Measurements of adhesive bondline effective thermal conductivity and thermal resistance using the laser flash method

Thermal modeling of device packages requires accurate thermophysical property data for package materials. Accurate data for the thermal resistance of the adhesive bondline used to attach a high power device to a substrate is critical because this thermal resistance can be a significant part of the total thermal resistance in the heat flow path from the device junction to the package case or ambient. The bondline thermal resistance can in principle be calculated by dividing the expected or measured bondline thickness by the adhesive thermal conductivity measured on a free-standing cured sample. However, at a typical bondline thickness of 15-75 mu m, the contact thermal resistance between the adhesive and its adherents can be significant compared to the intrinsic thermal resistance of the adhesive and thus cannot be ignored. Also, the thermal conductivity measured on a free-standing cured sample may not be equivalent to the thermal conductivity of the adhesive in the bonded assembly. This paper will investigate some of the variables that determine the adhesive bondline effective thermal conductivity and contact resistance. The results of multi-layer laser flash diffusivity measurements will be presented for a range of available adhesives in "sandwich" sample assemblies that simulate the package. Thermal conductivity measurements of the adhesives free-standing will also be obtained by the laser flash method.