Effects of moisture and delamination on cracking of plastic IC packages during solder reflow

Some studies have indicated that "popcorn" cracking of plastic IC packages during solder reflow is due to excessive stress caused by the vaporization of moisture in the interface between the die pad and the plastic encapsulant. The numerical stress analyses done to date, however, have assumed that the vapor pressure in the delaminated pad-encapsulant is equal to the saturation pressure corresponding to the temperature at the interface during solder reflow. In fact, this vapor pressure in the delamination is developed gradually by the diffusion of moisture into the delamination. In this paper, this transient pressure rise at the delaminated pad-encapsulant interface is obtained using finite element simulation of the heat and moisture diffusion processes that occur simultaneously. It is shown that the maximum vapor pressure developed in the delamination is much lower than the saturation pressure values assumed by other researchers. However, even at this lower vapor pressure, it is shown using finite element analysis employing a crack-tip element, that the maximum principal stress developed in the encapsulant exceeds its strength. Thus, cracking of the encapsulant would still occur. It is also shown that delamination of the pad-encapsulant interface is necessary condition for package cracking. More significantly, it is established that the primary effect of moisture in "popcorn" cracking is the degradation of the adhesion of the pad-encapsulant interface, and not so much the increase in stress due to moisture vaporizing in the delaminated interface.