Impact of substrate finish on Sn/Ag/Cu alloy solder joint

In recent years, implementation of lead-free solder in the electronic industry is gaining momentum. Lower processing temperature and reliability of a Sn/Ag/Cu alloy has made it a viable alternative and the industry is aligning towards this alloy. This work investigates impact of substrate finish on Sn/Ag/Cu alloy for ball grid array packages. The scope of This work includes study of lead-free alloy Sn/Ag/Cu, Sn/Ag & Sn/Cu, its interaction with substrate plating finish, intermetallic at substrate-ball interface, shear force and failure mode. The type of substrate used in this study is buildup substrate with electroless nickel gold finish and laminate substrate with electrolytic nickel gold finish. Experimental results show that copper containing alloys exhibited failure mode at IMC/nickel interface, when shear test was performed on the units reflowed three times at 260deg. The conclusion from the above study is, Sn/Ag/Cu solder alloy used in ball grid array can result in fracture at IMC /electrolytic nickel interface. Impact force from testers or drop test can result in such brittle fracture between IMC and nickel. Increasing the copper percentage in Sn/Ag/Cu alloy can increase the chance of this brittle fracture between the IMC and electrolytic nickel interface.