Effect of PCB finish on the reliability and wettability of ball grid array packages

Organic protective coatings (OPC) and metallic plating chemistries have emerged as alternatives to traditional hot air solder leveling (HASL) to ensure the solderability of printed circuit boards (PCB). This study examined a number of commercially-available printed circuit board finishes to determine their intrinsic solderability and their effect on the solder joint reliability of Ball Grid Array (BGA) packages. The PCB finishes included OPC (two different chemistries), immersion Au over electroless Ni (four chemistries), electroless Pd over Ni, immersion tin over Cu, immersion Bi over Cu and HASL. Solderability tests were performed by reflowing 20 mil diameter solder spheres on various aged test coupons and then quantitatively measuring the area and shape of the solder after cooling. The immersion Au finishes exhibited generally excellent wettability as a function of N₂ reflow aging as compared with the OPCs and HASL. The Pd-Ni, tin, and one of the Ni-Au finishes exhibited poor wetting when exposed to an 85°C/85% RH environment. Solder joint reliability was measured for a 41 I/O ceramic BGA using cyclic thermal shock (nominally -55°C to 125°C) and three point bending. All finishes tested achieved good results in bending, but the Au finishes performed poorly in thermal shock as compared to OPC and HASL. The wetting and reliability results are correlated with the structure and composition of the finishes and the solder joints