Ph.D. Dissertation Defense by
Monday, November 28, 2005
(Dr. Jianmin Qu, Chair)
"Electrical and Thermomechanical Modeling and Reliability of Electrically Conductive Adhesives "
Isotropic electrically conductive adhesives are viewed as a replacement for traditional tin-lead solders. Still, before conductive adhesives can be widely used, their electrical conduction mechanism and reliability issues under harsh environmental conditions need to be fully understood.
The first part of the dissertation focuses on the understanding and modeling of conduction mechanism of conductive adhesives. The research starts with an investigation of the contact resistance between filler particles in 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 the electrical conduction in conductive adhesives. The bulk resistivity of conductive adhesives is calculated from the computer-simulated model and is found to agree well with experimental measurement. 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. Geometric parameters of the filler particles that have significant impact on the overall electrical conductivity are identified for conductive adhesives with spherical and flake particles.
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. Fatigue tests are performed on conductive adhesive samples. It is found that the electrical conduction failure occurs prior to mechanical failure. The experimental data show that electrical fatigue life model can be described well by the Coffin-Manson equation. The fatigue strain amplitude, strain ratio and strain rate all affect the electrical fatigue life. 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 bulk resistivity is found not to be affected by the moisture absorption, but 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 with 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 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.