Glass Transition Temperature Effects on Thermal Conductivity

This paper explores and quantifies advantages in thermally conductive printed wiring board laminate and prepreg obtained through of high glass transition temperature resin systems.

Whether by power cycling or conveyor Infra-Red oven temperatures for assembly, excursions above glass transition temperature cause unrecoverable strain in the resin matrix in which thermally conductive ceramic fillers are incorporated. Resin systems which optimize glass transition temperature help minimize this effect, allowing a composite system to better maintain its thermal conductivity throughout processing and its life cycle.

Thermal Conductivity in organic laminate systems is predicated on particle to particle contact. After manufacture, but before further processing, these composites are tested and are at their highest thermal conductivity. However, during assembly, devices and attendant substrate materials are subject to often repeated thermal excursions to at least 220°C for eutectic tin/lead solder or up to 260°C for many lead-free solders. During thermal excursions or cycling, direct particle interaction and thermal conductivity are reduced by unrecoverable deformation of the resin system, as its own expansion competes with ceramic filler and fiberglass reinforcement.

Those systems which comprise a higher glass transition temperature resin matrix are better at maintaining performance faithful to their literature values for thermal conductivity. 99 Series Laminates available from Arlon with either copper foil cladding or pre-bonded to aluminum plate use a select high glass transition temperature epoxy and a combination of ceramic fillers to retain better particle interaction, achieve reliable throughput at assembly (by reducing thermal expansion) and maintain designed thermal conductivity for required heat rejection.