Fluxless wetting properties of the UBM-coated Si-wafer to Pb-free solders under different atmosphere

The wetting balance method was used to estimate the fluxless wetting properties of UBM (Under Bump Metallurgy)-coated Si-wafer to binary lead-free solders (SnAg, SnSb, SnBi, SnIn, SnZn). The wetting property estimation of UBM-coated Si-wafer was possible with new wettability indices from the wetting curves of one side-coated specimens: F_min, F_s, and t_s. Au/Cu/Cr UBM is better than Au/Ni/Ti UBM from the point of wetting time. At general reflow process temperature, the wettability of high melting point solders (SnSb, SnAg) is better than that of low melting point ones (SnBi, ShIn). The nitrogen atmosphere was more effective for improvement in fluxless wettability than in fluxed wettability. The contact angle of one side-coated Si-plate to solder can be calculated from the force balance equation by measuring the static state force and the tilt angle.