Special properties of molding compounds for large surface-mounting devices

Package cracking has become a problem in surface-mount technology as a result of larger package sizes, chip sizes and lead counts. To correct this problem, molding compounds suitable for use in surface mounting applications are being designed. One approach is to increase the cross-linking density of the organic resin to achieve a very-high strength at mounting temperatures. Another approach is to increase the resistance to stress using silicon modification and a very finely dispersed sea-island structure. Combining these two approaches yields a compound that delivers excellent performance as an encapsulant for SMT packages. The effects of the physical properties of this encapsulant, especially the flexural strength of the molding compound at soldering temperature, on preventing package swelling and package cracking at a temperature of 215 to 260 degrees C has been studied. Results are presented for 80-pin quad flat-package devices subjected to an 80 degrees C 85% RH environment after mold-cure and to a vapor phase soldering temperature of 215 degrees C after moisture absorption.<>