Mechanics of interfacial delamination under hygrothermal stresses during reflow soldering

Inherent voids or defects at the interface between die pad and plastic encapsulant become stress concentration sites when the IC package absorbs moisture or undergoes reflow soldering. If the stress intensity factor at the interfacial voids exceeds the critical fracture toughness of the pad-encapsulant interface, delamination occurs, eventually leading to popcorn cracking. In this paper, the mechanics of interfacial delamination under hygrothermal stress during solder reflow are discussed. A methodology for computation of the resultant total stress intensity factor due to the combined effects of hygro- and thermal stresses is presented. The methodology is then applied to study the effect of prebaking on delamination during solder reflow. To verify the methodology, a series of experiments was conducted in which several plastic IC packages were fabricated with a very small defect at the pad-encapsulant interface and preconditioning at 85°C/85% RH for 168 hours. The packages were then prebaked at 150°C for varying periods such as 30 minutes, 6.5 hours and 24 hours before being placed in varying high temperature environments in an oven. Oven temperatures ranged from 185°C to 200°C. The packages were then examined for delaminations by scanning acoustic microscopy. It was found that when the prebaking period was short, delamination occurred. The methodology developed was applied to predict the minimum period of prebaking required to avoid delamination during solder reflow. Good agreement was obtained between predicted and observed values