Investigation of the popcorn phenomenon in overmolded plastic pad array carriers

The overmolded plastic pad array carrier (OMPAC) is becoming a standard carrier in packaging technology. The OMPAC type surface mount package, however, has a drawback: popcorning during reflow soldering to mount to the motherboard. This phenomenon is ascribed to rapid expansion of water inside the plastic materials, which easily absorb moisture. The popcorning is readily confirmed as a delamination and/or a crack in a plastic package that reaches the outside of that package. Many studies of the package structure and assembly processes have been undertaken to eliminate the popcorn phenomenon. In this study, we investigated the raw materials to eliminate the popcorn phenomenon without changing the conventional OMPAC structure and assembly process. For the OMPAC PCB material, new materials with high glass transition temperature and/or low moisture absorption are investigated in comparison with the glass reinforced BT (bismaleimide triazine) resin most commonly used. For the mold compound, a new type of biphenyl material is studied in comparison with the customary multifunctional epoxy mold compound used. Since the popcorn phenomenon occurs between die and PCB, the die attach epoxy between them plays a very important part in the elimination of popcorning. For this purpose, we found a material with high glass transition temperature and excellent adhesive strength between die and PCB. Using this adequate combination of PCB, mold compound, and die attach epoxy, we realized an excellent package which exceeds JEDEC level 2 moisture sensitivity.