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

摘要

Thermal underfills are crucial to support integrationdensity scaling of future integrated circuit packages.Therefore, a sequential process using hierarchical selfassemblyof micro- and nanoparticles is proposed to achievepercolating thermal underfills with enhanced particle contacts.The three main process steps hereby are assembly of fillerparticles by centrifugation, formation of nanoparticle necksby capillary bridging, and the backfilling of the porousstructure with an unfilled capillary adhesive.Numerical simulations predicting trajectories anddistributions of micron-sized particles dispensed into arotating disk are presented. The trajectories exhibit a strongdependence on the particle size; thus in the case ofpolydisperse filler particles nonuniform particle beds mayresult. An efficient centrifugal disk design with spiral-likeguiding structures is experimentally validated. Defect-free,percolating particle beds in confined space with fill fractionsof 46 vol-% to 66 vol-%, i.e., close to the theoretical limit, arealso presented.The self-assembly of nanoparticles, forming enhancedthermal contacts between the percolating filler particles, isdiscussed. Two consecutive evaporation patterns during thecapillary bridging process were identified: 1) dendriticnetwork growth and 2) collapse of capillary bridges. Theconcave neck topology could only be achieved attemperatures below the boiling point. An optimal evaporationtemperature of 60°C with respect to in-plane uniformity andneck shape was identified. Existing thermal gradients normalto the cavity surface resulted in strongly asymmetric neckformation in the cavity. Hence, uniform heating in an oven isthe preferred method to initiate evaporation. Two types of bimodaldielectric necks are demonstrated. Polystyrene acts asthe adhesive between thermally conductive alumina particlesto form mechanically stable dielectric necks after anannealing step at 140°C. Interstitial and core-shell necks arepresented.Finally, a benchmark study was performed to compare theeffective thermal conductivity of the percolating thermalunderfill with and without necks with state-of-the-art capillaryunderfills. A close to fivefold improvement could be obtainedfor diamond filler particles with silver necks (3.8 W/m-K).