This work presents enhanced composite joints that support both electrical or thermal transport in electronic packages. The joints are sequentially formed by applying a nanoparticle suspension, evaporating a solvent, self-assembling of nanoparticles by capillary bridging, and the formation of so called “necks” between micron-sized features. This sequence is used to either form low temperature electrical joints under copper pillars or enhanced percolating thermal underfills with areal contacts between filler particles of the composite. The report discusses processing aspects of the capillary bridges evolution and of uniform neck formation, it discusses strategies to achieve mechanically stable necks, and it compares the performance of the achieved joints against state-of-the-art solutions.
The capillary bridge evolution during liquid evaporation was investigated in copper pillar arrays and random particle beds. The vapor-liquid interface first penetrates locations of low pillar or particle density resulting in a dendritic fluid network. Once the network breaks up individual necks form. For aqueous nano-suspensions highly uniform necks with high yield were obtained by evaporation at 60°C. Isothermal conditions were preferred to yield equal neck counts at the cavity’s top and bottom surfaces. Mechanically stable silver necks required an annealing at only 150°C, dielectric necks an annealing at 140°C with a bi-modal approach. Therein polystyrene nanoparticles occupy interstitial positions in densly packed alumina necks, then melt and adhere to the alumina. The electrical necks showed a shear strength of 16 MPa, equivalent to silver joints used in power electronic packages. The thermal necks yielded a thermal conductivity of up to 3.8 W/mK, 5-fold higher than commercially available capillary thermal underfills.