Effect of Co on microstructure evolution and thermal fatigue stability of lead-free solder alloys of SACBSN series.

The majority of studies on high-reliability solder alloys have focused on the six-element system of Sn, Ag, Cu, Bi, Sb, and Ni. However, there is limited research on the influence of Co element within this system. In this study, a Sn 3.0Ag 0.5Cu Bi Sb Ni xCo (x = 0 wt%, 0.02 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%) alloy (referred to as SACBSN-xCo alloy) was prepared using a melting method. The mechanical properties of SACBSN-xCo alloy solder joints were evaluated through ultimate shear strength testing. The composition analysis of the alloy, phase composition examination, intermetallic compound (IMC) investigation and interfacial layer morphology analysis were conducted using ICP, XRD, SEM and EDS techniques respectively. Furthermore, the evolution process of solder structure and solder joint interface layer under different aging times was observed in detail. Results indicate that with the addition of Co element in the alloy solder system two heat release peaks appear during the solidification process; specifically when adding 0.05 wt% Co element content to the mixture it reduces supercooling degree by 15.17 °C to only 1.03 °C; Moreover wettability improvement can be achieved to some extent when adding either 0.02 wt% or 0.05 wt% Co content. The addition of trace Co can inhibit the excessive growth of IMC in the solder alloy matrix and refine the alloy structure. It can promote the growth of Cu 6 Sn 5 -based IMCs and inhibit the growth of Cu 3 Sn layer in intermetallic compound layer (IMCs). In addition, the mechanical properties and thermal fatigue stability of the solder joints are steadily improved by Co element. After adding Co element, the shear strength of the alloy solder joint is increased by about 14.84 %. After aging at 150 °C for 25 days, the shear strength of SACBSN-xCo alloy solder joints is increased by about 20.4 %, which significantly improves the thermal fatigue stability of the solder joints after high temperature aging treatment. The results show that when Co content is 0.05 wt%, the alloy solder has better comprehensive properties. • The addition of Co element can increase the nucleation point in the solder matrix, refine the structure, reduce the formation of large size intermetallic compounds and inhibit their overgrowth during high temperature treatment. • The addition of Co element can increase the nucleation density of IMC, and promote the growth of Cu6Sn5-based IMCs. Thicker Cu6Sn5-based IMCs act as a good diffusion barrier, preventing the mutual diffusion of Sn and Cu atoms. Thus impeding the growth of the Cu3Sn layer. • The addition of Co can effectively improve the shear resistance of the solder joint, and make the shear resistance of the solder joint show excellent stability in the process of high temperature aging treatment. [ABSTRACT FROM AUTHOR]