Ever since the discovery of the “popcorn” failure of plastic-encapsulated integrated-circuit (IC) packages in the 1980s, much effort has been devoted to understanding the failure mechanism and modeling it. It has been established that such failures are due to the combined effects of thermal stresses and hygrostresses that arise during solder reflow of plastic IC packages. In recent years interfacial fracture mechanics has been applied successfully to the analysis of delamination or crack propagation along interfaces in plastic IC packages. This paper presents some fundamental aspects of interfacial fracture mechanics and describes some of the numerical techniques available for calculating the strain energy release rate and mode mixity at the tips of cracks at interfaces in plastic-encapsulated IC packages. A method of calculating the combined effects of thermal stress and hygrostress on the energy release rate is also described. Some case studies are presented that illustrate how the techniques are applied to predicting delaminaton in IC packages. Some experimental verification of predictive methodology is also presented.