Project Abstract

Randomly dispersed ferromagnetic particles in a material can be forced to align due to the presence of an externally applied field. These self-aligning materials are examples through which order is obtained through structure created internally which alters the bulk properties of the material. One such property is anisotropic conduction, in other words selectively passing current in one direction while isolating in all other dimensions. This property comes from self-aligning metallic rods that quickly "grow" inside of the bulk material. The anisotropic nature of the material ensures that only the pads vertically over one another will be connected. Neighboring pads will not be electrically connected because current cannot flow sideways in this medium.

Fig. 1 - A diagram of the metallic particles aligned by the applied magnetic field. Notice that neighboring pads are not connected, demonstrating the anisotropic nature of conduction.

As seen in the above movie the metallic particles quickly respond to the magnetic field forming columns of gold particles. The metal particles are aligned into these columns along the field lines with a regular density. In one-direction the columns conduct current, while not disturbing the electric fields that are parallel to the material. There is no connection between neighboring columns, so no current flows sideways. A wide range of bond (column) lengths from very short up to over 1mm are possible since the rods in the material conform to the geometry. This is in contrast to techniques using pressure or temperature which are constrained in bond length..

These materials have successfully been tested as a replacement for flip-chip bonding silicon to silicon wafers up to 90 GHz. It can be used as a "microwave glue" which easily attaches high density RF interconnects using a technique comparable to epoxying the parts together. This self-alignment material has several advantages over conventional anisotropic conductive adhesives; specifically (1) it does not require pressure in the curing process, (2) it can be applied without interconnect scale patterning, (3) the vertical interconnects grow to the length required, therefore precision height adjustment is not needed, and (4) the interconnects exhibit small electrical parasitics. In particular, a novel self-assembly process was introduced, enhancing the quality of interconnects and reducing packaging cost in fine-pitch electronic circuitry. This self-assembly process creates selectively agglomerated vertical columns on I/O pads and was first demonstrated using the magnetization interaction properties between ferromagnetic conductive particles and I/O pads in the presence of an external magnetic field. Specifically, this self-assembly process was demonstrated to still work at the extremely small pad size of 12 µm x 19 µm as shown in Fig. 2. Therefore, novel high-density interconnects using the self-assembly process are a very promising alternative to conventional technologies for high reliability and power applications.

Fig. 2. (a) Top-view images of self-assembly interconnects on 12 x 19 µm2  pad and (b) resistance change of self-assembly interconnects depending on pad dimension.

The IDEAS lab is working with Eric Hoppenjans from Indiana Microelectronics Company on using these properties to form anisotropic conductive adhesives to bond a variety of materials in high density high frequency systems.

Currently Involved Students

Sungwook Moon (postdoc)

Recent Publications

Journal Papers

[1] S. Moon and W. J. Chappell, “Novel Self-Assembly Process Using Magnetically Aligned Z-Axis Anisotropic Conductive Adhesive for High-Density Vertical Interconnection,” IEEE Trans. Compon. Packag. Manuf. Tech., Jan. 2012, Accepted.

[2] S. Moon and W. J. Chappell, “Analysis of a Failure Rate by Column Distribution in Magnetically Aligned Anisotropic Conductive Adhesive,” IEEE Trans. Comp. Pack. Manuf. Tech., vol. 1, no. 5, pp. 784-791, May 2011.

[3] S. Moon and W. J. Chappell, “Novel Three-Dimensional Packaging Approaches Using Magnetically Aligned Anisotropic Conductive Adhesive for Microwave Applications,” IEEE Trans. Microw. Theory Tech., vol. 58, no. 12, pp. 3815-3823, Dec. 2010.

[4] S. Moon, H. H. Sigmarsson, Y. Huang, T. Bruemmer, S. K. Khanna, and W. Chappell, “Magnetically Aligned Anisotropic Conductive Adhesive for Microwave Applications”, IEEE Trans. Microw. Theory Tech., vol. 56, no. 12, pp. 2942-2949, Dec. 2008.

[5] E. Y. Chow, C.-L. Yang, A. Chlebowski, S. Moon, W. J. Chappell, and Pedro P. Irazoqui, “Implantable Wireless Telemetry Boards for in-vivo Transocular Transmission”, IEEE Trans. Microw. Theory Tech., vol. 56, no. 12, pp. 3200-3208, Dec. 2008.

Conference papers

[1] T.-Y. Lin, B. G. Kim, D. Ha, S. Moon, E. Hoppenjans, and W. J. Chappell, “3D Packaging Technique for a Flexible and Biocompatible Antenna Using Z-axis for Wireless Monitoring of Intraocular Pressure (IOP),” Proceedings of Microelectronic Packaging and Materials for Medical Devices Workshop, IMAPS, Minneapolis, USA, June 10, 2010.

[2] S. Moon and W. J. Chappell, "Adjustable Dielectric Using Magnetically Aligned Conductive Particles for Microwave Applications," IEEE MTT-S Int. Microwave Symp., Anaheim, USA, May 2010, pp. 1288-1291.

[3] S. Moon and W. Chappell, "Column Distribution Analysis and RF Characterization of High-Density Vertical Interconnections Created by Magnetically Aligned Anisotropic Conductive Adhesive," in International Symposium on Microelectronics, International Microelectronics and Packaging Society (IMAPS), San Jose, USA, Nov. 2009, pp. 705-710.

[4] S. Moon, S. K. Khanna, and W. J. Chappell, "Multilayer Silicon RF System-in-Package Technique Using Magnetically Aligned Anisotropic Conductive Adhesive," IEEE MTT-S Int. Microwave Symp., Boston, USA, 7-12 June 2009, pp. 797-800.

[5] S. Moon, T. Bruemmer, S. K. Khanna, and W. Chappell, "High-Frequency Characterization of Self-Aligned Z-axis Interconnects on Silicon substrates," in Union Radio Scientifique Internationale (URSI), Chicago, USA, Aug. 2008.

[6] S. Moon and W. Chappell, "Vertical Interconnects using Magnetically Aligned Anisotropic Conductive Adhesive for RF packaging," in International Symposium on Microelectronics, International Microelectronics and Packaging Society (IMAPS), Providence, RI, USA, Nov. 2008, pp. 970-974.

[7] Y. Huang, X. Gong, T. Bruemmer, S. K. Khanna, and W. J. Chappell, “Magnetically aligned anisotropic conductive adhesive for high frequency interconnects,” in IEEE MTT-S Int. Microw. Symp. Dig., Jun. 2005, vol.1, pp. 861-864.