High Reliability Mid-Temperature Pb-Free Alloy for Multi-Step Soldering

High silver Sn-Ag-Cu solder alloys, such as Sn-3Ag-0.5Cu (SAC305), result in solder joints with high mechanical and thermal reliability, and can sustain relatively high operational temperatures. However, their melting point (generally above 217 degrees C) requires at least 240-250 degrees C reflow temperatures, putting additional stresses on printed circuit boards. Solder alloys allowing for lower reflow temperatures can be very advantageous, adding benefits such as long-term reliability, lower energy cost and reduced cycle time and enabling multi-step soldering. Here we present our new Pb-free solder alloy that allows for a 25-30 degrees C reduction in peak reflow temperatures. We investigate the effect of various alloying additions into the mechanical properties of three alloys, 1N, 3D and 3E. We also investigate the thermal cycling performance of these alloys when used as solder paste. The results presented here show that solder alloy 3D enables peak reflow temperatures as low as 215 degrees C, in combination with higher mechanical properties and improved fatigue life that is significantly higher than SAC305.