Photopolymerizable liquid encapsulants for microelectronic devices

Photopolymerizable liquid encapsulants (PLEs) for microelectronic devices m ay offer important advantages over traditional transfer molding compounds, including reduced in-mold cure times, lower thermal stresses, and reduced w ire sweep. In this contribution, we discuss an encapsulation process based upon a low-viscosity resin that cures rapidly upon exposure to UV light. Th ese highly filled PLEs are comprised of an epoxy novolac-based vinyl ester resin (similar to 25 wt.%), fused silica filler (70-74 wt.%), photoinitiato r, silane coupling agent, and, in some cases, a thermal initiator. We have characterized the degree of cure, flexural strength, flexural modulus, coef ficient of thermal expansion, glass transition temperature, and thermal str ess parameter of these novel PLEs. In addition, we investigated the effect of the fused silica loading, UV illumination time, and postcure time on the se properties. The results indicate that a photocurable encapsulant contain ing 74.0 wt.% fused silica is very promising for microelectronic encapsulat ion. These liquid encapsulants cure (to an ejectable hardness) in less than 2 min for an initiating light intensity of 200 mW/cm(2) and exhibit approp riate values for the thermal and mechanical properties listed above. (C) 20 00 Elsevier Science Ltd. All rights reserved.