Suppression of Kirkendall voiding in Sn-3.5Ag/Cu solder joints by preannealing process

摘要

In this study, electroplated Cu films were pre-annealed at T (T = 673, 773, 873 K) for varying times.Sn-3.5Ag solder reflowed over the Cu films and subsequently aged at 150°C for 240 hrs. Effects of preannealingon the microstructure of Cu films, as well as contents of organic impurities incorporated in theCu films were investigated. After solder reflow, the formation of Kirkendall voids at Cu_3Sn/Cu interfacewas observed from SEM micrographs. Results show that the pre-annealing process significantlysuppressed Kirkendall void formation in the Sn-3.5Ag/Cu solder joints. A line fraction of voids at theCu_3Sn/Cu interface was definitely suppressed in the case of 500°C and 600°C pre-annealed samplescompared to as-deposited Cu sample. SIMS analyses revealed that pre-annealing reduced the level ofimpurities in the Cu films, especially S and C. The mechanism of suppressing Kirkendall voids at theCu_3Sn/Cu interface was presented by schematic diagram, and it could be seen that pre-annealing methodhas a potential to enhance solder joint reliability.