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Column Distribution Analysis and RF Characterization of High-Density Vertical Interconnections Created by Magnetically Aligned Anisotropic Conductive Adhesive
Keywords: RF packaging, Anistropic conductive adhesive, magnetic self-alignment
In this paper, approaches to analyze a magnetically aligned anisotropic conductive adhesive (ACA) are addressed, to create interconnections between a chip and substrate with small pad size. When compared to the conventional ACA, the magnetically aligned ACA has several advantages such as low patristic impedance, no bumping process, and pressure-free bonding without any additional patterning of individual I/O pads bonding during chip assembly. In order to utilize this material for high-frequency applications, it is required to characterize both the column distribution and the RF performance of the vertical interconnections, created by the self-alignment of randomly dispersed conductive particles in the presence of an induced static magnetic field. First, we analyze the spatial distribution of the created columns using an image-processing technique from top-view images of the cured epoxy. The columns size, spacing and density are obtained, and then finally a failure rate of created interconnects is also calculated from their distribution. Using this approach, it is possible to efficiently estimate the optimal pad size for guaranteeing the highly successful rate of created interconnections in a bonding layer instead of direct experiments. In addition, in order to analyze the RF performance of the ACA, the interconnect is modeled as an equivalent lumped components consisting of parasite parallel capacitance, series inductance and resistance. These parasite components are extracted by comparing the ABCD parameters of HFSS simulation results with the measured result of an assembled silicon sample. In conclusion, the imaging-processing technique and the extraction from ABCD parameters enable a better understanding of the characteristics of magnetically aligned ACA. This modeling will also facilitate the development of Z-axis ACA for advanced packaging such as interconnections between fine pitches less than 100 micron and vertical IC stacks for system-in-package.
Sungwook Moon, Graduate Student
Purdue University
West Lafayette, IN

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