Role of adhesive modulus on reliability of FCBGA with heat spreader

Finite element models of a flip chip ball grid array (FCBGA) package are utilized to determine the initiation of CTE mismatch induced defects, starting in the solder mask layer of the substrate and penetrating the copper traces to cause reliability failures of flip chip packages, with different adhesive fillet conditions and adhesive moduli. Due to insufficiency of the mechanical properties of the composition materials, analytic predictions are usually barely competent to grab the trend of the thermo-mechanical behavior of packages and sometimes even result in directions different from the real trend. This work employs finite element methods to perform a parametric study using the temperature dependent moduli of adhesives to reveal the stress status, which is hidden from what can be expected through the available data sheet of the constitutive materials of an FCBGA package with a heat spreader. The parametric study is performed on modulus ratios that indicate the stiffness difference of the adhesives below and above the glass transition temperature to explore the behavior of the package. The predicted stress status in the solder mask is capable of highlighting the potential failure of the solder mask, which is confirmed by the failure mode found in samples after temperature cycles. The same approach can be adopted to predict the potential failure modes during package development stages.