Microstructure and Mechanical Properties of Sn58Bi/SAC305 Hybrid Solder Joints Bonded by Intense Pulsed Light Soldering Processes

The Sn-3.0Ag-0.5Cu (SAC305) solder, widely used in the electronics industry, exhibits excellent wettability and mechanical stability. However, its relatively high melting point of 217 °C can cause thermal damage and interfacial delamination during repeated reflow processes. As an alternative, the Sn-58Bi solder, with a low melting point of 139 °C, has attracted significant attention. Sn58Bi solder is cost-effective and shows favorable mechanical properties and wettability. Nevertheless, its high Bi content (58 wt%) leads to brittleness and the formation of coarse Bi-rich phases, which degrade joint reliability. To overcome these limitations, a hybrid solder structure combining Sn58Bi and SAC305 has been proposed, allowing the integration of the low processing temperature of Sn58Bi with the mechanical robustness of SAC305. However, conventional reflow soldering still involves a high thermal load and long processing time, which result in excessive energy consumption and accelerate intermetallic compound (IMC) growth and warpage. To address these issues, this study adopts intense pulsed light (IPL) soldering, which utilizes the photothermal effect of a Xe flash lamp as an alternative process. The IPL process enables localized heating within a few seconds to induce solder melting and bonding, allowing the formation of a thin IMC layer with precise temperature control. In this study, Sn58Bi solder paste was printed on Cu pads, and SAC305 solder balls were mounted on top to form a hybrid joint structure. The joints were then fabricated using both reflow and IPL soldering processes, and their microstructural and mechanical properties were compared according to the process conditions. As a result, the IPL process produced a thinner IMC layer than reflow soldering and, although it exhibited slightly lower shear strength, the increased shear displacement indicated improved ductility of the joints.