Effect of Bismuth Addition to SAC305 on the Microstructure and Mechanical Properties of Solder Joints

Abstract Two types of solder alloys—96.5Sn-3.0Ag-0.5Cu (SAC305) and SAC305 with the addition of 3.3% bismuth (Cyclomax)—are thermally aged, and their comparison is carried out in terms of solder joint microstructure and mechanical performance variation. Samples of both solder compounds were bonded to copper pads, thermally aged at 150 °C and 100 °C for up to 1000 h, and compared to untreated samples. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) microstructural investigation showed considerable differences between the two alloys. The Cu6Sn5 intermetallic compound (IMC) layer in Cyclomax is more stable and homogeneous, enhancing joint integrity and mechanical performance compared to the bulkier and uneven layers in SAC305. Due to these structural changes, the ultimate shear strength (USS), modulus of elasticity, and thermal reliability of Cyclomax increase under aging. Although Cyclomax has the highest initial strength, its homogeneous IMC layer increases brittleness over time, as reflected in a significant loss of ductility and energy absorption capacity after 1000 h. Testing of shear strength demonstrated that Cyclomax outperformed SAC305, especially in the early phases of thermal aging, reaching peak stress of 76 MPa compared to 48 MPa for SAC305. The strain at fracture research indicated that SAC305 retains its ductility better with age and therefore is probably superior for flexible applications. Bi improves the solder junction's mechanical and thermal performance of SAC305, hence making Cyclomax a good option for high-reliability applications. This again underlines the relevance of alloy composition in solder joint design in cases of long-term temperature exposure, balancing strength, and ductility.