Abstract:

Plastic ball grid arrays (PBGAs) have been gaining increasing industry acceptance as potentially the lowest cost packages for high pin count applications.  The main factors for their success include the inherent low cost of plastic packages, the high yields achievable with existing assembly equipment, the density advantages of an area array and, in many cases, superior electrical performance.  The reliability of these packages with respect to solder joint fatigue has been during assembly, such as popcorning.  In this study, two types of plastic ball grid array packages, a 225-lead full matrix array and a 256-lead perimeter array, which were preconditioned by baking at 125ºC for 24 hours followed by exposures to 85ºC and 30% relative humidity for 168 hours, were then subjected to three passes of simulated infra-red re-flow at 1ºC/s and 0.67ºC/s.  Subsequent examination using scanning acoustic microscopy and environment scanning electron microscopy revealed considerable delamination and cracking in both packaging types re-flowed at the higher ramp rate.  These damages originated in the die attach and propagated along the weakest interfaces in the package.  At the lower ramp rate, a much smaller amount of delamination was observed.  This suggests that there is a critical ramp rate below which popcorn cracking can be inhibited.