Interfacial reaction between 42Sn-58Bi solder and electroless Ni-P/immersion Au under bump metallurgy during aging

Abstract

The interfacial reaction between 42Sn-58Bi solder (in wt.% unless specified otherwise) and electroless Ni-P/immersion Au was investigated before and after thermal aging, with a focus on the formation and growth of an intermetallic compound layer, consumption of under bump metallurgy (UBM), and bump shear strength. The immersion Au layer with thicknesses of 0 mu m (bare Ni), 0.1 mu m, and 1 mu m was plated on a 5-mu m-thick layer of electroless Ni-P (with 14-15 at.% P). The 42Sn-58Bi solder balls were then fabricated on three different UBM structures by using screen printing and pre-reflow. A Ni3Sn4 layer formed at the joint interface after the pre-reflow for all three UBM structures. On aging at 125 degrees C, a quaternary phase, identified as Sn77Ni15Bi6Au2, was observed above the Ni3Sn4 layer in the UBM structures that contain Au. The thick Sn(77)Ni(15)Bi(6)AU(2) layer degraded the integrity of the solder joint, and the shear strength of the solder bump was about 40% less than the nonaged joints.