Electroless Ni-Cu-P barrier between Si/Ti/Al pad and Sn-Pb flip-chip solder bumps

This work investigates the barrier effect of the electroless Ni-Cu-P deposit between Al conductor and Sn-Pb solder bump, as well as the interfacial reaction with the Sn-Pb solder. For the Ni-Cu-P/63Sn-37Pb system, a (Ni, Cu)/sub 3/Sn/sub 4/ compound with three different morphologies: fine-grain, whisker, and polygon are formed at the Ni-Cu-P/63Sn-37Pb interface after reflow at 220/spl deg/C for 15 s. These (Ni, Cu)/sub 3/Sn/sub 4/ crystals transform into polygon shape with smooth appearance during 150/spl deg/C aging. For the Ni-Cu-P/95Pb-5Sn system, equiaxial (Ni, Cu)/sub 3/Sn/sub 4/ crystals are formed at the Ni-Cu-P/95Pb-5Sn interface after reflow at 350/spl deg/C for 15 s, and they also transform into polygon shape during 150/spl deg/C aging. In addition, the Ni-Cu-P deposit will crystallize to form Ni/sub 5/ P/sub 2/ during 350/spl deg/C reflow. The growth of the (Ni, Cu)/sub 3/ Sn/sub 4/ compound by solid state reaction is a diffusion controlled process for both Ni-Cu-P/63Sn-37Pb and Ni-Cu-P/95Pb-5Sn systems. A 4 /spl mu/m Ni-Cu-P deposit can provide adequate barrier function between an Al conductor and two Sn-Pb solders under 150/spl deg/C aging for 1000 h.