Formulation of percolating thermal underfills using hierarchical self-assembly of micro- and nanoparticles by centrifugal forces and capillary bridging

Thermal underfills are crucial to support integration density scaling of future integrated circuit packages. Therefore, a sequential process using hierarchical selfassembly of microand nanoparticles is proposed to achieve percolating thermal underfills with enhanced particle contacts. The three main process steps hereby are assembly of filler particles by centrifugation, formation of nanoparticle necks by capillary bridging, and the backfilling of the porous structure with an unfilled capillary adhesive. Numerical simulations predicting trajectories and distributions of micron-sized particles dispensed into a rotating disk are presented. The trajectories exhibit a strong dependence on the particle size; thus in the case of polydisperse filler particles nonuniform particle beds may result. An efficient centrifugal disk design with spiral-like guiding structures is experimentally validated. Defect-free, percolating particle beds in confined space with fill fractions of 46 vol-% to 66 vol-%, i.e., close to the theoretical limit, are also presented. The self-assembly of nanoparticles, forming enhanced thermal contacts between the percolating filler particles, is discussed. Two consecutive evaporation patterns during the capillary bridging process were identified: 1) dendritic network growth and 2) collapse of capillary bridges. The concave neck topology could only be achieved at temperatures below the boiling point. An optimal evaporation temperature of 60°C with respect to in-plane uniformity and neck shape was identified. Existing thermal gradients normal to the cavity surface resulted in strongly asymmetric neck formation in the cavity. Hence, uniform heating in an oven is the preferred method to initiate evaporation. Two types of bimodal dielectric necks are demonstrated. Polystyrene acts as the adhesive between thermally conductive alumina particles to form mechanically stable dielectric necks after an annealing step at 140°C. Interstitial and core-shell necks are presented. Finally, a benchmark study was performed to compare the effective thermal conductivity of the percolating thermal underfill with and without necks with state-of-the-art capillary underfills. A close to fivefold improvement could be obtained for diamond filler particles with silver necks (3.8 W/m-K).