Improved drop reliability of Sn–Ag–Cu solder joints by Zn addition to a Cu wetting layer

Abstract

Cu–20 wt% Zn solder wetting layers were fabricated by electroplating in cyanide and non-cyanide solutions, and board-level drop impact reliability was evaluated for Sn–Ag–Cu (SAC) solder joints formed on Cu–Zn layers. Adding Zn to the Cu wetting layer suppressed the formation of a large Ag3Sn plate in the solder and delayed interfacial intermetallic compound (IMC) growth in the solder/Cu–Zn interfaces. Formation of Cu3Sn IMC and microvoids was not observed in the Cu6Sn5/Cu–Zn interface during thermal aging. The drop impact reliability of the Cu–Zn/SAC/Cu–Zn solder joints was superior to that of Cu/SAC/Cu solder joints, both before and after aging. The drop impact performance of the Cu–Zn specimens was similar whether their Cu–Zn layers were electroplated in cyanide or non-cyanide solution. Drop impact reliability of both Cu and Cu–Zn specimens decreased after aging due to IMC growth and/or microvoid formation in the solder interfaces. Interfacial microstructure influenced the failure mode of the solder joints. For Cu specimens, the dominant failure sites were a Cu6Sn5/Cu interface before aging and a Cu3Sn/Cu interface after aging. The formation of a large Ag3Sn plate facilitated crack growth. For Cu–Zn specimens, the crack always propagated inside the Cu6Sn5 layer. Microstructural changes in the solder interfaces arising from the addition of Zn contributed to the improvement of drop impact reliability of SAC solder joints.