Determination of critical defect size for delamination failure of the pad/encapsulant interface of plastic IC packages undergoing solder reflow

During the solder reflow process thermal stresses are induced in a plastic-encapsulated IC package due to the mismatch of coefficient of thermal expansion (CTE) between the plastic encapsulant, the silicon chip and the leadframe-pad. Inherent voids or defects at the interface between leadframe-pad and the encapsulant become sites of stress concentration which can result in the delamination of interfaces during solder reflow. In order to assess whether a defect of a particular size would lead to delamination of the pad/encapsulant interface, a necessary prerequisite for popcorn failure, it would be useful to determine what the critical defect size might be. This is the main objective of this paper. A 160-leaded PQFP was used as the test vehicle in this investigation. The mechanics of delamination growth along the pad/encapsulant interface was studied by varying the initial delamination from 0.1 mm to 3.5 mm. The analysis was done for packages with and without the die. It was found that for those packages without the die, G increased monotonically with delamination size while for those with a die, G increased and reached a maximum before declining with further increase of delamination size. It is significant to note that the point when the maximum G occurred was when the tip of the delamination was just below the die edge. The effects of using different leadframe materials were studied. Alloy 42 was found to give a higher value of G than copper, leading to increased tendency to delamination. However, for both leadframe materials, the critical delamination length was unchanged if the interface toughness remained the same. For packages with a die, it was found that there are two critical crack lengths which divide the whole interface into three distinctive regions. In the first and last area no crack propagation will be observed. Once the first critical crack length is exceeded, the crack will continue to grow until it reaches the second critical crack length when it will stop.