The formation and evolution of IMC and its effect on the solder joint properties

This paper made a review of the kinds, morphology and distribution of intermetallic compound (IMC) formed in the bulk solder and at the solder-pad interface during reflow soldering of components and printed circuit boards, and then the formation or evolution of them was investigated after aging. The effect of IMC on the mechanical properties and fracture mechanism of the joint was also discussed in this paper. The results show that during the soldering, the kinds of IMCs are controlled by the diffusion rate of metals, which correspond to the saturation solubility of the metals in the eutectic solder alloys and thermodynamics (Gibbs energy change) and diffusion kinetics of the new formed TMCs, the amount of Cu in SnAgCu could affect the compositions of IMCs when Ni is used as substrate; the morphology of IMC is related to the heating and cooling speed during reflow soldering and the temperature gradient at the solder joint interface, also related to the content of Ag for Sn3.5Ag or SnAgCu eutectic solder, large plate Ag₃Sn formed easily with more than 3.5%Ag; the distribution of IMC depends on the composition of solder alloys and the diffusion rate of metallic elements in the solder joint. The evolution of IMCs or the formation of new IMCs during aging is related to the composition of the solder alloys and the touch pad, welding technique, aging temperature and time. The microstructures of solder and the IMCs formed during reflowing trend to coarsen commonly. New IMCs are formed or the compositions of the original IMCs change with the diffusion of the metallic elements such as Au, Ag, Cu, Ni. The study of mechanical properties and fracture mechanism of the solder joint shows that fracture pattern of the Pb-free solders is mainly the interfacial fracture. The high Ag low Cu solder are bad for the mechanical properties of soldering joint, and high reflow temperature and slow cooling rate is also bad for the mechanical properties of soldering joint.