Analysis of SAC305/SnBi Hybrid Solder Interface and Comparison of the Relationship Between Hardness and Shear of Solder

Recent research has focused on hybrid solders as a promising alternative to SAC305 solders, although studies examining long-term thermal reliability have been limited. This study investigated the Sn-3.0Ag-0.5Cu/Sn-57.6Bi-0.4Ag hybrid solder, combined with a PCB featuring an Electroless Nickel Immersion Gold (ENIG) surface finish, subjected to thermal aging to assess the relationship between microstructural changes and mechanical properties such as hardness and modulus in the solder joint. The findings revealed that Cu<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf>Sn<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf>, (Ni, Cu)<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>Sn<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf>, and AuSn4 intermetallic compounds (IMCs) formed due to the interaction between Sn solder and Cu, Ni, and Au, which diffused from the electrode. Prolonged thermal exposure led to Cu diffusion into the solder joint, forming Cn3Sn, while an increased density of Ni-(Sn)-P modified the microstructure. This extended exposure also caused the Bismuth Mixing Ratio (BMR) region. As a result, the hardness and modulus of the solder decreased, while the hardness and modulus of the IMCs increased. Shear test results indicated that as solder's ductility increased, shear strength improved until reaching a critical point at approximately 1000 hours of aging, where rapid IMC growth caused a significant decline in shear strength. These results suggest that controlling BMR in hybrid solder joints could play a key role in enhancing long-term thermal reliability in electronic packaging applications.