Reliability modeling of chip scale packages

The chip-scale package (CSP) is an increasingly popular small size, high performance package developed by the electronics industry. The advantages of such a package are that it offers considerable space savings over fall-sized ball,al-id array or peripherally leaded devices while maintaining the convenience and die protection of a packaged device. In this paper, a finite element based approach for estimating the thermal cycling reliability for chip scale packages is presented. The methodology is based on Anand's viscoplastic constitutive law for the solder response and Darveaux's crack growth rate model for solder fatigue. A Weibull two-parameter failure distribution is assumed. Three-dimensional finite element models are built for several different CSP package configurations. Two- and three-fold symmetry is used to reduce the model size and computer run-time. In addition, to facilitate rapid development of the finite element models, a basic building block approach is used. These building blocks consisted of a solder joint (including pad dimensions) and package geometry. These units are then used repeatedly to construct the overall model. The number of cycles to fifty percent package reliability is estimated for a twenty-minute air-to-air thermal profile of 0 degreesC to 100 degreesC. (five minute dwells at each temperature extreme). Good correlation between the measured and predicted life is observed. All of the packages studied have measured life within the expected +/- 1.5X error band of the method for the Weibull slopes considered.