THE EFFECT OF CU6SN5 WHISKER PRECIPITATES IN BULK 60SN-40PB SOLDER

Hollow, Cu//6Sn//5 intermetallic rods form within molten 60Sn-40Pb solder when it reacts with Cu. It is suggested that these rods form at the Cu surface by a screw dislocation mechanism and break off into the bulk solder. Hollow hexagonal intermetallics result when the core of the rod dissolves away and fills with molten solder. The mechanical properties of bulk 60Sn-40Pb solder with and without the Cu//6Sn//5 intermetallic rods were tested in tension, at minus 196 degree C, 20 degree C, and 125 degree C. The intermetallics had no effect on strength, but decreased elongation at the lower temperatures. The intermetallics had a large effect on the fracture characteristics. At minus 196 degree C failures initate by interfacial separation between the intermetallic and solder matrix. At 20 degree C failures initiate at cleaved intermetallic rods. At 125 degree C the intermetallic rods appear to have little effect on the mechanical properties.