Photopolymerizable liquid encapsulants for microelectronic devices

Photopolymerizable liquid encapsulants (PLEs) for microelectronic devices may offer important advantages over traditional transfer molding compounds, including reduced in-mold cure times, lower thermal stresses, and reduced wire sweep. In this contribution, we discuss an encapsulation process based upon a low-viscosity resin that cures rapidly upon exposure to UV light. These highly filled PLEs are comprised of an epoxy novolac-based vinyl ester resin (∼25 wt.%), fused silica filler (70–74 wt.%), photoinitiator, silane coupling agent, and, in some cases, a thermal initiator. We have characterized the degree of cure, flexural strength, flexural modulus, coefficient of thermal expansion, glass transition temperature, and thermal stress parameter of these novel PLEs. In addition, we investigated the effect of the fused silica loading, UV illumination time, and postcure time on these properties. The results indicate that a photocurable encapsulant containing 74.0 wt.% fused silica is very promising for microelectronic encapsulation. These liquid encapsulants cure (to an ejectable hardness) in less than 2 min for an initiating light intensity of 200 mW/cm2 and exhibit appropriate values for the thermal and mechanical properties listed above.