Anisotropic Conductive Film (ACF) bonding: effect of interfaces on contact resistance

Anisotropic conductive film (ACF) is used for finepitch bonding in liquid crystal display (LCD) packaging. The film is typically composed of 3-10 μm conductive particles incorporated into an epoxy based film matrix. Smaller particles (e.g., 3 μm) are used to obtain a finer pitch, driven by a demand for higher resolution. Using smaller particles leads to a reduction in the effective contact area resulting in an increase in the resistivity. Smaller particles also exhibit a reduced tolerance for variations in the bond line thickness which may affects the repeatability of interconnects and reliability. In this paper, we investigate the relation of particle deformation to the resistance of both free particles (electromechanical test) and particles embedded in an adhesive film (ACF bonding). The presence of adhesive film has resulted in a much higher particle contact resistance and affected interconnect repeatability. The effect of the adhesive film is strongly dependent on the bump surface topography, and a smooth surface tends to increase the contact resistance.