Interfacial reaction of printed solder bump with UBM

This work is to investigate the interfacial interaction between the UBM and solder bump produced with print bumping process. The solder bump manufactured with printing by using PI (polyimide) as the patterning mask was investigated for its interfacial reaction with electroless nickel UBM. The solder paste is a 63Sn-37Pb solder with 90% of metal content. The solder powder is of type 5 with particle sizes range from 15 μm to 28 μm. Reflow of the solder paste was conducted at 230°C for 30 sec. The electroless nickel was deposited on Cu electrode of silicon chip. A thin layer of electroless Au was deposited on the electroless nickel to provide wetting property and oxidation resistance for the electroless nickel. The solder bump was tested, respectively, under 150°C and 85°C/85RH for up to 1000 hours. The interdiffusion behavior of the constituent elements Cu, Ni, and Sn across the interface was investigated with elemental line scanning. It was found that the electroless nickel with a thickness of 5 μm functions well as a diffusion barrier for Cu and Sn. No interdiffusion was seen between Cu and Sn after 1000 hours of test under the two testing conditions