Solderability Challenges in Emerging BGA Packages

Abstract

The final step of reflowing solder spheres to solderable pads on the bottom of substrates in FCBGA, CSP, and WLCSP packages is often considered to be a trivial step. However, with the increasing complexity and number of assembly processes taken prior to this final soldering step, the formation of a reliable final joint is far from certain. In order to eliminate this problem, many OSATs and ODMs are adopting the non-value-added process of prefluxing, reflowing, cleaning, and drying substrates immediately prior to the final ball-attach process. This paper details the sequence of processes seen in typical BGA assembly, and examines the effects of each set of prior processes on the solderability of the final pad. It also details investigations of the solderable surface and metallurgy of the substrate pad. The introduction of a “one-step” pin-transfer ball-attach flux is shown to be a means of reducing both process cost and time, and also reducing the risk of significant warpage in the finished package.