An eco-friendly Cu-Zn wetting layer for highly reliable solder joints

We successfully fabricated an eco-friendly Cu-Zn wetting layer for Sn-Ag-Cu (SAC) solders by electroplating in a cyanide-free solution. The reliabilities of solder joints formed on the Cu-Zn solder wetting layer were evaluated through the drop impact test and thermal cycling (T/C) test. First, board-level drop impact test was performed with the SAC solder joints formed on electroplated Cu or Cu-Zn layers. Drop test samples were fabricated by connecting SAC solders with Cu or Cu-20 wt% Zn wetting layer formed on printed circuit boards (PCBs). Cu-20 wt% Zn layers were electroplated in cyanide and cyanide-free solutions. Drop impact resistance of Cu-Zn/SAC/Cu-Zn solder joints was superior to that of Cu/SAC/Cu solder joints. Next, for T/C test, Cu pillar bumps with Sn-2.5 wt% Ag capping layers were fabricated on Si chip and joined with Cu or Cu-Zn layer on PCB substrate by flip chip bonding. T/C test was carried out between -55°C and 125°C up to 3000 cycles. The flip chip joints formed on either Cu or Cu-Zn layer showed excellent T/C reliability in the underfilled condition. The failure was not observed in both samples after 3000 cycles. When underfill was not applied in the flip chip joints, Cu-Zn samples exhibited better results than Cu samples even though both samples had short thermal fatigue life. The improvement of reliability in Cu-Zn samples was ascribed to the beneficial effect of Zn. Our results implied that electroplating of Cu-Zn layers in a cyanide-free solution was successful and the reliability of solder joints can be enhanced by applying a Cu-Zn solder wetting layer.