Lead-free reflow soldering for electronics assembly

Results for reflow soldering are presented from a three-year EC funded project "IDEALS" to develop lead-free soldering solutions. On the basis of fundamental data from the literature, a shortlist of candidate lead-free solders was selected, and results from tests on physical and soldering characteristics, and wetting balance testing, led to the choice of SnAg3.8Cu0.7, melting at 217 degreesC. Implications for solder paste medium development are discussed. Differences in alloy density, melting point, and surface tension relative to conventional solders were found to give higher levels of internal voids, reduced spread on copper, and rougher, duller joints. Reflow process window studies showed that sound reliable joints could be obtained with a peak temperature as low as 225 degreesC. Reliability was tested on soldered test boards using thermal shock cycling, power cycling, and vibration. Overall the SnAg3.8Cu0.7 gave results approximately equivalent to conventional solders, and different board finishes had no significant effect. The effects of Sb and Bi were also evaluated. No justification was found for minor additions of Sb, but 2-5 per cent Bi was found to allow a reduction of the peak reflow temperature, though at the cost of reduced reliability if any Pb was present.