Liquid structure of Sn-Ag- x Cu solders and its effect on the formation and growth of interfacial Cu 6 Sn 5

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 Cu6Sn5-type clusters which can stably exist below 330 °C. The parameters of the ordered SRO and MRO clusters were calculated. Then, Sn-3.5Ag-xCu (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 Cu6Sn5 intermetallic compound (IMC). The Cu6Sn5 layers were found to be thicker with higher initial Cu content in the solders. It is proposed that the Cu6Sn5-type MRO clusters provided potential nuclei for the formation of Cu6Sn5, 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.