ACF vs FPC: Anisotropic Conductive Film in Flex Assembly

An anisotropic conductive film is a bonding material, not a circuit board. It is a thin adhesive loaded with conductive particles that conducts in one direction only, so a row of fine pitch pads on a flex circuit can be joined to a rigid board without solder and without any risk of bridging between neighbours. ACF and FPC are therefore not alternatives: the film is the material that attaches the flexible circuit.

How the Film Conducts

The adhesive contains metal coated polymer spheres, typically 3 to 10 µm in diameter, dispersed at a controlled density. When the film is pressed between two pads, the particles are trapped and crushed, forming a conductive path through the thickness of the adhesive. In the plane of the film there are far too few particles to connect adjacent pads, so the material is anisotropic by design.

The resin then cures and holds the joint together mechanically. Conduction depends on the particles remaining compressed, which is why the cured adhesive must retain its mechanical properties through thermal cycling and why the bond is sensitive to the pressure used during assembly.

Where ACF Is Used

The technique dominates display and camera module assembly, where a flex tail is bonded to a glass panel or a small rigid board carrying dozens of fine pitch connections in a very small area. It is also used for touch panels, sensor modules and any product where a connector would be too tall or too expensive.

Compared with a board to board connector, ACF saves height and cost, and it cannot vibrate loose. Compared with soldering, it avoids the thermal load of reflow on a heat sensitive assembly, which is important when the flex is bonded to a display or a component that cannot survive an oven.

Bonding Process and Parameters

Bonding happens in two stages. The pre-bond tacks the film to the substrate with light pressure and moderate heat, which holds the alignment while the parts are transferred. The main bond applies the full temperature, pressure and time through a heated bar, and then the assembly is cooled under load so the adhesive sets before the pressure is released.

Typical main bond conditions sit between 150 °C and 200 °C at the interface, with bonding pressure in the range of 1 to 3 MPa and a dwell of 5 to 15 seconds. The values depend on the film specification and on the stiffness of the parts, so the supplier’s recommended window should be treated as the starting point and refined on the actual assembly.

Substrate and Pad Requirements

The pads must be flat and coplanar, because the particles only deform if the gap between the two surfaces is uniform. Solder mask should be thin and pulled back from the bond area, and the copper surface should be finished with a thin gold layer rather than a thick solder coating that would leave an uneven surface.

Pad height variation across the bond row should stay within a few micrometres, and the flex circuit should not carry plated through holes or stiffeners inside the bonded region. Alignment marks on both parts are essential, because the bonding tool cannot see the pads once the film is in place. The geometry rules are the same as for any fine pitch interface, as described in the notes on PCB pad design standards.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/20L-4阶HDI-无镍-钯金PCB.jpg" alt="Hot bar bonding head pressing a flex circuit onto board pads” />

Pitch Limits and Particle Size

At a 0.2 mm pitch, conventional films with 5 to 8 µm particles work reliably. Below about 0.15 mm the pad width becomes comparable to the particle size and the risk of a particle bridging two pads rises, so finer films with smaller particles and a lower particle density are required. The alignment accuracy of the bonder then becomes the limiting factor rather than the film.

The particle density also affects the number of particles captured per pad. Too few and the joint resistance varies from pad to pad; too many and the risk of lateral conduction appears. Suppliers specify a capture count per unit area for each film, and that figure should be checked against the smallest pad on the design.

Failure Modes and Inspection

An open joint comes from too few captured particles, insufficient pressure or a cured adhesive that has relaxed and released the compression. A short comes from excess particles or from a particle displaced sideways during bonding. Delamination and bubbles come from contamination, moisture or a bond performed outside the temperature window.

Inspection is a challenge because the joint is buried under the flex. Daisy chain test structures on the flex confirm continuity, and cross sectioning a sample shows particle deformation and fill. Shear and peel tests quantify the mechanical strength. X-ray inspection, which works so well on solder, gives little information here. Protective measures applied after bonding, such as the options described for conformal coating, help against moisture but will not repair a weak interface.

Design and Sourcing Considerations

Leave room for the bonding bar. The tool needs to reach the bond area without touching components, and it needs a flat backing on the other side of the assembly. Plan the keep-out area on both parts before the layout is finished, because moving a component after tooling has been made is expensive.

Thermal expansion differences between the flex and the substrate load the joints during temperature changes. Keeping the bond area short, using a flex material with a matched expansion coefficient and avoiding large temperature excursions all help. Rework is difficult: the cured film has to be removed mechanically and a fresh piece bonded, which usually means the flex tail must be replaced rather than reused.

ACF Compared With a Connector

A board to board connector is removable and can be replaced in the field, which is decisive when a flex tail or a display is likely to be serviced. An ACF bond is permanent. For products that are repaired rather than replaced, that difference often outweighs the cost and height advantages of bonding.

Connectors also tolerate wider tolerances. Pad height variation, surface finish and alignment all have more margin, and the assembly process is simpler because no heated tool is involved. ACF wins when the connection count is high, the available height is a fraction of a millimetre or the parts cannot survive soldering.

Price follows the same pattern. Film and bonding tooling add cost, while connectors add height, insertion force and a second set of joints to inspect. The decision should be made with the assembly house before the layout is frozen, because the pad geometry and the keep-out area differ between the two approaches. Documentation practices for either route follow the manufacturable design guidelines.

FAQ

Can an ACF joint be reworked? Only by removing the flex and re-bonding with new film. The cured adhesive cannot be softened selectively, so the original tail is normally scrapped rather than reused.

Is ACF suitable for high current? No. Each joint carries only a small current through a few micrometre scale particles. ACF is used for signals and low current supplies, not for power connections.

Why is the surface finish so important? The particles must deform against a flat, hard surface. A thick or uneven coating leaves gaps that reduce the number of captured particles and raise the joint resistance.