New High Thermal Conductivity Thermoplastics for Power Applications

Abstract

The trend towards higher density, higher frequency, higher power active devices in placing increasingly difficult demands on device packaging. Materials with high thermal conductivities are replacing the traditional ceramics in hermetic, high power packages, and MCM/ hybrid modules. Thermally enhanced plastic packages more frequently feature heat sinks embedded in the package for direct attachment of the power devices. Today's challenge in electronic packaging is to dissipate the heat from the source, the device itself, without affecting its electrical performance or reliability. The material directly contacting the device is the die attach medium. On lower power packages, the die bond line is not usually the highest thermal resistance in the thermal path. With highly conductive substrates and heat sinks, the die attach material now becomes the critical element directly in series with the highly conductive substrate. Fundamental limitations in thermal properties of about 3 W/mK exist in present‐day organic adhesives, primarily of the thermosetting type. This thermal conductivity (k) does not meet the current demands of thermally enhanced plastic laminate packages, MCMs, or direct die attach to heat spreaders or heat sinks. This paper describes the development, properties and application of electrically conductive thermoplastic adhesive pastes having thermal conductivity values as high as 35 W/mK, and able to produce thin, void‐free bond lines for maximum thermal transfer. The key material variables are isolated and evaluated for their impact of the k value. DOEs (design of experiments) were run to optimise the combination of the key variables, namely size/shape of the filler and the volume fraction to produce the highest k without sacrificing other functional properties such as adhesion. The effect of polymer chemistry (thermoset and thermoplastic) was also studied. The properties of the newly developed, enhanced conductivity thermoplastic adhesives are compared with other material technologies and examples of current applications reviewed.