Thermal Characterization and Modelling in Microsystem Packaging Applications

Licentiate thesis, 2005

As the electronics circuits grow denser and the power consumption per area unit is increasing, new more efficient technologies for heat removal is necessary. Heat fluxes from the integrated circuit, IC, in the order of 100W/cm2 are not rare. Two possible approaches to improve the cooling of electronic packages are to improve the thermal conductivity of the packaging material and increase the heat removal from the package. In this thesis both approaches are explored. Anisotropic conductive adhesives are used for electrical interconnections but have a poor thermal conductivity. This can be improved by adding a third, electrically insulating but thermally conductive phase into the adhesive. The experimental work included producing thermally conductive composite from various micro- and nanosized fillers. Experimental effort was also put on developing the Transient Hot Wire Method for the measurement of the thermal conductivity. The thermal conductivity increased with increased amount of conductive filler and filler surface treatment. The experiments also indicated higher conductivity for alumina nanoscale particles than for microscale particles. Carbon nanotubes provided the highest conductivity at low filling levels. However, for carbon nanotubes, high filling levels were difficult to achieve due to high viscosity of the uncured composite. In order to explore whether the cooling effect of microchannels could be improved by fins mounted inside the channels, thermal and fluid dynamic modelling was carried out. The modelling based on experimental results was carried out in FEMLAB Multiphysics™. The water was modelled as laminar with Navier-Stokes macroscopic models. Thermal modelling showed that the thermal resistance between hot “active” chip side and water could be reduce from 0.98 K/W to about 0.43 K/W when fins were used. Potential improvements of the fin layout were revealed. For example it could be seen that a small extent of the water stream was flowing close to the chip surface. Although the system was not optimized it showed great potential. System in a package, SiP is a new type of packaging concept for electronics but the thermal performance of the concept has not been studied significantly. The research has focused on the thermal performance of a system consisting of a chip mounted in a substrate cavity, with copper traces as interconnects embedded in epoxy. A parametric study on material properties, geometries and heat transfer coefficient was carried out with 3D Finite Element Modelling. The results variable was the chip temperature. It was shown that heat transfer coefficient had the largest influence for the joint temperature. The substrate conductivity also played an important role. The encapsulant conductivity was important only if the substrate had good conductivity. Seen from a thermal perspective, for the present system, the optimal copper thickness was found to be about 30micron.