Effects of residual impurities in electroplated Cu on the Kirkendall void formation during soldering

Additions of bis-(sodium sulfopropyl)-disulfide (SPS) to the Cu electroplating bath strongly affected the characteristics of Kirkendall void formation when the Cu film is soldered with Sn–3.5Ag solder and subsequently aged. Voids were predominantly distributed near the Cu3Sn∕Cu interface with SPS, but randomly distributed in the Cu3Sn layer without SPS. In situ Auger electron spectroscopy of voids at the Cu3Sn∕Cu interface revealed surface segregation of S atoms, which came from SPS put into the bath as an additive. The S segregation to Cu3Sn∕Cu interface lowers the interface energy, thereby accelerating the void nucleation. Assisted by the high surface diffusivity of Cu and the presence of excess vacancies arising from the Kirkendall effect, voids tend to localize at the interface, which would result in serious degradation of the joint reliability.

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