High-reliability epoxy molding compound for surface mount devices

Surface mounting packages have recently been developed with more and more increasing speed. And surface mounting type molding compounds have also been developed accordingly; compared with conventional molding compounds, new ones have been greatly improved in properties. With the development of surface mounting materials, however, there has occurred great change in the occurrence of soldering cracks. Until now soldering cracks were caused in the following way: the interface between the molding compound and the die-pad delaminated under thermal stress, absorbed moisture expanded suddenly, and stress concentrated at the edge of the die-pad caused cracks. On the other hand, soldering cracks are now caused by the delamination of the interface between the silver paste and the die-pad, not by the delamination of that between the molding compound and the die-pad. We examined what was the cause of the difference in the soldering crack occurrence mode. First we examined the mechanism of a new soldering crack occurrence mode and conducted model experiments for the mechanism, which proved that adhesion; heat strength, water absorption properties, thermal expansion, etc. Had a great influence. Then we determined the material design in R&D and reviewed the properties of hardeners and fillers. After that, we made experiments by using low molecular weight hardeners with a structure in which substituents were combined between phenols in pendant form. These hardeners are distinguished by excellent flexibility, water absorption properties, and curability as well as low viscosity. It was proved that they also showed various properties when changing the molecular weight of substituents. As for properties of fillers, from the matrix of the particle size distribution and the specific surface area we found a close relationship between the n value of the Rosin-Rammler diagram and the spiral flow and optimized properties of fillers. Materials developed in this way prevented not only the conventional crack mode, but also a new crack mode peculiar to compounds subjected to new soldering cracking; they showed excellent resistance to soldering PCT and resistance to temperature cycle