Lead-free wave soldering development for PCB assembly

The scope of work entailed determining the performance of Sn0.7Cu and Sn3.8Ag0.7Cu lead-free alloys for wave soldering applications in combination with lead-free components, board finishes, different flux types and operating atmospheres using a production scale wave soldering machine. The vertical solder filling of through holes using passive components such as connectors was excellent for PCBs with NiAu, Ag and OSP finishes. Lead-free soldering issues were observed for IC components with large thermal mass and the OSP PCB finish appeared vulnerable for such components. These components required higher soldering temperature, longer solder contact time and higher pre-heat temperature to match the performance of tin-lead soldering. A large process window was seen for SMT chips soldered to the bottom side of PCBs. Nitrogen atmosphere soldering was found to be necessary for ROL0 (no-clean, low activity) fluxes while ROL1 (no-clean, medium activity) and water-soluble fluxes were found to be acceptable in air soldering. The solder joint reliabilities of SnCu and SnAgCu wave solder assemblies were investigated. In-situ resistance monitoring of surface mount chip components found no failures up to 3000 cycles of a temperature cycling test (-40 degreesC to 125 degreesC). However, solder joint cracks appeared in the temperature cycling test both in lead-free and tin-lead assemblies for through hole and surface mount components. Solder cracking in through-hole solder joints with SnCu alloy was more severe than with SnAgCu alloy after 3000 cycles of temperature cycle testing. The solder joint cracks formed in joints with SnAgCu alloy were similar in extent to those found in control samples with SnPb alloy. Less severe through-hole solder joint cracks were formed when the SnCu alloy was applied with PCBs made of high Tg FR4 material. The cracks observed after 3000 cycles were similar to those of control samples with SnPb alloy. High Tg PCB material is recommended for lead-free wave soldering using SnCu alloy. Conventional FR4 PCB material appeared sufficient for lead-free wave soldering using SnAgCu alloy.