Effects of Ni/Ag coating on the wettability of Sn-3Ag-0.5Cu alloy on Cu substrates at different temperatures

With the development of high density interconnection and fine pitch technology, good wettability effectively guarantee the reliability of solder joins. Hence, it is essential to carry out researches on wetting behavior between molten solders and metal substrates. The effect of Ni, Ag coating and temperature on wetting behaviors of Sn-3Ag-0.5Cu solder on Cu substrate after reflowing at 523 K, 553 K and 583 K were studied in this paper. The results showed that when temperatures are constant, Ag coating are favorable to the wetting of Sn-3Ag-0.5Cu alloy on Cu substrates. Because the diffusion rate of Ag itself to Sn-3Ag-0.5Cu solder is much faster than Cu and deeper grooves in scallop-like IMC provide a more convenient access for Cu atoms to dissolve into molten solders. These two factors lead to faster IMC formation rate and better wettability due to the reaction products are the diffusion frontier. While the slower diffusion of Ni to molten solder and none groove-like IMC formation lead to slower growth rate of IMC and worse wettability of Sn-3Ag-0.5Cu alloy on Cu/Ni substrates than pure Cu substrates. In addition, an increase in temperature promotes the substrate atoms diffusion and reaction rate with Sn-3Ag0.5Cu alloy, decreases the viscosity as well as surface tension of melting solder and contributes to better wettability significantly.