With the driving force of enhanced performance and reduced size of electron system, replacing discrete passive components with embedded passives becomes crucial for the next generation electronic packaging. Among all integral passives, embedded capacitors call for special attention due to their wide applications in signal de-coupling, noise suppression, filtering, and tuning. Embedded capacitor technology not only offers increased silicon packaging efficiency and reduced assembly cost, but also improved electrical performance. To incorporate capacitor into the substrate requires the development of high dielectric, low loss, and printed wiring board (PWB)-compatible materials for capacitors, which is one of the major challenges for materials scientists working on embedded passives. Filled polymer composites, e.g. polymer-ceramic composites are promising methods to fulfill such requirements as polymer provides good processability and filler offers desirable electrical properties. However, for ceramic filled composites, to achieve dielectric constant over 100 usually requires very high ceramic loading level, i.e.～95 wt%, which will pose difficulties in sample preparation and lead to poor mechanical properties. It's imperative to find new candidate materials for embedded capacitors. This paper presents the development of a novel aluminum-filled high dielectric constant composite. Aluminum is well known as a fast self-passivation and low-cost metal. The thin passivation layer forms a boundary layer outside of the metallic spheres, which has dramatic effects on the electrical, mechanical, and chemical behaviors of the resulting composites. Influences of aluminum particle size and filler loading on the dielectric properties of composites were studied. Due to the self-passivation nature of fine aluminum spheres, high loading level of aluminum can be used while the composite materials keep being insulating. Dielectric property measurement demonstrated that, for composites containing 80 wt% 3.0 fun aluminum, dielectric constant of 109 and low dissipation factor of about 0.02 (@10 KHz) were achieved. At such loading level, materials still show good processability and good adhesion toward the substrate. Bulk resistivity measurement, high resolution transmission electron microscope (TEM) observation, and thermogravimetric analysis (TGA) were conducted to characterize the aluminum powders. Dielectric mechanism of the materials was discussed, based on the comparison with aluminum oxide-filled composites. Bimodal aluminum filled composites were also systematically studied to optimize the dielectric constant. Rheology study was performed to find the bimodal weight ratio that gives the lowest viscosity at the same loading. Such bimodal weight ratio leads to the highest loading level for the specific system. A dielectric constant of 160 (@ 10 KHz) was achieved with optimized bimodal aluminum composites.
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