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An Overview of Isotropic Conductive Adhesives Filled with Metal-coated Polymer Spheres
Keywords: isotropic conductive adhesives, metal-coated polymer spheres, characterization
Isotropic conductive adhesives (ICAs) have been commercially available as an alternative to solder interconnects in electronic packaging for decades. An ICA consists of a non-conductive adhesive matrix filled with conductive particles. Traditional ICA compounds are loaded with 25-30 vol% (up to 80 wt%) of solid Ag particles (typically in flake-form) to ensure the required electrical conductivity. Such a high Ag particle loading causes significant changes in the mechanical properties of the adhesives, including increased bulk modulus and reduced ductility (more brittle response). A novel idea for reducing the brittleness, and hence improving the reliability of the adhesive, is to replace the solid Ag particles with metal-coated polymer spheres (MPS). The idea is supposed to reduce local stresses concentrated at the interfaces between the conductive particles and the epoxy matrix, due to better match in the mechanical properties, such as CTE and elastic modulus. A number of studies have recently been carried out to characterize the rheological, the mechanical and the electrical properties of ICAs filled with MPS. In addition, the reliability of the ICAs under rough environments has been investigated. MPS-based ICAs have been developed based on an epoxy matrix filled with Ag-coated monodisperse polymer spheres (AgPS). A particle volume fraction of 45 % has been selected to represent a complete ICA filled with MPS, based on the characterization of effects of particle fraction on the properties of an epoxy filled with polymer spheres. Interesting results are to be summarized in the paper. + The rheological properties of the ICA with 45 vol% AgPS were found to be suitable for stencil/screen printing and dispensing processes. + The AgPS-based ICA exhibited improvement in die shear performance over conventional ICAs filled with Ag flakes. Furthermore, the mechanical strength at elevated temperatures was satisfactory, showing no dramatic decrease at the glass transition temperature of the novel ICA. + The studies of the electrical properties of ICAs filled with AgPS showed a percolation threshold in the range of 24-33 vol%. Importantly, the electrical conductivity of an adhesive containing 45-50 vol% of AgPS was close to that of conventional ICAs filled with Ag flakes, even though the Ag content was significantly lower. Effect of Ag coating thickness on the electrical properties was also investigated, in both macroscopic ICAs and microscopic conductive particles. + With regard to the reliability in environmental stress tests, an excellent impact performance of MPS-based ICAs was observed. Interconnects based on ICAs with 50 vol% MPS passed a firing test where test structures were exposed to an acceleration of more than 60000 g. These results are promising with regard to the reliability of ICAs based on MPS. In summary, MPS-based ICAs have been found to exhibit excellent impact strength, improved mechanical shear strength, sufficient electrical conductivity and a good potential for processing. Therefore, these novel ICA materials have the potential for applications in rough environments, in which electronic devices are subjected to thermal shock and thermal cycling, mechanical vibration and mechanical shock (e.g. drop).
Hoang-Vu Nguyen,
IMST-Department of Micro and Nano Systems Technology, HiVe-Vestfold University College
Borre, Vestfold

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