Improvements of solder ball shear strength of a wafer-level CSP using a novel Cu stud technology

The solder ball shear test has been widely adopted in the electronics industry to estimate the strength of solder ball attachment of advanced electronic packages. A solder ball with low shear strength is usually considered as a weak solder joint in package reliability testing. Consequently, demands for increasing the solder ball shear strength have risen in recent years. This work attempts to enhance the solder ball shear strength of the wafer level chip scale package (WLCSP) by forming a Cu stud on the surface of the solder pad. The novel Cu stud design technology has been achieved by using a simple semiconductor manufacturing process. To investigate the impact of Cu stud, a three-dimensional (3-D) nonlinear finite element method is used for Cu stud design. Furthermore, the shear force-displacement curves, obtained by computational analysis, are compared with the experimental results to demonstrate the accuracy of the finite element models. This investigation also explores the effects of various parameters including the Cu stud's dimension, shape, and material properties on solder ball shear strength. The analytical results establish that a suitable size of Cu stud in a solder ball can effectively enhance the ball's shear strength.