Dependence of shear strength of Sn-3.8Ag-0.7Cu/Co-P solder joints on the P content of Co-P metallization

The mechanical properties of solder joints highly depend on the interfacial reaction between the solders and the metallization on substrates. In this work, we electroplated Co-P films with various compositions on the Cu pads of printed circuit boards and fabricated Sn-3.8 wt% Ag-0.7 wt% Cu/Co-P ball grid array (BGA) solder joints. The BGA solder joints were annealed at 150 degrees C for 100, 200, 500, and 1000h and the shear strength of these joints was measured. When the P content of the Co-P metallization was increased from 2.3 to 18.8at.%, the shear strength after 1000h annealing initially rose to 107.9MPa at a P content of 8.5at.%, then decreased to 84.3MPa at a P content of 12.5at.%, and again increased to 96.0MPa at a P content of 18.8at.%. The enhancement of the shear strength of the joints with Co-8.5at.% P, Co-12.5at.% P, and Co-18.8at.% P films was 109.5%, 63.7%, and 86.4% in comparison to the joints without Co-P metallization, respectively. The interfacial reaction between the Sn-Ag-Cu (SAC) solder and Co-P films during annealing and the fractured surfaces of the solder joints after the shear test were studied. For the joints with Co-8.5at.% P and Co-18.8at.% P films, a thick layer of CoSn3 was formed at the interfaces during annealing, which enhanced the shear strength. For the joints with Co-12.5at.% P metallization, a thin layer of Co-Sn-P was formed at the interfaces and was peeled off layer by layer with prolongation of the annealing time. The spalled Co-Sn-P was mixed with the solder matrix, increasing the shear strength of the solder joints. The shear strength of the SAC/Co-12.5at.% P joints was less than that of the joints with Co-8.5at.% P and Co-18.8at.% P films because no CoSn3 formed. Therefore, the composition of Co-P metallization played an important role in the interfacial reaction of the SAC/Co-P solder joints, which in turn affected the shear strength of the solder joints. Our experimental results show that the electroplated Co-P film is a promising candidate as the metallization for BGA solder joints.