Effect of phosphorus content and solid state aging on intermetallic formation between lead-free Sn-Ag-Cu solder and electroless nickel/immersion gold under bump metallurgy

We have examined the effect of phosphorus content in the electroless nickel/immersion gold (ENIG) under bump metallurgy and the effect of solid-state aging at 150/spl deg/C on the formation and resultant morphology of the intermetallics formed between Sn-Ag-Cu (lead-free) solder and electroless Ni-P/immersion Au (ENIG) surface finish. Structural examination and measurements of the average thickness of the growing intermetallics on cross-sectioned solder joints made from the Sn-Ag-Cu solder revealed that the thickness of these intermetallic compounds increases with reducing the phosphorus content in the electroless nickel. This is in contrast to the increase in thickness of intermetallics with increasing phosphorus content when a Pb-Sn eutectic solder is used. The results also showed that the intermetallic compounds grew faster with the eutectic Pb-Sn solder than for the Sn-Ag-Cu solder during reflow and solid-state aging. The resulting intermetallics in the Sn-Ag-Cu solder were composed mainly of Ni/sub 3/Sn/sub 4/ at the solder/Ni interface after reflow. The morphology of this intermetallic appears to be affected by the solid-state aging process. It was also found that aging of samples made of Sn-Ag-Cu solder joints led to the growth of intermetallics such as (Ni, Cu)/sub 3/Sn/sub 4/ and (Cu, Ni)/sub 6/Sn/sub 5/ at the solder/Ni-Au, indicating that copper may have dissolved into the Ni/sub 3/Sn/sub 4/ during the aging process.