Fluxless wetting properties of UBM-coated Si-wafer to Sn-3.5wt%Ag solder

The fluxless wetting properties of Under Bump Metallurgy (UBM)-coated Si-wafer to Sn-37wt%Pb and Sn-3.5wt%Ag solder were estimated by wetting balance method. With wettability indices from the wetting curves of one-side-coated specimens, the degree and rate of wetting of different UBM structure were evaluated. Wettability of Au/Cu/Cr and Au/Ni/Ti UBM shows similar results in terms of the degree of wetting while the wetting rate of Au/Cu/Cr is better than that of Au/Ni/Ti. Related with the thickness of Cu layer in Au/Cu/Cr UBM, wettability of 0.1 μm-thick Cu film is superior to that of 0.5 μm-thick one from the viewpoint of the degree of wetting but the effect of the Cu layer thickness on wetting kinetics is negligible. Using flux has advantage over Au coating not so much in the rate of wetting as in the degree of wetting. The degree and the rate of wetting of nickel base UBM is better in Sn-Ag solder than in Sn-Pb solder.