Analysis of warpage and residual stress in plastic ball grid array package after post mold cure

摘要

Purpose - The purpose of this paper is to provide a design and material selection guideline for a plastic ball grid array (PBGA) package in order to improve its reliability and manufacturing ability after post mold cure. Design/methodology/approach - Numerical experiments based on a three-dimensional (3-D) viscoelastic finite element method have been conducted to evaluate governing damage mechanisms after post mold cure (PMC) for PBGA packages. The parametric studies for the PBGA package with various molding compounds have been performed. A wide range of the modulus (1MPa, 15GPa) and the coefficient of thermal expansion (CTE) (10ppm , 300ppm) are evaluated to see feasibility of a new class of material set in the molding compound. Effects of thermo-mechanical properties of selected molding compound on the warpage and residual stress of the PBGA are analyzed. Findings - The present study shows that the material properties such as modulus and CTE of molding compounds play an important role in warpages and reliability of PBGA packages. After post mold cure, compressive normal stress sxx is observed in the silicon die, while tensile stress occurs in the rest of the PBGA package. The maximum normal stress sxx is observed at the center of the silicon die and decreases near the edge of the package. As the coefficient of thermal expansion of the silicon die is substantially less than that of the molding compound or substrate, the molding compound and the substrate are trying to shrink more when temperature decreases and in turn compressing the silicon chip. The molding compound with low modulus produces low stresses in the Si die and the die attach. Moreover, for the low modulus case, the CTE of molding compound does not affect the warpage of the PBGA package and the stresses in the silicon die or the die attach. However, for the high modulus case, the warpage and stresses are increased significantly by increasing the CTE of molding compound. Research limitations/implications - It is suggested that adhesion strengths of die attaches should be studied in future studies, since those affect the delamination between dies and substrates. Practical implications - The findings can be used as general design guidelines for a PBGA package. Originality/value - The results presented in the paper will be very useful to designers of PBGAs.