The basic process of molten droplet solder bumping and interfacial reaction between molten SnPb droplet and Au/Ni/Cu under bump metallurgy (UBM) pad was investigated in the present paper. The morphology of intermetallic compounds was strongly influenced by the initial temperature of the solder droplet. When the initial temperature 350uC was reached, a continuous AuSn 2 and needle like AuSn 4 were formed at the solder bump/pad interface. When the initial temperature 450uC was reached, all the AuSn 2 converted into AuSn 4 . Rod like AuSn 4 grew strongly, some acicular AuSn 4 was distributed inside the solder near the interface. Under the condition that the substrate was preheated, interfacial reaction is more intensive at the molten stage. Results of the calculation have shown that compared with the initial height of droplet, the contact temperature is mainly dependent on the initial temperature. To elucidate intermetallic formation mechanism at the interface, the pad/solder temperature field was analysed using finite element method (FEM).