Synergetic effect of strain rate and electroplated Cu film for shear strength of solder/Kovar joints

The effects of strain rate and electroplated Cu on the shear fracture behaviors of Sn37Pb/electroplated Cu (EPC)/Kovar solder joints were investigated after reflowing at 250 °C for 5 min. The Kovar was electroplated with 4.1 µm thick layer of Ni and 1.9 µm thick layer of Au. The copper electroplating time were 2, 5, 10 and 20 min, respectively, resulting in a different deposited thicknesses of Cu film on the Kovar substrate. The shear tests of Sn37Pb/EPC/Kovar joints were performed with different strain rates ranging from 8.3 × 10⁻² to 1.6 s⁻¹ to study the shear fracture behavior. Experimental results showed that the shear strength and failure mode of solder joints were affected by the thickness of electroplated Cu film and the applied strain rate. The shear strength of solder joint demonstrated increment at first and then decrement as the applied strain rate increased. It was observed that the shear failure mode of solder joints is well known to have experienced a transition from ductile fracture with the low strain rates (8.33 × 10⁻² s⁻¹ and 3.33 × 10^− 1 s⁻¹) to a mixed fracture with the intermediate strain rate (6.67 × 10⁻¹ s⁻¹) and high strain rates (1 s⁻¹, 1.33 s⁻¹ and 1.67 s⁻¹). Studies in the shear properties of solder joints with electroplated Cu layer indicated that the electroplated Cu on the Kovar substrate has a significant improvement in the shear strength of solder joints. With increasing in the deposited thickness, the shear strength of solder joints increased when the deposited thickness was no more than 3.3 µm. Subsequently, the shear strength started to drop as the deposited thickness further increased.