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-3.5 wt%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 220¼C, which is below the normal reflow temperatures of around 240 250¼C 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 220¼C 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 +125¼C, 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.