Reduced temperature soldering of capacitors using Sn-Bi plated Sn-3.5%Ag

Purpose - This paper seeks to decrease the soldering temperature of capacitors using Sn-Bi plated Sn-3.5 wt%Ag solder. Design/methodology/approach - Sn-Bi layers were electroplated on Sn-3.5wt%Ag solder. As soldering examples, type 1608 capacitors electroplated with Sn, and printed circuit boards (PCBs) with a surface coating of electroless-plated Ni/Au, were selected. Sn-3.5Ag foil coupons plated with Sn-95.7 wt%Bi were inserted as solder between the capacitors and the lands on the PCBs. The samples were reflowed at 220 degrees C, which is below the normal reflow temperatures of around 240 similar to 250 degrees C used with Pb-free solders. During heating, Bi in the plated layer diffuses into the Sn-3.5Ag core solder resulting in a transient decrease in soldering temperature based on the concept of transient liquid phase bonding. Findings - The joints made with the Sn-95.7%Bi plated Sn-3.5Ag solder at 220 degrees C showed good appearance, and evidence of significant Bi segregation was absent in the microstructure, The shear strengths of the capacitor joints bonded with Sn-95.7%Bi plated Sn-3.5%Ag solder were approximately 5,000-6,000 gf. After 1,000 thermal cycles between -40 and +125 degrees C, the shear strengths of the joints decreased approximately 5-10 percent from the strengths in the as-reflowed state for all plated solders. This confirmed that the soldered joints were stable and not significantly degraded by thermal cycles. Originality/value - Reduced temperature soldering using Sn-Bi plated Sn-3.5%Ag solder was applied to attach capacitors to PCBs. In a production application, the foil coupons could be replaced by pre-solder on the PCB pads.