ADHESIVES

As the name suggests, electrically conductive adhesives serve as pathways for electric current while performing their primary function of holding parts together. According to adhesive manufacturers, this mixture of conductivity and adhesion is the right formula for many applications.

By eliminating the need to add separate conductive elements to a product, electrically conductive adhesives can simplify both design and manufacturing, notes George Cramer. He is vice president of marketing and commercial development for Adhesives Research Inc. (Glen Rock, PA).

In the medical device industry, companies use electrically conductive adhesives for product assembly and device attachment. The materials are also helping manufacturers cope with the demand for ever-smaller devices. For example, using small circuits that contain conductive adhesives instead of hard circuit boards will help to make some of tomorrow's innovative products smaller. This article explores the ways in which conductive adhesives could transform the device industry.

Conductivity Basics

Electrically conductive adhesives are polymers loaded with materials such as carbon or metal 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. The Carpinteria, CA–based firm produces 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. In addition, he notes, silver is still conductive when it oxidizes—unlike nickel, which turns into nonconductive nickel oxide. Brenner is research and development manager for adhesive maker Master Bond Inc. (Hackensack, NJ).

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 stabler 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.”

Although the 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.

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% of the weight of the material,” says Robin Tirpak, manager of electronics technology for adhesive manufacturer Lord Corp. (Cary, NC).

The conductivity of an adhesive also 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 an electrical current in order to produce optimal conductivity. As for the polymer matrix, an epoxy may shrink as it cures, bringing more filler particles in contact with each other, thereby boosting the adhesive's conductivity.

In some cases, Tirpak adds, electrical conductivity is an unintended result of adhesives designed to provide thermal conductivity. Thermally conductive adhesives 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. The conductivity is needed for monitoring functions or to transmit electric current from the device to the patient, Cramer explains.

In the manufacture of electronic devices, electrically conductive adhesives can help to create a ground path that dissipates static electricity. Conductive adhesives can also replace solder in the production of delicate devices that cannot withstand the high temperatures of the soldering process. For example, 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 Düsseldorf, Germany–based adhesive supplier.

Electrically conductive materials are used to bond resistors, capacitors, and other components to circuit boards in the production of various medical electronic products. 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 exceeding 180°C, notes Jay Richardson, an application engineer for Ellsworth Adhesives (Germantown, WI), an adhesive manufacturer and distributor. According to Richardson, the temperature can rise to 220°C or higher in processes that use lead-free solder, which is gradually replacing conventional solder owing to lead-related health and environmental concerns. By contrast, he says, electrically conductive adhesives can be cured at temperatures as low as 110°C, making them appropriate for the assembly of temperature-sensitive devices.

Drawbacks of Conductive Adhesives

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

Electrically conductive adhesives are also relatively expensive. According to Cramer, the higher cost can mostly be attributed to the component materials in the adhesives as well as the techniques used to manufacture them.

Silver-filled conductive adhesives are certainly more expensive than tin-lead solder material—they can be as much as five or six times as expensive as tin-lead solder paste. So “cost is a huge disadvantage” when comparing conductive adhesives with conventional soldering options, Richardson explains.

Electrically conductive adhesives are also about four times more expensive than lead-free solder. Companies that switch from solder to electrically conductive adhesives must replace their reflow ovens and solder machines with adhesive-dispensing equipment. Consequently, several adhesive companies are developing adhesives that can be applied using a soldering technique instead of via a dispensing operation. These adhesives go through reflow ovens at temperatures lower than those required by lead-free solder. A few such adhesives are currently on the market, but some development challenges still must be addressed before those products are perfected, Richardson says.

New Developments

Development work is also focused on the manufacturability of electrically conductive adhesives. Solder joints can be created in a matter of seconds, but 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. Fast cures bring processing times more in line with those of conventional manufacturing methods.

Another concern for developers of electrically conductive adhesives is the degree to which the materials can withstand harsh conditions, both in the assembly process and in the application environment. Tirpak says that adhesives often must be capable of withstanding either high temperatures or a combination of high temperatures and humidity without degradation that would adversely affect the functionality of the component. To address such requirements, formulators like Lord are coming up with adhesives that can handle higher moisture levels and temperatures during processing and in use. “Performance under harsh conditions is getting better for a lot of these materials,” Tirpak maintains.

Also of concern to manufacturers is ensuring that connections are reliable. 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 trying to develop electrically conductive materials that will forge a bond comparable to that produced by traditional soldering methods.

Silicone Strengths

Besides bond strength, reliability can also 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 adhesive makers 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 address 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. They provide good temperature stability compared with the epoxy-based adhesives that are commonly used in electronics applications. Bruner adds that silicone adhesives are also compatible with the silicone materials that are widely used to encapsulate implanted electronic medical devices (to protect them from the body). These silicone encapsulation materials are sometimes 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 resistivity 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.

Some companies are creating films that serve as the backing for adhesives. These films are proprietary polymer formulations that contain conductive carbon fillers. Films used in conjunction with 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, the 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, which are expected to reach the market in the next 5–10 years. These polymers will feature a structure that provides electrical conductivity without the presence of silver or other fillers, says Brenner.

No wait is necessary for medical device manufacturers to take advantage of electrically conductive adhesives with disinfectant properties. Brenner says that 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 the devices are handled during installation.

Moving Forward: New Applications

While formulators work on new products, their customers are finding new uses for electrically conductive adhesives. 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. These adhesives are rubber-based formulations that don't contain acids that could corrode the silver-chloride components in iontophoretic devices, Cramer explains.

With medical devices of all kinds getting smaller, manufacturers are looking for ways to reduce space requirements. Some medical firms are using electrically conductive adhesives to create circuit patterns on compact and flexible substrates that take the place of multilayer 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. Electrically conductive adhesives are even being used in devices that help users fight signs of aging, Cramer says.

Conclusion

Adhesives manufacturers continue to mix conductivity and adhesion to satisfy the requirements of a variety of medical applications. These adhesives have been used to bond electronic components in implantable medical devices, as well as to transmit current from a device to the patient. Adding conductivity to adhesives has helped device makers to avoid adding separate conductive elements to their products, simplifying both the design and manufacturing processes.

In the future, electrically conductive adhesives may not even require fillers such as carbon or metal. The adhesives can also be instrumental in managing the trend toward miniaturization in medical devices. 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.”

Copyright ©2007 Medical Device & Diagnostic Industry