Influence of material combination on warpage and reflow crack resistance of PBGA

The plastic ball grid array (PBGA), which can increase the I/O pin counts by arranging the solder balls on the package back side, is advantageous in surface mount type IC packages. However, this package seems to have problems such as warpage and reflow crack resistance depending on the package structure and material combination. We studied the influence of material properties on the warpage and reflow crack resistance of PBGAs in order to optimize the properties of each material and material combination. It was found that PBGA warpage is closely correlated with the properties of the molding compound, i.e. polymerization shrinkage and the coefficient of thermal expansion, rather than those of the substrate and the die attach material, and that the warpage was drastically reduced by using a molding compound with low polymerization shrinkage. On the other hand, reflow crack resistance was heavily influenced by the existence of voids in the die attach layer, which are produced by vapor from the water absorbed by the substrate during the die bonding process. Hardening the die attach material before the vapor penetrates the die attach layer can reduce voids and is effective in improving reflow crack resistance. By using a molding compound with low polymerization shrinkage by introducing a rigid molecular structure such as naphthol hardener and an acrylic type die attach material, it was found that the warpage is less than 60 μm and the reflow crack resistance meets JEDEC level 2 (85°C/60%RH/168 hr) for the 352 I/O model PBGA package