Photopolymerizable liquid encapsulants for microelectronic devices: Thermal and mechanical properties of systems with reduced in-mold cure times

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 (similar to 25 wt %), fused silica filler (70-74 wt %), photoinitiator, silane coupling agent, and, in some cases, a thermal initiator. We have characterized the material properties (flexural strength and modulus, coefficient of thermal expansion, glass transition temperature, and thermal stress parameter) of PLEs cured with UV illumination times of 60, 90, and 120 s, as well as, the thermal conductivity and adhesive peel strength of PLEs photocured for 90 s. In addition, we investigated the effect of the fused silica loading and the initiation scheme on these properties. The results indicate that the PLEs are very promising for microelectronic encapsulation. These liquid encapsulants cure (to an ejectable hardness) in 1 min for an initiating light intensity of 200 mW/cm(2), and exhibit appropriate values for the thermal and mechanical properties listed above. (C) 2001 John Wiley & Sons. Inc.