Study of the compound of properties of percolating and neck-based thermal underfills

Percolating and neck-based thermal underfills with significant improvements in thermal conductivity compared with capillary underfills are currently under development. They could be applied between dies to improve the heat dissipation through a 3D chip stack. In this parametric study, we provide insights into the thermal, mechanical, thermo-mechanical and electrical properties achievable by this new composite material class. The primary objective of the investigation is the linear buckling phase of monodisperse spherical filler particles confined between two parallel plates with a fill fraction range of 48.7% to 61.3% as observed by experiment. The introduction of necks between the point contacts of the filler particles had the most significant impact on the composite effective material properties, resulting in an increase in thermal conductivity, stiffness and a drop in the thermal expansion coefficient. The high stiffness could cause delamination of the underfill in chip corners because of high shear forces and hence may have to be mitigated. Finally, two design points for the composite were proposed, respecting the target values for the percolating and neck-based thermal underfill, with a predicted effective thermal conductivity of 1.9 and 3.6 W/m-K.