ACF has been used for years for creating electrical conductive adhesive bonds between flexible and rigid circuit boards, glass panel displays, and flex foils. The process essentially means heating and cooling an adhesive containing conductive particles under pressure. ACF bonding is one of the best methods for bonding fine pitch connections, and is now being used for a growing number of new automotive safety and driver assistance systems. The advent of autonomous driving systems and increasing use of displays and camera’s will only increase this trend, since it will require even more communications and sensor applications.
ACF bonding basics
ACF bonding is the process of creating electrical conductive adhesive bonds, with anisotropic conductive adhesive/film, between flexible and rigid circuit boards, glass panel displays, and flex foils. This interconnection technique is mostly used for connecting to printed circuit boards (PCBs).
The ACF material comes in reels and has three functions: electrical connection, insulation of the adjacent terminal, and adhesion.
Figure 1. ACF bonding, basic principles
Anisotropic conductive adhesives contain small, spherical particles that, when compressed and heated, form an electrical connection between parts. The conductive material in the adhesive can be foil, flex, or paste. The conductive particles are distributed homogeneously, maintaining consistent particle density and thickness.
Before bonding, the particles are separated by an isolating matrix of adhesive. The parts to be joined are first brought together with the adhesive in between and tacked, in a step called ACF laminating.
A heating element (thermode) then presses the top and bottom circuit board together with the adhesive in between, causing the adhesive to flow and trapping the conductive particles, resulting in an electrical connection. The particles that are trapped between the conductors form a conductive interface between the pads on the two mating surfaces and conduct only in the Z axis.
Figure 2. Position of the thermode, flex, and adhesive in the ACF process
The joint is stabilized by subsequent cooling and full curing of the adhesive while still in the compressed condition. Because of the low filler content, there is no short-circuiting between adjoining tracks.