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

There is growing interest in using anisotropically conductive adhesives (ACAs) not only for interconnections between LCDs and PCBs but also in higher current contacts such as flip chip and rigid-flex interconnects, for example in automotive applications. 1/f noise measurements are a common diagnostic tool for investigation of chip metallization lifetime. Resistance fluctuations (which mean noise) are induced by electron mobility fluctuations due to, for example, scattering on moving atoms (electromigration). Another noise source is resistance fluctuations that are dominated by 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 are exposed to current, and noise is measured before and after these damaging processes. Measurements show that anisotropically conductive contacts exhibit a transition from mixed film/spot contact behaviour to film dominated contacts when the gap between the contact pads is increased, transgressing the diameter of the conducting particles. After current damaging, we find a noise spectrum consisting of a 1/f portion and an additional 1/f² component, which is relaxed during a following zero current period. The increase in noise power after damaging is significantly higher than the increase of resistance. The tests were carried out on FR-4 substrates with Cu traces and Ni/Au metallization. Adhesives were filled with Au particles