The effect of moisture on the critical defect size for delamination failure at the pad/encapsulant interface of plastic IC packages undergoing solder reflow

Abstract

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. Moreover, if the package has absorbed moisture before solder reflow, it is well known that moisture does increase the likelihood of delamination and popcorn failure. 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 and how this is affected by moisture. 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 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. It was also found that increasing levels of moisture decreases the critical crack size for delamination. © 2005 IEEE.