Characterization of anisotropically conductive adhesive interconnections by 1/f noise measurements

Today there is growing interest in using anisotropically conductive adhesives (ACAs) not only for interconnections between liquid crystal displays and printed circuit boards but also in contacts conducting higher currents such as flip-chip and rigid-flexible interconnections, for example in automotive applications. 1/f noise measurements are commonly known as a diagnostic tool to investigate the lifetime of chip metallizations. Resistance fluctuations (which mean noise) are induced by electron mobility fluctuations due to, for example, scattering on moving atoms (electromigration). Another source of noise is resistance fluctuations that are dominated by the current constriction in a point contact. Both mechanisms play a role in ACA contacts. A technology to prepare anisotropically conductive interconnections with only a few conducting particles per contact to separate different failure mechanisms and to realize the noise measurements is presented. Samples manufactured in this way are exposed to current. The noise is measured before and after these damaging processes. Our measurements show that anisotropically conductive contacts exhibit a transition from a mixed film/spot contact behaviour to film dominated contacts when the gap between the contact pads is increased through the point where it surpasses the diameter of the conducting particles. After current damaging we find a noise spectrum consisting of a 1/f portion and an additional 1/f/sup 2/ component, which is relaxing during a following zero current period. The increase of noise power after damaging is significantly higher than the increase of resistance.