Low silver, lead free solder paste, new alloy developments

The compliancy with RoHS directives as well as the trend towards green technologies and green processes has been the main driver for the paste manufacturers. One direction that has proven to be worth while exploring is the one towards low melting point alloys that will enable energy savings during the manufacturing processes. Last but not least these emerging alloys will add an extra benefit to the economy of the production process, the elimination of wave soldering process. The solder paste used: OM520, 42%Sn/57.6% Bi/0.4%Ag, type 3 powder by IPC J-STD-005, the study was completed on a FR2 PCB with OSP finish. A SAC305 paste - OM338PT, SAC305, type 3 powder by IPC J-STD-005 - was used as benchmark for the measurements. The interconnection structure elements at level two in case of surface mount technologies are electronic components terminals - pin -, the dedicated metallic surface on the printed circuit board - pad -, and the solder joints. The solder joints are the result of the soldering process consisting of the temperature gradient action over the solder paste volume at the interface between pins and pads. The solder joint overall performance is described by the intermetalic formation at the interface of pad and solders volume as well as the pin and solder volume. The implementation of a lead free process needs to take in account the sum of all parameters that affect the soldering process and the interdependence between them. In the paper will be presented the results of termocycling of the boards assembled with the two alloys for the BGA components using reflow process under air. Scientific and practical conclusions shall be drawn in order to prove the improvements brought by the bismuth alloy in reliability and energetic efficiency of the reflow soldering process.