WHITE PAPER - Voiding Behavior in Mixed Solder Alloy Systems
By Yan Liu, Derrick Herron, Joanna Keck, and Ning-Cheng Lee.
Jun 21, 2013
The voiding behavior of mixed solder alloy systems using a nitrogen reflow atmosphere during BGA assembly was investigated.
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The solder joints were comprised of a combination of Sn62/Pb36/Ag2 (Sn62), Sn63/Pb37 (Sn63), Sn98.5/Ag1/Cu0.5 (SAC105), and Sn96.5/Ag3/Cu0.5 (SAC305) as the solder ball or solder paste, except for the SnPb solder ball, which used lead-free solder 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; a narrower pasty range; smaller joint overheating; when the top ball remained solid during reflow; and by using flux alone instead of solder paste.
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