What are Anisotropically Conductive Adhesives?
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 ACAs
Dealing with the Challenges
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 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
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.
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References:
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- 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