Voiding mechanism and control in BGA joints with mixed solder alloy system

The voiding behavior of mixed solder alloy systems during BGA assembly was investigated with the use of nitrogen reflow atmosphere. The solder joints were comprised of a combination of Sn62Pb36Ag2 (Sn62), Sn63Pb37 (Sn63), Sn98.5Ag1Cu0.5 (SAC105), and Sn96.5Ag3Cu0.5 (SAC305) as the solder ball or solder paste, except for SnPb solder balls with lead-free pastes. Assembly of each BGA employed a high temperature (HT) profile for lead-free and mixed systems, and a low temperature (LT) profile for mixed and SnPb systems. Mixed systems had higher voiding than lead-free systems when reflowed at HT. SnPb systems had a wider voiding range when compared with mixed systems reflowed at LT. Voiding was found to decrease with reduced alloy surface tension, earlier melting of the bottom solder paste, the top ball remaining solid during reflow, a narrower pasty range, smaller joint overheating, and using flux alone instead of solder paste.