Qualification and optimization of Sn based soft solders: A refinement for Bi based lead free soft solders for EFM volume production

Continuous drive for environmental friendly manufacturing (EFM) has driven lead free soft solder to come on board at a larger scale world-wide since it was first debated publicly in 1999. Several renowned lead free soft solder suppliers have proposed Bi based soft solder as the most suitable candidate to replace the current widely used tin-lead (SnPb) solder. Experience with Bi out of material characterization lab has proven otherwise when used for die attach in power application devices. Composition of BiAgNi or equally synthesized alloys having Bi more than 45% have garnished poor wettability performance and presented brittle characteristic in term of solder wire handling. Oxidation is also seen prevalent in high temperature die mounting, and thus resulting in die bond failed to secure a good adhesion. As such, rule against stringent requirement of minimum 260°C secondary pre-conditioning temperature should be reconsidered. Other low liquidus point lead free soft solders like Sn based SnSb & SnSbCuNi (241°C to 247°C) are put on test for the qualification process. This paper delineates the qualification processes & its response tests for the aforementioned soft solders between supplier and end-user, a 3 factors 2 levels DOE in optimizing the finalized lead free soft solder for die attach process and its reliability performance on thermal resistance shift. Results shown Sn based soft solders are sustainable and performed better than Bi based soft solders for volume EFM production.