| The electronics packaging and testing industry is interested in new methods for making contacts to electronic chips to allow improvement or replacement of existing interconnect technologies. One approach involves the use of flexible contact structures integrated with the package or the testing apparatus which allow the device to be fully contacted by placing and pressing the interconnect array into contact. This technoΩlogy depends on the properties of low-force electrical contacts. The research presented is a careful characterization of the electromechanical properties of such contacts, using instrumented MEMS force sensors.; This work provides a sensor, measurement system and methodology for low force contact resistance data collection. This research includes the design, fabrication, and characterization of a micromechanical force sensor integrated with a 4-wire electrical contact characterization capability, a set of parametric measurements in the 10nN-10mN regime, and the development of qualitative design rules for small force thin film gold electrical contacts. The sensor consists of a silicon cantilever beam with a piezoresistive force sensor suitable for high-accuracy force measurements in the mN-nN range. The contact tips consist of a glass spheres for a controlled contact geometry as well as polystyrene spheres for highly-compliant contact structures. The contact halves are coated with varying thicknesses of evaporated, sputtered, or plated thin film gold. Combined with AFM scans and nanoindentation hardness measurements, correlations are found between the contact behavior, resistance measurements, and material characteristics over varied contact sizes, film types, thicknesses, and substrates.; Mean stable contact resistance (<10) is achieved with less than 100μN force for 1800Å to 1.2μm thick gold. Differences in contact resistance from Hertzian elastic contact theory are attributed to the presence of asperities on the contact spheres, plasticity in the films, and differences in material properties for thin film vs. bulk form. The two most significant factors affecting contact resistance are film hardness and surface roughness. The contact resistance decreases and adhesion force increases with lowered film hardness and surface roughness. Hardness and roughness are functions of substrate properties, film manufacture method, and thickness. Low adhesion forces and low contact resistance are both desirable and present a design tradeoff. |
