IMC growth and shear strength of Sn-Ag-Cu/Co-P ball grid array solder joints under thermal cycling

Co-P films with P contents of 4.0 and 8.0 at.% were electroplated on printed circuit boards and used as surface finish of ball grid array (BGA) packages. Sn-3.0 wt% Ag-0.5 wt% Cu (SAC305) solder balls were reflowed on the Co-P finish to form SAC/Co-P solder joints. The morphology and growth of intermetallic compounds (IMCs) in SAC/Co-P joints under thermal cycling were systematically analyzed, and the effects of IMCs on shear strength and fractured surface of the joints were studied. SAC/Cu BGA solder joints were also prepared as a comparison. It was found that CoSn3 and (Cu, Co)(6)Sn-5 grew at the interface of SAC/Co-4 at.% P solder joints during thermal cycling. The shear strength of SAC/Co-4 at.% P joints exceeded that of SAC/Cu joints after 100 thermal cycles, because the granule size of (Cu, Co)(6)Sn-5 was smaller than that of Cu6Sn5 and no Kirkendall void existed at the interface of SAC/Co-4 at.% P joints. For SAC/Co-8 at.% P joints, a layer of smooth and void-free Co-Sn-P formed at the interface, and the shear strength of SAC/Co-8 at.% P joints increased by 8.4 % in comparison with that of SAC/Cu joints after 300 cycles. In addition, the fractured surface of three kinds of joints after thermal cycling was examined. The experimental data demonstrated that Co-P surface finish enhanced the shear strength of solder joints after thermal cycling and improved the reliability of BGA packages.