Mechanical properties of Pb-free SnAg solder joints

Keller J., Baither D., Wilke U., Schmitz G.

Abstract

The mechanical stability of Pb-free SnAgCu solder connections is studied in comparison to conventional eutectic SnPb. Shear tests are performed with solders of different near-eutectic compositions. In addition, the hardness of bulk solder alloys is measured. The strength and ductility of the Pb-free joints depend significantly on Ag content. In general, however, Pb-free solder reveals superior mechanical properties in terms of maximum strength and ductility when compared to SnPb. Microstructure characterization after reflow is performed by electron microscopy. It is demonstrated that failure always appears across the solder alloy, while the intermetallic region providing adhesion remains intact. Thus, the strength of the joints is determined by the bulk properties of the solder. Plastic deformation of the latter appears via a dislocation mechanism. It is demonstrated that particle hardening in interdendritic zones of the solidified structure is the most important factor in understanding the strength.