A new Cu-Zn solder wetting layer for improved impact reliability

The excessive growth of intermetallic compound (IMC) and the formation of microvoids in the interface between Sn-Ag-Cu (SAC) solder and Cu substrate have been a serious problem which may degrade the solder joints reliability. Recently, a new Cu-Zn alloy solder wetting layer for the Pb-free solders has been developed to reduce the IMC growth rate. In this paper, the kinetics of IMC growth and the shear strength of SAC solder joints formed on electroplated Cu-Zn wetting layer were investigated. And, drop impact reliability of SAC/Cu-Zn joints was evaluated. The solder joints were fabricated by reflowing Sn-4.0Ag-0.5Cu (SAC 405) solder balls on electroplated Cu-20 wt% Zn wetting layers. An experiment of the SAC solder joints on electroplated Cu layers was also carried out for comparison. After soldering, Cu₆Sn₅ IMC formed on Cu or Cu-Zn wetting layer. The typical double layer of Cu₆Sn₅ and Cu₃Sn with microvoids formed during aging at the solder/Cu interfaces. On the contrary, only Cu₆Sn₅ formed at the SAC/Cu-Zn interfaces with aging. Neither Cu₃Sn nor microvoids were found in these specimens. The growth rate of IMCs at the SAC/Cu-Zn interfaces was much slower than that at the SAC/Cu interfaces. Cu-Zn wetting layer was very effective to reduce the IMC growth. IMC growth was controlled by volume diffusion. The shear test was carried out with SAC 405 solder bumps formed on Cu or Cu wetting layers. The shear strength of SAC solder bumps on Cu or Cu-Zn layers decreased with aging. The shear strength of SAC solder on Cu-Zn pads was slightly higher than that of SAC solder on Cu layer at the same aging time. Since all specimens showed ductile fracture in the solders, the shear strength of the solder bumps depended on the strength of solder ball. Drop test were conducted with the assembly which was fabricated using SAC 405 solder balls and two printed circuit boards. The mean number of drops- to failure of Cu/SAC/Cu and Cu-Zn/SAC/Cu-Zn specimens was about 150 and about 350 drops, respectively. Though the mean number of drops to failure decreased with increasing aging time, the mean number of drops to failure of Cu-Zn/SAC/Cu-Zn specimens was higher than that of Cu/SAC/Cu specimens.