Abstract
A wafer level package (WLP) without the underfill layer was introduced in recent years to address the demand of the electronic packaging industry for increased density and performance, as well as lightness, thinness, small size and cost-effectiveness. However, the solder joint reliability under thermal cycling conditions becomes a critical problem when mounting the WLP onto a PCB when the underfill layer has been eliminated. Consequently, enhancing the board level reliability is of primary concern in current WLP design. At the same time, 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 a weak solder joint in package reliability testing. Consequently, demands for increasing the solder ball shear strength have risen in recent years. In order to solve these problems, this research proposed a new WLP design, based on forming a Cu stud on the center of the surface of the solder pad. The solder pad with a round Cu stud was made using a semiconductor manufacturing process. Therefore, this novel Cu stud design technology is workable. To investigate the impact of a Cu stud on the solder ball shear strength and solder joint reliability, 3-D non-linear finite element models were used for the Cu stud design. In shear analysis, this investigation explored the effects of various parameters including the Cu stud’s dimension, shape, and material properties on the solder ball shear strength. Furthermore, the shear force-displacement curves, obtained by computational analysis, were compared with the experimental results to demonstrate the accuracy of the finite element models. In thermal cycling analysis, this research investigated the effects of various parameters, including the Cu stud’s dimension, shape, material properties and the die and PCB thicknesses on the solder joint’s reliability. To demonstrate the accuracy of the finite element models, the analytical results were compared with the experimental results and the experimental data reported in the literature. Comparing the experimental data with the results from the finite element analysis revealed that the finite element analysis was reliable. The analytical results established that a suitable size of Cu stud in a solder ball could effectively enhance the ball’s shear strength. Moreover, the solder joint reliability could be significantly improved by forming large Cu studs on the surfaces of the solder pads of WLP and PCB substrate, and could be further enhanced by combining large Cu studs with thin die. In addition, this research also explored the growth of intermetallic compounds (IMC) under aging for eutectic Sn-Pb solder reflowed on a Cu pad with an Au/Ni surface finish. The effects of the intermetallic layer on the solder ball shear strength were examined for various solder ball sizes, Cu pad sizes and Au layer thicknesses. The IMC growth is dominated by the diffusion-controlled mechanism, in which the vacancy diffusion is the main diffusion mode. The vacancies and atoms can interchange locations continuously. Experimental results indicated that the degradation of the solder ball shear strength was found to be mainly caused by brittle interfacial fracture, due to the formation and growth of the Au0.5Ni0.5Sn4 intermetallic layer. Decreasing the Au layer thickness can reduce the Au weight in the solder and the Au0.5Ni0.5Sn4 thickness, and so avoid the degradation of the solder ball shear strength. The findings of this research can offer designers and manufacturers an index to adjust the design for advanced ball grid array package to enhance their package reliability.