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 extensively studied; however. little has been published on their reliability with respect to moisture-related failure mechanisms during assembly, such as popcorning.

In this study. two types of plastic ball grid array packages. a 225-lead fuel matrix array and a 256-lead perimeter array. which were preconditioned by baking at 125C for 24 hours followed by exposure to 85⁰C and 30%. relative humidity for 168 hours, were then subjected to three passes of simulated infra-red reflow at 1⁰C/s and 0.67⁰C/s. Subsequent examination using scanning acoustic microscopy and environmental scanning electron microscopy revealed considerable delamination and cracking in both package types reflowed at the higher ramp rate. This damage 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.
 
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