Neat epoxy polymers are highly electrically insulating. However, when these same epoxy resins are filled with electrically conductive filler materials, the resulting polymerized composite material is able to achieve low levels of electrically resistivity that compete with solder in many electronics applications.
Although solder is still able to outperform any epoxy purely in terms of electrical conductivity, thermosetting epoxies offer some physical and assembly benefits that are superior to solder alternatives.
As a result, electrically conductive epoxies find themselves in a growing number of electronic applications, including, but not limited to, "cold solder" applications.
This post will address the following questions with regards to electrically conductive epoxies:
- What are they?
- What are the different types?
- Where are they used?
What Are Electrically Conductive Epoxies?
As we've mentioned in the intro, electrically conductive epoxies are simply neat epoxy resins loaded with electrically conductive filler materials.
The theory behind these types of products is simple:
- Epoxies are highly insulating materials, but make great rigid adhesives.
- Metals (silver, gold, aluminum, etc.) are highly electrically conductive, but need a bonding agent
- Combining the two (by loading an epoxy resin with metal particles) results in a composite material that acts as a great adhesive that is also highly electrically conductive.
Of course there is more to formulating electrically conductive epoxies than I've suggested above. As with all polymer chemistry, formulating a electrically conductive epoxy is a balancing act. A polymer chemist has to make a decision about particle material, shape, size and loading percentage (not to mention the oligomers, monomers, and modifiers of the base resin material).
At the end of the day, however, electrically conductive epoxies are simply electrically conductive thermoset materials.
- They may be applied as liquid, either as a one or two part formulation (some cationic UV cure epoxy variations exist)
- They will polymerize to form rigid polymer products
- One parts require heat (normally 60 - 100C)
- Two parts will generally cure at room temperature after mixing
- They'll provide substantial electrical conductivity across a bond interface
Common electrically conductive filler materials (in order of decreasing electrical conductivity) include: silver, copper, gold, aluminum, nickel, platinum, and graphite.
Due to the superior electrical properties achieved with silver, most electrically conductive epoxies are formulated with silver flakes or powders.
We should note that although epoxies are commonly the resin of choice, electrically conductive adhesives can also comprise of UV acrylate or silicone resins.
What Are The Different Types Of Electrically Conductive Epoxy Products?
Electrically conductive epoxies can either be isotropic or anisotropic:
- Isotropic conductive adhesives will conduct electricity in 3 dimensions. These products are formulated to contain very high filler loadings (up to 35% by volume). The goal of isotropic adhesives is to load the resin to an extent that the filler particles are able to touch and provide a direct electric pathway (remember epoxy is still electrically insulating). To this end, isotropic adhesives are normally formulated with long plate shaped particles, such as those in the image below. Most electrically conductive epoxies are isotropic.
- Anisotropic conductive adhesives, on the other hand, are formulated to exclusively provide electrical conductivity across the bondline. Filler loadings are reduced (to about 1-5% by volume) and fillers are normally spheres with a diameter the exact thickness of the intended bondline. In this way, when the the interface materials are pushed together the bondline is reduced so that the filler particles make contact with both materials. The graphic below created by Epoxy Technology clearly illustrates this concept.
Where Are Electrically Conductive Epoxy Products Used?
The main use of electrically conductive epoxies is as a solder replacement. When compared to solder epoxies generally...
- Are more environmentally friendly
- Allow for simpler processing (no flux)
- Allow for lower processing temperatures (can also bond to heat sensitive components unlike solder)
Electrically conductive epoxies are generally considered to be a "cold solder" alternative due to the lower processing temperature requirements.
Some other applications include:
- Static discharge
- EMI/ RFI, electrical shielding
- Specialty heat sinks (electrically conductive fillers are usually also highly thermally conductive)
- Solar cell manufacturing
- LED packaging
- Wireless headsets
- Membrane switches
- Antenna assemblies
- Copper/polyimide (PI) circuits
- Wire tacking
- Communication systems
- Electronic test equipment
- Hybrid microelectronic packaging