Anisotropic conductive adhesives (ACAs) are a group of electrically conductive adhesives that exclusively conduct across the bond-line. Filler loadings are reduced (to about 1-5% by volume) compared to isotropic adhesives and fillers are normally spheres with a diameter the exact thickness of the intended bond-line. In this way, when the the interface materials are pushed together the bond-line is reduced so that the filler particles bridge the interface gap.
ACAs can either be provided as a thin film material, or as a liquid resin. Films are typically several hundred microns thick and the filler particles are already aligned to provide a bridge between substrates. Liquid systems, on the other hand, will align themselves on application.
First the liquid resin is placed between two substrates. At this point, filler particles are disorganized and will not conduct across the bondline.
Then the substrates are pushed together, which will begin to align the filler particles.
Finally, once the interface gap is reduce to the diameter of the filler spheres, the particles will bridge the interface gap.
Liquid anisotropic adhesives are usually silver filled epoxy resins. Although these material are typically less costly than isotropic alternatives they require a precision assembly process and don't conduct 3 dimensionally (only conduct along the z-axis), and as a result are more rarely used.