Reliability aspects of molded BGA's related to material properties

BGA packages have become increasingly popular for microprocessor device packaging and MCM assembly. This is due to the advantages the ball grid array offers with regard to high lead count combined with large pitches, processable with standard SMT equipment. For BGA package encapsulation, epoxy resins are mostly used, either as glob top or molding compound. For cost effective encapsulation of large numbers of BGA packages, the transfer mold process offers various advantages. The process is fully automated with short process times and the molding compound glass transition temperature (Tg) is very high (>200 degrees C) compared to glob top encapsulants ( 160 degrees C) offering higher reliability at elevated temperatures. However, BGA packages challenge both material suppliers and packagers as this package type is asymmetric and is thus very sensitive to encapsulant material properties. Properties to consider are reduced encapsulation material shrinkage after molding to provide low warpage, and optimized molding compound adhesion to the materials present in a BGA to avoid delaminations that might cause electrical failures. Fraunhofer IZM and Toshiba Chemical have designed test vehicles to investigate different BGA molding compounds with regard to substrate adhesion with various types of solder resist after molding and after package reliability testing. Results of these investigations are reported and investigations regarding humidity sensitivity (according to JEDEC Std. 020) of the encapsulated BGA and the relationship between adhesive strength of EMC to solder mask and humidity uptake of the package are described.