The fundamentals of Anisotropic Conductive Film (ACF)

What is anisotropic conductive film (ACF)?

Anisotropic Conductive Film (ACF) serves as an adhesive for securing and electrically connecting electronic components, like integrated circuits (ICs), to circuit boards. Originally introduced by Sony Chemicals (now Dexerials Corporation) in 1977, ACF is now a staple in nearly all digital devices that employ flat panel displays—think smartphones, tablets, and high-definition TVs.

ACFs are typically used to connect the display panel to the flexible substrate that transmits signals to the panel or to the IC. Common applications of ACF include:

• COG (Chip-on-Glass), which connects the IC and glass substrate
• FOG (Flex-on-Glass), which connects the flexible substrate and glass substrate
• FOB (Flex-on-Board), which connects the flexible substrate and rigid substrate

Other applications of ACF extend to smart cards, camera modules for CCDs (charge-coupled devices), and CMOSs (complementary metal oxide semiconductors).

How ACFs can bond, conduct, and insulate

ACFs serve three main functions: adhesion, conduction, and insulation. One of the significant advantages of ACFs is their ability to connect a large number of pads simultaneously, enabling finer pitch connections than the traditional soldering method used for bonding electronic components. Moreover, ACFs facilitate bonding at lower temperatures, ranging from 110°C to 180°C.

ACFs are composed of conductive particles dispersed in a thermoset resin. There are various types of conductive particles in ACFs to cater to different needs. A typical conductive particle structure includes a polymer coated with nickel or gold, which conducts electricity efficiently, further covered by an insulating coating. When heat and pressure are applied, the opposing pads capture the conductive particles, breaking the insulating coating to establish electrical connections between the pads.

Particles not sandwiched between pads are transferred between pads within the base resin of the film, maintaining the insulating coating and preventing short circuits. In the figure below, the electricity between the pad and the substrate is conducted only in the vertical direction.

The ACF structure and bonding process

Here is the ACF bonding process:

Step 1: Clean the surface of the board that will be attached.
Step 2: Apply heat and pressure to ACF with the release liner still attached to it.
Step 3: Peel off the release film.
Step 4: Align the pads of the IC chip to ACF.
Step 5: Apply heat and pressure again.

Dexerials’ ACFs are sold in reels, as shown in the photo below. They come in a wide range of film thicknesses from 10 to 45μm, widths from 0.5 to 20mm, and lengths from 10 to 300m.
The figure below illustrates two types of ACFs: a three-layer type, where ACF is sandwiched between a protective film and a release film, and a two-layer type where only the release film is used. Each type offers different benefits. The three-layer type reduces the risk of dust contamination, while the two-layer type eliminates the need to remove the protective film during bonding.

The advantages of bonding components with ACFs

As technology advances each year, circuit boards are becoming increasingly fine pitch, the area of connecting pads is shrinking, and the spacing between them is narrowing. While reflow soldering and connector components are traditionally used for bonding electronic components such as ICs to circuit boards, ACFs provide a solution to these trends.

However, a disadvantage of ACF is that it is challenging to bond components of different shapes simultaneously, which is common in reflow soldering. Also, since ACF is an adhesive, it cannot be freely removed and reattached like mechanical connectors.

Benefits of ACF

• Can bond components on glass substrates
• Enables batch connections of multiple pads
• Enables fine pitch connections
• Pb-free
• Quick bonding at relatively low temperatures
• Thinner bonding area

The history of ACF and its innovation

Dexerials has been continually enhancing ACF since its introduction in 1977. Initially, the conductive particles used were carbon fiber and solder particles. However, in 1988, Dexerials developed particles plated with nickel and gold. By the 1990s, Dexerials had successfully developed a technology to coat the surface with insulating material.

As digital devices have evolved towards higher definition, Dexerials has adapted to the trend of fine pitch connections by reducing the particle size from 5μm to 2.8μm. In 2014, Dexerials developed a technology to uniformly align conductive particles in thermosetting resin, and in 2016, began marketing the product as “particle-arrayed Anisotropic Conductive Film” (ArrayFIX). This product contributes to the miniaturization, slimming design of digital devices, and higher resolution displays.

Selecting the best type of ACF

When selecting the best ACF, several factors should be considered, including the type of adherend, connection area, distance between pads, height of pads, and heat resistance.

ACF is composed of either epoxy resin or acrylic resin that cures with heat. Dexerials recommends acrylic ACF for applications where Flexible Printed Circuits (FPCs) and substrates (PCBs or glass substrates) are to be bonded, and where the design is intended to be reworked after the main bonding. (Please note that Dexerials does not recommend reworking itself. Dexerials advises that rework should only be done after reliability evaluations have been conducted to ensure that there are no issues.) Dexerials does not recommend ACF with epoxy resin if there is a possibility that the customer will rework. The primary reason is that it often damages the substrate to be reworked. This is because cured epoxy resin does not dissolve in solvents, and the process involves scraping off the residue.

Reworking after bonding typically involves heating the bonding area with a heat gun, hot iron, hot plate, etc., and then peeling off the FPC. Afterward, the cured resin residue between the pads on the board should be carefully removed using a solvent such as Methyl Ethyl Ketone (MEK) or N-Methyl-2-pyrrolidone (NMP) and a cotton swab. However, Dexerials does not recommend reusing FPCs because FPCs often curl during the peeling process and cleaning the resin residue between the pads on the FPC side is challenging.

With over 40 years of leadership in ACF technology, Dexerials can provide services including the initial setup, proposing an appropriate ACF, and post-connection analysis and evaluation. If you have any questions about ACF application, please feel free to contact Dexerials.