Size effect on microstructure and tensile properties of Sn3.0Ag0.5Cu solder joints

In the current study, the size effect on microstructure and tensile property of Sn3.0Ag0.5Cu (SAC305) solder joint is investigated. Experiments were performed to solder joints with thickness from 50 μm to 2 mm. The intermetallic compound (IMC) growth rate of solder joints is found to be dependent on the size of solder joints. After soldering procedure, the IMC of smaller size solder joints is thinner than that of larger size solder joints. It is also found that the average grain size of IMC increases with decreasing of the joint size. To better understand the difference of IMC growth during solid-state and liquid-state thermal aging, experiments were performed at 210 and 300 °C, respectively. IMC thickness of solder joints after 210 and 300 °C of thermal aging was measured. It is found that the size effect on IMC growth rate at solid-state and liquid-state is different. The IMC growth rate of smaller size solder joints is higher than larger size solder joints under solid-state thermal aging. Under liquid-state reaction, the IMC growth rate of smaller size solder joints is slower. ‘H’ type and hexagonal prism of Cu₆Sn₅ were observed in 2 and 1 mm thickness size solder joint, respectively. Fewer Cu₆Sn₅ prism was found in 50 μm thickness solder joints. Tensile strengths of solder joints with different sizes were measured. The solder joints showed ductile to brittle fracture transition with the decreasing of joints thickness. The H-field fracture criterion is employed to predict the tensile strength of Sn3.0Ag0.5Cu solder joints with different thickness.