A Drop-in High-Temperature Pb-Free Solder Paste that Outperforms High-Pb Pastes in Power Discrete Applications

Sn-based high-temperature lead-free (HTLF) solder pastes have been developed as a drop-in solution to replace the high-Pb solder pastes in power discrete applications. The pastes were designed to combine the merits of two constituent powders. A SnSbCuAg powder, with the melting temperature above 320°C, was designed to maintain a high-temperature performance. A SnAgCuSb powder, with a melting temperature around 228°C, was added to the paste to enhance wetting and improve joint ductility. In the design, the final joint will have the low-melting phase (the melting temperature >228oC) in a controllable quantity embedded into the high-melting SnSbCuAg matrix. HTLF-1, one of the designs, maintained the bond shear strength up to 15MPa, even around 290°C. Another design, HTLF-2, has a similar bond shear strength as Pb92.5/Sn5/Ag2.5 around 290°C, but exceeds substantially below 250°C. The power discrete components had been built with both HTLF solder pastes for both die-attach and clip-bond through the traditional high-Pb process, which demonstrated the drop-in processing compatibility. The components survived three additional SMT reflows (peak temperature of 260°C) and passed moisture sensitivity level 1. This confirmed that the maintained joint strength (comparable to or stronger than high-lead), helped to keep the joint integrity within the encapsulated components, even with melting phases in a controllable quantity existing above 228°C. Both HTLF solder pastes outperformed Pb92.5/Sn5/Ag2.5 in RDS(on) even after 1000cycles of TCT (-55/175oC), which is attributed to the intrinsic lower electrical resistivity of Sn of both HTLF pastes. Microstructural observation had shown no corner cracks for both die-attach and clip-bond joints after TCT.