CHARACTERIZATION OF ANISOTROPICALLY CONDUCTIVE ADHESIVE INTERCONNECTIONS BY 1 F NOISE MEASUREMENTS/

Today there is growing interest in using anisotropically conductive ad hesives (ACA's) not only for interconnections between liquid crystal d isplays and printed circuit boards but also in contacts conducting hig her currents such as flip-chip and rigid-flexible interconnections, fo r example in automotive applications. 1/f noise measurements are commo nly known as a diagnostic tool to investigate the lifetime of chip met allizations. Resistance fluctuations (which mean noise) are induced by electron mobility fluctuations due to, for example, scattering on mov ing atoms (electromigration), Another source of noise are resistance f luctuations 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 co nducting particles per contact to separate different failure mechanism s and to realize the noise measurements is presented, Samples manufact ured in this way are exposed to current. The noise is measured before and after these damaging processes. Our measurements show that anisotr opically conductive contacts exhibit a transition from a mixed film/sp ot contact behavior to film dominated contacts when the gap between th e contact pads is increased through the point where it surpasses the d iameter of the conducting particles. After current damaging we find a noise spectrum consisting of a 1/f portion and an additional 1/f(2) co mponent, which is relaxing during a following zero current period. The increase of noise power after damaging is significantly higher than t he increase of resistance. The investigations have been carried out on FR-4 substrates with Cu tracks and Ni/Au metallization. Adhesives wer e filled with gold particles.