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Vertical Interconnects using Magnetically Aligned Anisotropic Conductive Adhesive for RF Packaging
Keywords: RF packaging, Anisotropic conductive adhesive, Magnetic alignment
A novel anisotropic conductive adhesive (ACA) with ferromagnetic conductive particles is studied for vertical interconnects in high frequency applications. When the static magnetic field is induced, randomly dispersed conductive particles in an epoxy are aligned along the given magnetic field. Simultaneously, the created columns are relatively uniformly distributed horizontally due to the mutual interaction of magnetic polarity. Therefore, the anisotropic interconnects built by the magnetic alignment allow for current to flow only vertically. The ACA has several advantages over the conventional technologies such as wire bonding and solder bumping. It is lead-free, low temperature, thin-layer bonding which does not need patterning. By utilizing these features, the packaging based on the magnetically aligned ACA helps to achieve the performance required for highly integrated RF circuitry. To verify the RF characteristics of the ACA, coplanar waveguides (CPW) on silicon substrates with high resistivity were fabricated. Using a flip-chip bonder with +/- 5 m alignment accuracy, silicon substrates with a 100 m x 80 m transition region were assembled using the magnetically aligned ACA. The number of created columns on the transition region dominates the performance of assembled samples, as verified using measurements. Specifically, the S-parameter repeatability of assembled CPW lines on assembled silicon substrates strongly relates to the diameter of the conductive particles. It was also observed that the performance of assembled samples using the smaller particle (10 m average diameter) is more repeatable. In comparison to the silicon sample with solder bumps, the difference in the insertion loss is less than 0.5 dB up to 30 GHz. Therefore, the magnetically aligned ACA is expected to become a promising solution for high-frequency applications with less than 100 m pad dimension and narrow pitches.
Sungwook Moon , Graduate Student
Purdue University
West Lafayette, IN

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