Anisotropic conductive film (ACF) has been used for years to create 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.

ACF bonding 01 (cr) 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.

ACF bonding 02 (cr) Figure 2: Position of the thermode, flex and adhesive in the ACF process

The joint is stabilised 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.

ACF bonding 03 (cr) Figure 3: Scanning electron microscope image of a single hollow conductive particle compressed between two conductive tracks.

The scanning electron microscope (SEM) image shows one hollow conductive particle squeezed between two conductive tracks. The actual particle size is about 5μm. Normally there are between 100 to 1000 particles involved in one connection.

The adhesive usually consists of a mixture of thermoplastic and thermohardening (also called thermoset or duroplast) glues, to get the best of both substances’ properties.

The conductive particles can be either massive conductive particles, massive plastic particles coated with conductive material, or hollow plastic particles coated with conductive material. They are most frequently made of graphite, gold or gold-plated plastic, each of which has specific advantages and disadvantages.

Graphite particles are sharp, which can be a benefit if one of the materials being connected has a thin isolating oxide layer. One disadvantage is that that the particles are not elastic, causing higher resistance. Graphite particles are also hygroscopic (they absorb moisture from the air), which can influence the glue matrix and cause corrosion between the contacts.

Gold is not hygroscopic, and contact resistance is lower than graphite particles. However, gold can be more expensive than carbon.

Gold-plated plastic particles are compressible, so contact resistance is lower, because there can be more surface contact between the upper and lower track. The gold-plated particle also works as a spring: a small relaxation of stress in the glue is compensated by an extension of the particle, resulting in an extra safe connection.

 
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