Moisture absorption and desorption predictions for plastic ball grid array packages

Plastic electronic packages are known to absorb moisture when exposed to humid ambient conditions during storage in tape and reel in the factory. Reliability becomes a concern when packages are exposed to a humid environment for a prolonged time and then processed through a reflow oven. Thermo-mechanical and moisture induced interfacial stresses generated between the die attach and die may lead to die delamination and package cracking. Current package level moisture evaluation methods are based on experimental weight gain measurements and visual inspection after reflow for cracking or delamination. This study presents a finite element analysis (FEA) simulation of the moisture weight gain or loss in plastic ball grid array (PBGA) packages as a function of time, including the effects of package geometry and material selection. Experimental weight gain and bake-out data for a 68 I/O PBGA package are shown to closely match FEA model predictions. Diffusivity and solubility property data for common packaging materials are given as a function of temperature from 23 to 210°C. Solder pot dip tests performed at 230°C indicate that popcorn failures result when the moisture concentration in the die attach region exceeds 0.0048 g/cm³ for the specific package tested