Anisotropic Conductive Adhesives (ACA) for Electronics

What are Anisotropically Conductive Adhesives?

When people look at the liquid crystal display on their cell phone or television, they aren't thinking about how the pictures get there. Even if they were, few would entertain the idea that adhesives play a crucial role in turning electrical signals into images.

And, that's just what anisotropically conductive adhesives (ACAs) do!

By forming films that conduct electricity in one dimension but are insulating in the other two, they can connect matrices of electrodes that pass signals to the individual pixels of an LCDs. These products have been in existence for some time, but innovation continues apace as higher-resolution displays and slimmer electronics shrink the space between electrodes.

ACAs are typically made by filling relatively conventional epoxy insulating adhesives with conductive particles, which are either pure metal or metal-coated polymer. They must cure in seconds under high-temperature, high-pressure, thermocompressive bonding between an IC and display electrodes formed on the glass substrates in the LCD.

The conductive particles can form a connection when they're trapped between these raised electrodes, while the adhesive ensures that each connection is electrically isolated. As technology trends shrink the spacing in the matrix pattern of electrodes, the conductive particle size must be reduced. This poses challenges to formulators in this high-value market so great that some users have turned away from ACA films. However, some great developments have resolved this issue.

Continue reading and find out:

Effective Formulation Strategies for Anisotropically Conductive Adhesive
How to Deal with Performance Challenges

Effective Formulation Strategies for ACAs

Load Small Particle Metal Fillers

To make an adhesive into a conductor, it's usual to load small particle metal fillers into the formulation. In itself, this requires fine scientific judgement. This is because as the proportion of metal increases, so does conductivity, but at the same time adhesive properties diminish. To make matters worse, at high concentrations the metal filler can precipitate out. This risk can be reduced by adding structurally reinforcing fillers to the formulation. But, essentially to devise a conductive formulation demands compromise on adhesive characteristics.

Metal, Metal-coated Polymer and Insulator-coated, Metal-coated Polymer Filler

For anisotropic conductive films (ACFs,) metal contents are optimized in reactive, solvent-free, epoxy formulations.

Thermocompression Bonding of an Anisotropically Conducting Film ACF

The epoxy adhesives used for ACFs might typically be loaded with around 20 percent by weight of 4 μm diameter metal particles. Adding up to ten percent of a non-conducting filler, such as 0.8 μm diameter silica, helps improve an adhesive's anisotropy.

Changing the Rules

One of the developments that helps ensure that an adhesive remains anisotropic conducting are specifically developed insulator-covered conducting particles. These bizarre sounding particles consist of polymer coated in a mixture of nickel and gold, that is then coated with a further insulator layer. The insulator cover layer is fragile, and under the bonding pressure between the bump and substrate it breaks. This leaves the newly uncovered conductor particles in the compression zone, while in the remaining spaces the filler still behaves as an insulator.

Using such fillers has shown to reduce the minimum pitch - or distance in-between bumps - that specific displays can go down to without shorting from 15 μm to 10 μm.

Correlation b/w Anisotropically Conductive Adhesive Layer Thickness & Number of Particles on Bump

Correlation between Anisotropic Conductive Adhesive Layer Thickness and
Number of Particles on Bump in a 25 μm Thick Double-layer ACF

Another way of improving anisotropic conductivity is to dispense the adhesive in two separate parts. In such a double-layer arrangement the conducting adhesive is first placed onto the driver IC, and then a more conventional non-conductive epoxy is laid on top. The reasoning here is that when this assembly is cured under thermocompression, the conductive adhesive collects on the bumps as they are forced up through the dual adhesive layer. In the spaces between the insulating conductive filler is diluted, reducing the chances of any electrical shorting.

For such an arrangement the adhesive and associated dispensing equipment must be well tuned for the tiny dimensions involved. The gap between the IC and the glass of the LCD above would typically be only 25 μm in depth. Within this, carefully modifying the thickness of non-conductive and conductive adhesives dispensed can optimize the amount of conductive filler. Experiments suggest that a 7 μm conducting layer and an 18 μm thick non-conducting layer best balances ease of manufacture and best electrical performance.

Voids Avoid Trouble

100-400 nm spheres of insulating materials like cross-linked acrylic can be attached to 3-10 µm diameter conductive particles by hybridization processing at 16,000 rpm. Here, the mechanical-thermal energy of impact between the particles welds them together, creating 200-300 nm thick films 20-40 percent of whose volume is nothing but air-filled voids. The voids make the spheres easier to penetrate and lowers the resistance of the connection, while keeping the conductive particles insulated from each other.

As could be seen from the figure below:

Insulating Film Thickness v/s Electrical Resistance and Conductor Spacing

Correlation b/w Insulating Film Thickness and
a. Electrical Resistance b. Conductor Spacing at the Time of Short Circuit

Processing particles for 20 minutes provides the conventional insulator coating, labeled A. Reducing this to 10 or three minutes provides layers with voids, labeled B and C respectively. While both coatings B and C show lower resistance than coating A when exploited in an adhesive in a test connection, coating C can spontaneously peel off. This contributes to short-circuits at larger electrode spacings than seen with coating A. By contrast, coating B adheres to the conductive particle sufficiently to prevent short circuits while minimizing resistance in the connection between electrodes.

ACA films can also be kept working with finer-pitched electrode matrices through control of an insulating filler used in them alongside conductive fillers. Such silica, alumina and titanium dioxide particles are familiar in many formulations. Usually they serve to reduce the linear expansion coefficient and the water absorbing properties of adhesives.

If insulating filler particles are too much smaller than the conductive filler particles, then they can obstruct the path between the electrodes and prevent a connection being formed. However, if the conductive particles are too much bigger, they can cause short-circuits by bridging electrodes. Nevertheless, careful control over the size and amount of each particle used in an epoxy-based adhesive make it possible to ensure reliable connections.

Selecting Conductive Fillers for Adhesives & Sealants

Dealing with the Challenges

Insulating Filler Issues

Adhesives containing 0.5µm diameter insulating silica particles and 0.6 µm diameter conductive particles, produced by electroplating insulating particles with metal is a well known development. A mixture of these fillers, which could contain from 10-50 percent conductive filler, included at a proportion of 25% by weight in epoxy resin showed a good balance between desired conductivity and insulation properties. Increasing the proportion of filler - this time specifically containing 50% conductive particles - in the adhesive to 50 and 70 percent additionally can improve the adhesive's reliability.

The adhesive can mount and seal a semiconductor chip on a circuit board simultaneously by a single thermocompression bonding treatment. This means that the connection is not again affected by the application of heat and pressure after connection, reducing the number of manufacturing steps and improving the yield.

Micrograph of an Aluminum Foil by both Conducting and Insulating Filler Particles

Micrograph of an Aluminum Foil Formed on a Glass Panel after Thermocompression Bonding
with an Adhesive Containing Conventional Insulating Filler

A downside of using insulating fillers in ACAs comes because the electrodes that are used to connect are typically opaque metal mounted on glass sheets. To check if a connection has been made, the electrodes can be inspected through the glass sheet, with impressions made by the conductive particles providing confirmation. For especially fine pitch electrodes it is conceivable that a single impression could provide this visual confirmation. The extra insulating particles added to provide suitable electronic performance also make impressions. As a result, they provide false positives in the visual inspection.

Consequently, formulations with polymer-based insulating particles in a urethane-acrylate insulating adhesive have been used. These polyester or polyamide insulating particles melt and dissolve when the film is cured at 180°C, 3 MPa for 6 seconds. Only conductive particles can therefore cause impressions. The adhesive was still able to provide adequate bond strength and appropriate electronic properties for an ACA.

Manufacturing Issues

Outside of these electrical considerations the manufacturing issues surrounding LCDs and similar products will be familiar to most adhesive formulators. For example, the assemblies undergo thermal cycling that leads to expansion and contraction of the adhesive layers. Including thermally conductive fillers like silicon carbide in the formulation therefore helps improve the flow of heat in a way that reduces physical strain on the system and lessens the likelihood of a heat build-up thermally decomposing the adhesive.

Schematic image of display applications using conventional ACFs before and after ACFs bonding process

Schematic Image of Display Applications using Conventional ACFs
a. Before and b. After ACFs bonding process

Warping Issue

Another important issue for LCD panels is the ability to deal with warping. For this purpose, it is important that the adhesives are designed to have a low elastic modulus, reducing their resistance to warping and the resultant strain. Conveniently, this is achieved fairly well by the double-layer approach. However, with the existing adhesives a great deal of attention has been paid to optimizing curing conditions to keep crosslinking levels and hence elastic moduli low. Clearly there seems to be an opportunity here to optimize crosslinker content, so that critical elastic modulus levels are never reached.

Overall, with electronic companies holding many of the patents related to ACFs, the electrical performance of anistropically conductive adhesives has been well optimized. But, there appears to be a room for development in physical and adhesive characteristics. The market for flat screen displays is now well established and represents a huge opportunity for adhesive manufacturers.

Thanks to this, not only are anisotropically conductive adhesives a high-volume manufacturing product, but they are primed for further contributions from adhesive formulators to help improve processes and lower costs.

Increasing Conductivity of your Adhesive Further

Take the course by industry expert Edward M. Petrie and learn how to increase electrical & thermal conductivity of your adhesives formulation, while maintaining good adhesion, durability, combined resistance to high humidity & temperature by renewing your formulation strategy.

Electrically & Thermally Conductive Adhesives: Formulation Strategies

References:

Lee, S.-J.; Kim, J.-K. "Driving Apparatus, Display Apparatus Having the Driving Apparatus with Non-Conductive Adhesive Film and Method of Manufacturing the Display Apparatus", US Patent Application No. 20100182287, July 22, 2010
Rizvi, M. Y.; Lu, H. ; Bailey, C.; Chan, Y. C.; Lee, M. Y.; Pang, C. H. Microelectronic Eng., 2008, 85, 238–244
Akutsu, Y.; Namiki, H. "Conductive Particle, Anisotropic Conductive Interconnection Material That Uses The Conductive Particle, And Method For Producing The Conductive Particle"US Patent Application No. 20100051878, March 4, 2010
Yim, J. Y.; Paik, K. W. Int. J. Adhes. Adhes., 2006, 26, 304-313
Suga, Y. "Adhesive film", US Patent Application No. 20100290205, November 18, 2010
Tatsuzawa, T.; Kobayashi, K.; Ito, A.; Yokozumi, T. "Circuit Connecting Adhesive Film and Circuit Connecting Structure", US Patent Application No. 20100221533, September 2, 2010
Fu, Y. ; Liu, J.; Willander, M. J. Electron. Manuf 1999, 9, 4, 275–281
Yim, J. Y.; Hwang, J.; Paik, K. W.Int. J. Adhes. Adhes.2007, 27, 77–84
"http://www.andisil.com/oneptrtv.html" Rizvi, M. Y.; Lu, H. ; Bailey, C.; Chan, Y. C.; Lee, M. Y.; Pang, C. H. Microelectronic Eng., 2008, 85, 238–244

Anisotropic Conductive Adhesives (ACA) - Quick Formulation Tips