Centrifugal Formulation of Percolating Thermal Underfills for Flip-Chip Applications

In this work, a methodology for the formulation of densely packed percolating thermal underfills is presented. The methodology involves centrifugal acceleration of particles, which are placed in chip cavities of 60 micrometer in height due to the action of the applied centrifugal force. Each cavity mimics the gap between a chip and a laminate or between dies in 3D packages. High thermally conductive particles, including: diamond, boron nitride and graphite are used to fill the cavities; followed by an epoxy backfilling and curing steps. The filling fraction and the associated thermal conductivity of the composite material is characterized by means of optic and electron microscopy and measured using our thermal tester, respectively. We have found that the presented methodology is relatively insensitive to the shape and size of the particle being used. Very high filling fractions approaching those corresponding to the theoretical maximum were routinely measured, ranging between 45 to 69 vol% and thermal conductivities between 1.7 and 2.5 Wm(exp -1)K(exp -1); outperforming in this way, available commercial capillary thermal underfills by at least a factor of 2. The methodology presents an efficient approach to reduce die-to-substrate and die-to-die heat resistance, improving heat dissipation in mobile and 3D stacking applications.