The mechanical strength of reflowed solder connections.

In the solidified state, 60Sn/40Pb solder is comprised of grains that contain alternating lamellae of nearly pure tin and lead phases. The lead phase constitutes 30% of the solder volume, but the number of lamellae varies with solidification conditions. By contrast, 96.5Sn/ 3.5Ag solder consist of 3.6% by volume of Ag3Sn intermetallic particles in a nearly pure tin matrix. The number and morphology of these particles is also determined by solidification conditions. The lead phase in 60Sn/40Pb solder is quite ductile, whereas the Ag3Sn intermetallic compound in 96.5Sn/3.5Ag solder is very hard. The mechanical properties of these two phases are reflected in the strength and ductility of the respective solders. The mechanical properties of connections made with these two solders were studied by attaching phosphor- bronze clip-on leads to Ti-Pd-Au thin film using condensation soldering. Leads with both types of solder were reflowed at 253 degrees C and then aged at 170 degrees C for up to 100 hours. The mechanical strength of the solder connections was evaluated by pull testing the leads. A representative sample of the fracture surfaces were examined in a scanning electron microscope. The primary results of this investigation were as follows: Unaged 60Sn/40Pb solder joints had a 20% higher pull strength than 96.5Sn/ 3.5Ag connections. Aging reduced the strength of the 60Sn/40Pb solder connections, but had little effect on the 96.5Sn/3.5Ag solder joints. SEM examination of the fracture surfaces indicated that the 60Sn/40Pb solder connections failed by nucleation and growth of voids. The 96.5Sn/3.5Ag connections failed primarily by brittle cleavage accompanied by some void formation.