Mixed mode fracture toughness of lead-tin and tin-silver solder joints with nickel-plated substrate

This study uses modified compact tension specimens to evaluate the fracture toughness of solder joints under a combination of tensile and shear loads. Copper substrates were coated with electroless nickel plating before they were soldered together. The samples were reflowed to peak temperatures of 215 and 240 °C for Sn-37Pb and Sn-3.5Ag, respectively, with appropriate preheating temperatures, holding time and cooling rates. Fracture toughness tests were conducted as per ASTM E399-90. Cross-sectional microstructural and fractography studies were carried out on the tested samples. Earlier fracture studies which were based on J integral as per ASTM E813-89 did not produce valid results because of brittleness of the solder joints. The current results showed that the fracture of the solder joints follows the general principles of a mixed mode fracture mechanism map and shear is the preferred mode of failure regardless of different mode of loadings, type of solders and substrate coatings.