Viscosity characterization of highly filled photopolymerizable liquid encapsulants for microelectronic devices

In this paper the viscosity of novel photopolymerizable liquid encapsulants (PLEs) for microelectronic devices was characterized as a function of the particle size distribution of the fused silica filler. Microelectronic devi ces are typically encapsulated using a transfer molding process in which th e molding compound flows over the leadframe and wire bonds as it fills the mold. The molding compound should have a low viscosity to minimize problems such as: 1) incomplete mold filling; 2) lead frame movement during cavity filling; and 3) displacement of the wires that connect the die with the lea dframe (wire sweep). We have developed a photopolymerizable liquid encapsul ant using an epoxy novolac-based vinyl ester resin that may alleviate these problems. In this contribution, we have investigated the blending of two d ifferent particle size distributions of fused silica to tailor the viscosit y of PLEs for microelectronic applications. We have characterized the visco sity of highly filled PLEs containing 70.0, 72.0, and 74.0 wt% silica, and found that a blend of particle size distributions with a particle size rati o of 3.13 resulted in the best viscosity reduction. In addition, the PLE vi scosity decreased slightly with increasing concentration of a silane coupli ng agent. The resulting PLEs exhibit low viscosities at ambient temperature while maintaining desirable material properties for microelectronic applic ations.