Photopolymerization of silica-filled composites: Encapsulants for microelectronic devices

Encapsulation of microelectronic devices is typically accomplished using a transfer molding process in which the molding compound is cured by heat. Most transfer molding processes suffer from significant problems due to the high operating temperatures and high pressures required to fill the mold. Photopolymerizable liquid encapsulants (PLEs) offer important advantages over the conventional encapsulants such as reduced in-mold cure times, lower thermal stresses, and reduced wire sweep. In this contribution, we discuss a new strategy for encapsulating microelectronic devices based upon a low viscosity, epoxy novolac-based vinyl ester resin, with highly filled fused silica fillers, that cures rapidly upon exposure to UV light. For these PLEs, the viscosity, flexural strength and modulus, coefficient of thermal expansion, glass transition temperature, initiation scheme, and illumination time for systems containing 70.0 - 74.0 wt% fused silica have been characterized. A photocurable encapsulant containing 74.0 wt% fused silica was found to be very promising for microelectronic encapsulation. These liquid encapsulants cure (to an ejectable hardness) in less than two minutes for an initiating light intensity of 200 mW/cm(2) and exhibit appropriate the thermal and mechanical properties.