In applications where a lot of heat needs to be dissipated, the goal of the designer is to minimize the thermal resistance of bonds. So the natural idea is to use materials with good thermal conductivity. However this is not the whole story. Usually the bond line thicknesses generated are rather thin (<100μm) and engineers try to produce a bond line that is as thin as possible. However, adhesives are complex systems. The thermal resistance depends on the adhesive’s thermal conductivity at a given bond line thickness, the bond line thickness itself, and other parameters such as bonding pressure, surface roughness, processing conditions (in particular the cure), etc.
Most thermally conductive adhesives have thermal conductivities in the range of 0.7 - 4 W/m*K. To get the best thermal contact, it is key to clamp solid particles in between the two surfaces during the bonding process. This forms an intermetallic contact through the filler material. During the adhesive cure, the compression force is formed and supported by adhesive shrinkage and an adapted bonding process. Because of the high thermal conductivity of solid fillers such as Al, Cu, or Al2O3, low thermal resistance between the two joined surfaces can be achieved.