Development and validation of a lead-free alloy for solder paste applications

A lead-free solder alloy has been developed and, in form of a solder paste, validated for use in the assembly of surface mount printed circuit boards (PCB). Liquidus and solidus temperatures have been measured by thermal analysis and an optimum composition selected. Wetting balance tests have been done comparing the performance of the alloy with that of 60/40 tin/lead (SnPb), 99/1 tin/copper (SnCu) and 96.5/3.5 tin/silver (SnAg) alloys on bare copper (Cu) coupons. The tests showed faster wetting for the alloy than for the SnAg and SnCu alloys, with results under some conditions comparable to those achieved with 60/40 SnPb. A no-clean rosin base solder paste was made up with the alloy and used to assemble 20 standard mobile phone PCBs. The boards were successfully soldered by nominally raising the reflow temperature, but only by 10°C as compared with eutectic SnPb pastes. Solder joint reliability was tested by temperature cycling Pb-free and 63/37 soldered boards between -25°C and +125°C for 1000 cycles. Results as assessed by metallographic analysis were better for the Pb-free alloy than for the reference SnPb alloy