The growth behavior of intermetallic compound layer at solder/Cu joint interface during soldering

In this paper, the growth behavior of intermetallic compound (TMC) layer at solder/Cu interface during soldering had been studied by using three Pb-free solder, Sn-3.5Ag, Sn-3.5Ag-0.5Cu and Sn-0.7Cu alloys. Meanwhile, Sn63-Pb37 eutectic alloy was used for comparison. In order to study the effects of reaction time on the formation and the growth behavior of IMC layer between four different solder and Cu substrate, the specimens were reacted at the temperature of 260°C for 30s, 60s, 90s, 240s and 480s respectively. Optical microscope (OM) was used to measure the thickness of IMC layers and observe the microstructure of solder joint. The results show that the mean thickness of interfacial IMC layers increased with the increase of reaction time for the same solder, but the effects of reaction time on the thickness of IMC layer was not monotonous, when the reaction time prolonged to 480s, the mean thickness of IMC layer of Sn-0.7Cu and Sn63-Pb37 solder joints decreased mildly. The high content of Sn in Pb-free solder can cause a more rapid consumption of Cu. So, by contrast with three Pb-free solders, Sn63-Pb37/Cu joints have the thinnest IMC layer for each reaction time. The chemical reaction intensity of following four solders with Cu weakens as the order listed below: Sn3.5Ag, Sn3.5Ag0.5Cu, SnO.7Cu, Sn63Pb37. We found a surprising phenomenon that some Cu blocks/particles distributed in the IMC layer or near the IMC layer, and when the immerging time was long enough, the faultage usually emerged at the solder/substrate interface of the solder joints.