Thermal characterization of percolating thermal underfills: Bulk and cavity

Three dimensional chip stacking benefits from thermally conductive intra-stack bondlines. Accurately determining the thermal conductivity of percolating thermal underfill (PTU) layers, made up of dielectric filler particles and epoxy, is challenging due to their reduced dimensions. The constrained dimensions may also influence the effective composite material properties. This study investigates how bulk samples compare to constrained cavity samples of identical material combinations, characterized by a steady state measurement technique. It is shown that the bulk thermal conductivity is reduced by over 35% in cavity layers. Reduced particle fill fractions at the chip interfaces due to cavity boundary effects which limit possible particle settling locations are the primary reason for this observation. Reduced dimensions in the size range of the filler particles consequently play a very significant role in effective medium properties of composite materials and need to be taken into account during material development.