Adhesive Makers Add Conductivity to the Mix

Special fillers allow formulations to conduct electricity while connecting components.

As the name suggests, electrically conductive adhesives serve as pathways for electric current while performing their
primary function of holding things together. According to adhesive manufacturers, this mixture of conductivity and
adhesion is the right formula for many applications. The reason? By eliminating the need to add separate conductive
elements to a product, electrically conductive adhesives can simplify both design and manufacturing, notes George
Cramer, vice president of marketing and commercial development for Adhesives Research Inc. (Glen Rock, PA),
which makes conductive products.

In the medical industry, companies use electrically conductive adhesives for product assembly and device
attachment. The materials are also playing a new role that’s helping medical manufacturers cope with the demand for
ever-smaller devices.

Conductivity Basics

Electrically conductive adhesives are polymers loaded with materials that make them conductive. To get conductivity,
formulators must put in enough filler to get “point-to-point contact” of the particles through the adhesive material,
explains Steve Bruner, marketing director for NuSil Technology LLC (Carpinteria, CA), which sells silicone-based
adhesives.

In many cases, the filler materials are conductive metals. Silver is a common filler choice for a variety of reasons. It’s
malleable rather than brittle, less expensive than gold, and less reactive than copper, says Walter Brenner, research
and development manager for adhesive maker Master Bond Inc. (Hackensack, NJ). In addition, notes Brenner, silver
is still conductive when it oxidizes. This gives it an advantage over nickel, which turns into nonconductive nickel oxide.

Another filler option is carbon, which is used in most of the conductive materials made by Adhesives Research. The
company believes carbon can be more uniformly dispersed throughout an adhesive than metals, resulting in a more
stable product with more consistent properties. “Metal particles can become isolated by the adhesive itself,” Cramer
says. “And if the particles become isolated from each other, they’ll no longer conduct electricity through the material.”

Though isolation of filler particles is a concern, adhesive formulators must be careful not to overload adhesives with
fillers. Too much filler can cause a material to lose its adhesive properties, Cramer notes.

Filler is certainly a crucial component of electrically conductive adhesives, but there’s more to the production of these
materials than simply loading a polymer with metal or carbon. “We’ve formulated systems that weren’t very good at
conducting electricity, even though conductive metal [accounted for] 88 percent of the weight of the material,” says
Robin Tirpak, manager of electronics technology for adhesive manufacturer Lord Corp. (Cary, NC).

Besides the filler, the conductivity of an adhesive depends on factors such as the distribution of the filler and the
characteristics of the polymer matrix. For example, Tirpak says, silver flakes in an adhesive must be touching each
other and must form “efficient pathways” for electrical current in order to produce optimal conductivity. As for the
polymer matrix, an epoxy may shrink as it cures, which brings more filler particles in contact with each other and
thereby boosts conductivity.

In some cases, Tirpak adds, electrical conductivity is a byproduct of adhesives designed to provide thermal
conductivity. Thermally conductive adhesives help to provide a path for removing heat from devices. Like electrical
conductivity, thermal conductivity comes from metallic fillers added to adhesive formulations. Thermally conductive
materials that also offer electrical conductivity tend to be found at the high-performance end of the adhesive
spectrum, Tirpak says.

Conductive Adhesives in Use

Electrically conductive adhesives can be used to attach a medical device to the body of a patient. In this case, the
conductivity is needed for monitoring functions or to transmit electric current from the device to the patient, Cramer
explains.

In the manufacturing of electronic devices, electrically conductive adhesives can help to create a ground path that
dissipates static electricity. Conductive adhesives can also take the place of solder in the production of delicate
devices that can’t stand up to the high temperatures of the soldering process. Conductive adhesives have long been
used as a solder replacement in the assembly of pacemakers, says Douglass Dixon, electronics global marketing
manager for Henkel Corp., a Germany-based adhesive supplier.

In the production of electronic products like pacemakers, electrically conductive materials are used to bond resistors,
capacitors, and other components to circuit boards. The adhesive attaches the components to the boards, while the
conductive filler connects the components to the electrical circuit.

Circuit-board manufacturing processes using conventional tin-lead solder reach temperatures of over 180ºC, notes
Jay Richardson, an application engineer for Ellsworth Adhesives (Germantown, WI), an adhesive manufacturer and
distributor. According to Richardson, soldering temperatures rise to 220ºC or higher in processes that use lead-free
solder, which is replacing conventional solder due to health and environmental concerns about lead. By contrast, he
says, electrically conductive adhesives can be cured at temperatures as low as 110ºC, making them a good choice
for the assembly of temperature-sensitive devices.

On the downside, Bruner says, electrically conductive materials can be more viscous than other adhesives, making
them more difficult to dispense. In addition, there are limits to the conductivity of such materials. No adhesives are as
conductive as copper or gold wires, he notes.

Cost Considerations

Another significant disadvantage is the relatively high cost of electrically conductive adhesives. According to Cramer,
this is due in part to the techniques used to manufacture the adhesives and in part to their component materials.

Silver-filled conductive adhesives are certainly pricey compared to tin-lead solder material. So “cost is a huge
disadvantage” when comparing conductive adhesives to conventional soldering options, Richardson says.

Electrically conductive adhesives are also more expensive than lead-free solder. In addition, companies switching
from solder to electrically conductive adhesives must replace their reflow ovens and solder machines with adhesive-
dispensing equipment. So Richardson’s company and others are working on adhesives that can be applied in a
regular soldering process instead of a dispensing operation. These adhesives go through reflow ovens at lower
temperatures that those required by lead-free solder. A few adhesives like this are currently on the market, but some
development issues still must be addressed before the products are perfected, Richardson says.

New Developments

Development work is also focused on the manufacturability of electrically conductive adhesives. While solder joints
can be created in a matter of seconds, electrically conductive adhesives can require cure times of up to 48 hours,
according to Richardson. So adhesive manufacturers have developed electrically conductive materials that can be
“snap cured” in seconds, he says, thereby bringing processing times in line with those of conventional manufacturing
methods.

Another issue for developers of electrically conductive adhesives is the degree to which the materials can withstand
harsh conditions, both in the assembly process and the application environment. In many parts, Tirpak says,
adhesives must be capable of withstanding either high temperatures or a combination of high temperatures and
humidity without degradation that would adversely affect the functionality or reliability of the component.

So formulators like Lord are coming up with adhesives capable of handling higher moisture levels and temperatures
during processing and in use. “Performance under harsh conditions is getting better for a lot of these materials,”
Tirpak says.

Then there’s the issue of connection reliability. Joints made using electrically conductive adhesives aren’t as strong
as those created by soldering, according to Richardson. Therefore, he says, adhesive makers have been working on
electrically conductive materials that will produce a more “tenacious bond,” comparable to that produced by
traditional soldering methods.

Silicone Strengths

Besides bond strength, reliability can be affected by the rigidity of a connection. Shock and vibration can cause rigid
joints to give way, producing a short in the electrical system. So Richardson’s company and others have also been
working on electrically conductive adhesives that produce more flexible joints that won’t be damaged when devices
are subjected to shock and vibration during normal use. Products of this type include silver-filled silicones, which can
provide both the conductivity and flexibility required by some applications.

Silicones can also help manufacturers deal with the consequences of the trend toward miniaturized electronic
devices, which “tend to run a little warmer” than their larger predecessors, Bruner says. With processing and
operating temperatures on the rise, he notes, some manufacturers are turning to silicone-based conductive
adhesives, which provide good temperature stability compared to the epoxy-based adhesives that are commonly
used in electronics applications.

Besides temperature stability, Bruner  adds, silicone adhesives offer compatibility with silicone materials that are
widely used to encapsulate electronic medical devices in order to protect them from the body. Sometimes, these
silicone encapsulating materials are contaminated when they come in contact with other types of plastic, which can
inhibit the cure of the silicone. But silicone encapsulants won’t be contaminated by contact with a silicone-based
conductive adhesive used to assemble internal device components.

Other Advances

Conductive adhesives of all kinds may benefit from recent improvements in the conductivity of the materials. For
example, Tirpak says, formulators have had success in lowering the resisitivity of adhesive materials, which
translates into higher conductivity. Adhesive makers are also designing their materials so that conductivity through
the polymer stays consistent over time during exposure to the environment.

At Adhesives Research, some of the more significant developments in conductive materials involve films that serve
as the backing for adhesives. These films are proprietary polymer formulations that contain conductive carbon fillers.
Working together, films and adhesives can offer enhanced conductive properties, according to Cramer.

For example, he says, an electrically conductive adhesive could be coated onto a conductive film for use in an
electrode that’s part of a defibrillation system. When the electrode is attached to a patient’s body, he explains,
adhesive and film work together to disperse the electric charge uniformly over a relatively wide area so that it doesn’t
harm the patient or create a potentially troublesome “hot spot.”

Other products that may someday offer conductivity and adhesion are so-called inherently conductive polymers.
These polymers will feature a structure that provides electrical conductivity without the presence of silver or other
fillers, says Brenner, who expects such products to reach the market in the next five to 10 years.

No wait is necessary for medical device manufacturers to take advantage of electrically conductive adhesives with
disinfectant properties. According to Brenner, these adhesives can be used in implants, pacemakers, catheter
assemblies, and other medical devices both to provide conductivity and to mitigate the effects of contamination that
can occur when such devices are handled during installation.

New Applications

While formulators work on new products, their customers are putting existing electrically conductive adhesives to new
uses. For example, some medical device manufacturers are using the adhesives to make products that take a more
active approach to drug delivery. Unlike “passive” systems that rely on diffusion to move drugs from a patch through
a patient’s skin, iontophoretic systems actively deliver drugs in a process driven by electricity from a tiny battery.
Iontophoretic systems include electrodes that are bonded to circuits with electrically conductive adhesives. Made by
Adhesives Research, these adhesives are rubber-based formulations that don’t contain acids that could corrode the
silver-chloride components in iontophoretic devices, Cramer explains.

In the cosmetics industry, meanwhile, electrically conductive adhesives are playing a new role in the battle against
the effects of aging. According to Cramer, a company called Power Paper uses the adhesives to assemble
electrodes that are put on the face. The electrodes deliver electrical current as part of a process that’s supposed to
temporarily reduce wrinkles.

With medical devices of all kinds growing ever smaller, manufacturers are looking for ways to reduce space
requirements. So some medical firms are using electrically conductive adhesives to create circuit patterns on
compact and flexible substrates that take the place of multi-layer fiberglass circuit boards. To make these so-called
flex circuits, manufacturers use electrically conductive “inks” to print traces on materials such as Mylar and Kapton.
The conductive properties of the inks allow current to flow in the circuit, while the adhesive properties of the materials
attach them firmly to the substrates.

In hospitals, Richardson says, conductive inks can be found in pads that are attached to the chests of patients for
monitoring purposes. Each of these pads contains a small flex circuit lined with conductive-adhesive traces.

The bottom line? Instead of struggling to fit conventional hard circuit boards into their products, Richardson says,
makers of diminutive medical devices “can lay out all their circuitry on a tiny piece of plastic.”

This article appeared in Medical Device & Diagnostic Industry magazine.