Morphological transition of interfacial Ni3Sn4 grains at the Sn-3.5Ag/Ni joint

The transition in morphology of Ni3Sn4 grains that formed at the interface between liquid Sn-3.5Ag (numbers are in wt.% unless specified otherwise) solder and Ni substrate has been observed at 250–650°C. The morphological transition of Ni3Sn4 is due to the decrease of entropy of formation of the Ni3Sn4 phase and has been explained well by the change of Jackson’s parameter with temperature. According to the variation of solder joint strength with temperature, it decreased rapidly between 350°C and 450°C, where the thickness of the Ni3Sn4 intermetallic compound (IMC) layer was around 6.5 µm. However, the solder joint strength decreased slowly with an increase of soldering time without a significant drop, although the thickness of the IMC was larger than 6.5 µm. The notable drop of solder joint strength and the fracture mode transition with increase of soldering temperature appears to come from excessive lateral growth of IMC grains between 350°C and 450°C.