Materials interaction during soldering and isothermal ageing of Sn-Pb and Sn-Ag-Cu solders on electroless Ni/Au surface finish

Abstract

In flip chip interconnection an under bump metallurgy (UBM) is required to provide a diffusion barrier, an adhesion and a solderable surface. Among the various UBM systems available the electroless nickel / immersion gold (ENIG) has received greater attention in recent years due to its low cost and because it does not require a mask. Most of the time, however, the electroless nickel layer contains phosphorus when hypophosphite is used as the reducing agent. In the present study, the effect of phosphorus content in the electroless nickel/immersion gold under bump metallurgy has been examined and the effect of isothermal aging at 150 OC for up to 500 hours on the formation and resultant morphology of the intermetallics formed between eutectic Pb-Sn solder and Sn-Ag-Cu lead-free solder and electroless Ni-P / immersion Au UBM has also been investigated. Structural examination and measurements of the average thickness of the growing intermetallic compounds on cross-sectioned solder joints made from the Sn-Ag-Cu solder revealed that the thickness of these intermetallic compounds increases as the phosphorus content in the electroless nickel decreases. On the other hand, in Pb-Sn eutectic solder the intermetallic compound thickness increases with increasing phosphorus content in the electroless nickel layer. Thermal ageing of the soldered joints led to the formation of several types of intermetallic compounds with different morphologies.