Abstract
The effect of mixing process using roll mills on electrical property of conductive adhesives filled with silver particles was investigated in this work. Evaluation of different mixing conditions was based on the measurement of apparent critical volume fractions of silver particles. Our objective was to find out the best mixing process in order to obtain conductive adhesives with the least amount of conducting particles. Our results indicated that the use of three-roll-mill or heating slightly during mixing produced better results. It suggested that a more uniform mixture was achieved by enhancing the movement of particles, especially along the axial direction. The effect of grinding particles was also studied. We found that the addition of a small amount of ethylene glycol helped the uniform dispersion of Ag particles in the resin system. As a result, the apparent critical volume fraction decreased to 1.24vol%. In addition, the apparent critical volume fraction could be further decreased to 2.29vol% when we used nickel filaments having high aspect ratio as conductive fillers. In regards to the conduction mechanism, from the analysis of theory, the gap between conductive particles in our system was so large that the probability of electrons tunneling through the gap was extremely small. It was suggested that the conduction mechanism of conductive adhesives should be the particles, which connected to form conduction path. In the analysis of the effect of film thickness, we obtained the following equation to relate volume fraction of particles（Vc） with critical film thickness（or coherence length, t）. This equation also demonstrated satisfactory correlation to the data from literature.