Moisture diffusion modeling and its impact on fracture mechanics parameters with regard to a PQFP

Absorption of moisture by plastic packages increases the susceptibility of packages to interfacial delamination. Accurate determination of moisture concentration can be important. In this numerical study, a plastic package is subjected to level 1 moisture preconditioning and is subsequently exposed to solder reflow process. Local moisture concentration and fracture mechanics parameters (energy release rate, ERR and mode mixity) are determined through finite element simulation. The impact of the thickness of the package, the die attach layer, the assumption of the independence of saturated moisture concentration (Csat) on temperature and the assumption of spatial isothermal condition are analyzed. When Csat is assumed to be independent of temperature (standard method), the local moisture concentration remains the same or decreases during solder reflow depending on the thickness, whereas when Csat is dependent on temperature, the local moisture concentration can increase during solder reflow. Limited influence on the fracture mechanics parameters is observed. Under spatial isothermal condition, when the temperature of the package is assumed to be equal to the oven temperature, the local moisture concentration is relatively similar to that obtained from the standard method. However, the combined ERR resulting from thermal stress, hygro stress and vapor pressure is more than that obtained from the standard method. Generally, the die attach layer does not result in significant changes in local moisture concentration but in its absence, the ERR is generally higher than the standard method.