Multiangle approach for electromechanical characterization of silver coated polymer particles

Conventional isotropic conductive adhesives (ICAs) have been around for decades as an alternative to soldering in many types of electronic interconnects. The conductive pathways in such interconnects are usually provided by silver flakes embedded in an epoxy matrix for mechanical support. By replacing the solid silver with micron sized polymer spheres coated with hundred nanometers of silver, the total amount of silver needed to obtain conductive pathways throughout the epoxy matrix can be significantly decreased. The similar mechanical properties of the polymer cores and the surrounding polymer matrix of the adhesive will also reduce the effect of the mechanical mismatch between silver and polymer seen in conventional ICAs. The mechanical and electrical properties of the individual, silver coated polymer particles are thus important factors influencing the functionality of the bulk ICA. In this work a multiangle approach for investigating the mechanical and electrical properties of individual particles will be presented. At NTNU Nanomechanical Lab, a flat punch-based nanoindentation method previously developed to investigate the mechanical properties of polymer spheres has been extended for measuring the coupled electrical and mechanical properties of individual particles by customizing the nanoscale Electrical Contact Resistance (nanoECR) module of a Hysitron TriboIndenter 900. At NTNU NanoLab, micromanipulators (Imina miBots) capable of movement in 4 dimensions with nanoscale resolution can be inserted into FEI’s DualBeam FIB/SEM system. These miBots have been used for directly probing the electrical properties of individual particles while the process is monitored in real-time by high resolution SEM. The FIB can be utilized to customize the geometry and dimensions of the tungsten tips used for probing the particles. In both these techniques, the contact resistance between the probe and particle will be of importance to the total measured resistance. The roughness of the grains comprising the silver coating will yield a smaller real contact area than the apparent contact area between probe and particle, which will influence the contact resistance, especially at small deformations. By using the PeakForce Quantitative Nanomechanical Mapping (QNM) mode of the high-resolution Bruker AFM available at NTNU NanoLab, quantitative maps of surface topography, adhesion, and many other surface properties of the particles can be obtained.