sn-3.5Ag-0.7Cu melts at 260 °C and 330 °C while not at 400 °C. The MRO structure was considered to be related to Cu 6 Sn 5 -type clusters which can stably exist below 330 °C. The parameters of the ordered SRO and MRO clusters were calculated. Then, Sn-3.5Ag- x Cu ( x = 0, 0.7 and 1.5) solders were reflowed on Cu substrate for different durations to investigate the effect of liquid structure on the formation and growth behavior of interfacial Cu 6 Sn 5 intermetallic compound (IMC). The Cu 6 Sn 5 layers were found to be thicker with higher initial Cu content in the solders. It is proposed that the Cu 6 Sn 5 -type MRO clusters provided potential nuclei for the formation of Cu 6 Sn 5 , which promoted the IMC growth rate. The difference in liquid structure of the solders was regarded as a key reason for the different IMC growth rate as well as IMC type at the soldering interfaces.