Dissolution of electroless Ni metallization by lead-free solder alloys

An investigation has been carried out to compare the dissolution of the electroless Ni metallization of the ball grid array (BGA) substrate into the molten Sn-0.7 wt.%Cu, Sn-3.5 wt.%Ag-0.5 wt.%Cu and Sn-3.5 wt.%Ag solder. A fixed volume of the BGA solder ball (760 μm diameter) was used on a 4 μm thick electroless Ni(P) metallization over a Cu pad having a circular area with a diameter of 600 μm. The dissolution measurement was carried out by measuring the change of Ni(P) metallization thickness as a function of time and temperature. Scanning electron microscopy was used to examine the microstructure of the solder joint and to measure the consumed thickness of Ni(P). The dissolution in Sn-3.5%Ag solder is higher than in the other two lead-free solders. The presence of Cu in the solder plays a major role in inhibiting the consumption of electroless Ni in the soldering reaction. Despite the fact that the initial formation of intermetallic compounds (IMCs) is the highest in Sn-0.7%Cu solder, the formation of the IMCs and the crystallized Ni3P is much reduced in the later stage