Electrical, thermomechanical and reliability modeling of electrically conductive adhesives
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The first part of the dissertation focuses on understanding and modeling the conduction mechanism of conductive adhesives. The contact resistance is measured between silver rods with different coating materials, and the relationship between tunnel resistivity and contact pressure is obtained based on the experimental results. Three dimensional microstructure models and resistor networks are built to simulate electrical conduction in conductive adhesives. The bulk resistivity of conductive adhesives is calculated from the computer-simulated model. The effects of the geometric properties of filler particles, such as size, shape and distribution, on electrical conductivity are studied by the method of factorial design. The second part of the dissertation evaluates the reliability and investigates the failure mechanism of conductive adhesives subjected to fatigue loading, moisture conditioning and drop impacts. In fatigue tests it is found that electrical conduction failure occurs prior to mechanical failure. The experimental data show that electrical fatigue life can be described well by the power law equation. The electrical failure of conductive adhesives in fatigue is due to the impaired epoxy-silver interfacial adhesion. Moisture uptake in conductive adhesives is measured after moisture conditioning and moisture recovery. The fatigue life of conductive adhesives is significantly shortened after moisture conditioning and moisture recovery. The moisture accelerates the debonding of silver flakes from epoxy resin, which results in a reduced fatigue life. Drop tests are performed on test vehicles with conductive adhesive joints. The electrical conduction failure happens at the same time as joint breakage. The drop failure life is found to be correlated with the strain energy caused by the drop impact, and a power law life model is proposed for drop tests. The fracture is found to be interfacial between the conductive adhesive joints and components/substrates. This research provides a comprehensive understanding of the conduction mechanism of conductive adhesives. The computer-simulated modeling approach presents a useful design tool for the conductive adhesive industry. The reliability tests and proposed failure mechanisms are helpful to prevent failure of conductive adhesives in electronic packages. Moreover, the fatigue and impact life models provide tools in product design and failure prediction of conductive adhesives.
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