Nonlinear Analysis of a Ball Grid Array Package under Thermal Cycles

In this study, the stress-strain distributions and interface failure in a BGA (Ball Grid Array) package subjected to thermal cycling were investigated using 2D and 3D finite element analyses. The viscous behavior of the adhesive material and thermal mismatch between the dissimilar materials in the package were considered. The potential failure sites in the solider and adhesive joints were analyzed. Introduction The reliability analysis of ball grid array (BGA) package under a cyclic temperature environment has been widely investigated in recent years. A great concern in electronic package is the failure induced by thermal excursion during manufacture and operation processes. In a thin package, the coefficient of temperature expansion (CTE) mismatch between different materials is regarded as the main reason for warpage. On the other hand, the composite structure such as flip-chip assembly subjected to thermo-mechanical and/or hydrothermal loadings may introduce stress-related failure. Delamination and cracking were observed in the plastic packages [1, 2]. Thermally induced stresses in flip-chip assembly resulted in loss of interfacial adhension and solder joint fatigue [3]. Recently, because of their unique properties, adhesives have been increasingly used in microelectronics packaging. Generally, thermal loading can affect adhesive bonds through inducing thermal stresses in the assemblies and modifying the properties of the adhesives. Thermal fatigue can also cause viscoplastic deformation and failure in solder joints via void nucleation and growth. In this paper, numerical analysis was conducted in a BGA package under thermal cycling. A full scale three-dimension finite element (FE) model was built to analyze the potential failure sites both in adhesive assemblies and solder joints. The comparison between 3D and 2D analyses was conducted. The effects of thermal cycling and shape of solder on the stress-strain distributions in adhesive and solder joints was also investigated. Constitutive model The FE analysis was conducted in a typical full matrix plastic BGA package. Fig.1 shows the diagonal cross section of the package. Adhesive bond of 0.1mm thickness locates between the substrate, molding and the silicon die, while the dimensions and materials of the rest part of the package is the same as Yan et al. [4]. In the FE analysis, viscous behavior was assumed in both solder and polymer adhesive. The time dependent strain rate tensor in the solder alloy was described by th ij cr ij pl ij el ij ij ε ε ε ε ε & & & & & + + + = , (1) where the total strain rate tensor are the sum of the elastic, plastic, creep and thermal strain rate tensor. Under uniaxial loading, the stress as a function of time in a linear viscoelastic material can be given Fig. 1 Schematic of the BGA Assembly in Diagonal Direction