Lead free interfacial structures and their relationship to Au plating including accelerated thermal cycle testing of non-leaden BGA spheres

As lead is a harmful element, soldering in the electronics industry has moved toward elimination of lead from the solder alloy. Already there are a group of companies who have begun implementation of lead-free solder alloys. The scope of parts being tested for lead-free processes is not limited to the solder material; the complete absence of lead from electronics parts, including the substrate, parts, and module is being envisioned in the near future. Therefore, for BGA and/or CSP bumps as well, there is no other option but to evaluate lead-free materials. First in this paper, the microstructure and the shear strength of the solder bumps were examined. The following lead-free alloys and the alloy containing lead were selected for this first examination: Sn-5Sb, Sn-0.75Cu, Sn-3.5Ag, Sn-3.5Ag-0.75Cu and Pb-63Sn. The bumps were soldered on a copper/nickel substrate with gold plating and were evaluated before and after a thermal cycle test. Second in this paper, two types of lead-free alloys and the alloy containing lead were selected as CSP bump materials. The mechanical reliability of the soldering joint between the CSP package and PCB was evaluated before and after thermal cycle tests. One of these lead-free alloys is: Sn-Ag-Cu, the most influential candidate for eliminating lead from the current Sn-Pb solder. The other is: Sn-Zn-Bi which has a lower melting temperature than Sn-Ag-Cu. Also, three (3) types of plating on the substrate of PCB (Bare Copper, Alpha Level Silver and Flash Gold) were evaluated with CSP bump materials. The purpose of the evaluation was to select the best material for using lead-free solder alloy as a CSP joint.