Intermetallic growth on PWBs soldered with Sn3.8Ag0.7Cu

This paper describes the nature of the intermetallic phases observed at the interface and in the bulk of Sn3.8Ag0.7Cu solder immediately after reflow on PWBs coated with a variety of commercial plating systems. It also discusses the growth of these intermetallic phases after aging at homologous temperatures of 0.8Tm, 0.85Tm and 0.9Tm for 10, 100 and 1000 hours. The following board plating systems were investigated: organic solderability preservative (OSP) over bare copper, immersion tin, immersion silver, and immersion gold over electroless nickel. This study revealed that the composition, microstructure, and thickness of intermetallics at the interface were strongly dependent on the plating system. The effect of the bulk and interface microstructure on the shear strength of the joints was also investigated. For all plating systems, the shear strength of the solder joints did not degrade with aging, and the failure mechanism continued to be cohesive failure through the bulk of the solder.