Backward Compatibility Study of Lead Free Area Array Packages with Tin-Lead Soldering Process

摘要

In response to RoHS and other international environmental legislation, the semiconductor industry is moving toward the elimination of lead (Pb) from packages. During the transitional period, both leaded and lead-free components coexist with tin-lead and lead-free soldering processes. The compatibility of lead-free area array packages with tin-lead soldering processes is a critical issue if the transition becomes prolonged as several product categories may take advantage of the exemptions and continue to be built with tin-lead solder for some time to come. The issue of "backward compatibility" arises because for BGA packages, the contribution of solder balls to the solder joint material is very high (typically of 70% to 80%), and the assembly of lead-free BGA packages with tin-lead paste becomes a major concern from the perspective of solder joint metallurgical uniformity and reliability. In this study, the solder joint metallurgy of mixed alloys was characterized, and the amount of mixing of Pb through the solder joint was analyzed, for different package types and under various process conditions. The results showed that the solder paste amount (ultimately tin (Sn) percentage in the alloy) and the reflow temperature play critical roles in the mixed alloy assembly, both in terms of compositional homogeneity and voiding. Homogeneous solder joints were seen at various reflow temperatures ranging from 210℃ to 230℃, depending on the Sn percentage in the mixed solder alloy. This study will shed some light on the process optimization for backward compatible assemblies in order to improve yield and to create more reliable solder joints.