Drop impact reliability of Cu-Zn/Sn-Ag-Cu/Cu-Zn solder joints

The intermetallic growth in the Sn-Ag-Cu (SAC) solder joints fabricated on Cu or Cu-Zn solder wetting layer and its effect on the drop impact reliability have been studied. The drop test specimens were fabricated by connecting two printed circuit boards by reflowing SASC 405 solder balls on Cu or Cu-Zn wetting layers. And the drop test specimens were aged at 150degC up to 500 hrs. The well known bi-layer of Cu₆Sn₅ and Cu₃Sn with microvoids was formed at the SAC/Cu interfaces during aging. On the other hand, only Cu₆Sn₅ formed at the SAC/Cu-Zn interfaces with aging. No Cu₃Sn and microvoids were observed and the formation of large Ag₃Sn plates was suppressed remarkably. Also, the growth rate of IMCs at the SAC/Cu-Zn interface was much slower than that at the SAC/Cu interface. The number of drops to failure in the Cu-Zn/SAC/Cu-Zn solder joints was higher than that in the Cu/SAC/Cu solder joints before and after aging. The microstructure developed in the solder interfaces greatly influenced the drop impact reliability.