Electrical systems in e-textiles, biomedical devices, and consumer electronics applications are becoming smaller and more integrated into materials for easy human interfacing. Novel devices must fit more electronics into smaller spaces and keep them lightweight while also increasing the flexibility and durability of the device itself. Alongside flexible printed circuit boards made from conductive inks and thin metals, carbon nanotube (CNT) yarns are an excellent contender to help shave weight and increase flexibility in such devices. The excellent corrosion resistance, high tensile strength, and high temperature resistance of CNT materials will also be a big advantage for new lines of compact electronics.
There are some challenges that must be overcome to integrate conductive CNT yarns and films into electronic circuits. One drawback of these CNT conductors is the relative difficulty of connecting and terminating them to metal surfaces, in comparison to metal wire. For example, DexMat' Galvorn CNT yarns and fibers are unable to accept standard heat-activated solder or welded terminations.
In this blog post we demonstrate the difficulty of forming soldered connections to CNT materials, and discuss other termination methods that are more suitable. This article should serve as a starting point to help our customers achieve good electrical terminations for Galvorn CNT yarns and films using whichever methods are most suitable for their particular applications.
The video above shows the engineer trying to solder a Galvorn CNT yarn to a copper plate; the CNT yarn is not easily wetted by the liquefied solder medium, and the engineer is not able to achieve a good bond. The solder, when used in excess, will not successfully trap the yarn within itself but rather tries to push the yarn out of the pool of solder, leaving the connection cumbersome and almost impossible to achieve. As a reference, the engineer also places a copper stranded wire on the solder zone to show how easily a normal soldered connection can be achieved.
An alternative formulation of Galvorn makes this process a little easier. Since copper surfaces are so much easier to use for soldered connections, we found it fitting to try this same process with some of our copper-coated CNT yarns made with our electroplating process described in this past blog post. The video below shows that It is possible to solder a copper-coated CNT yarn in a similar fashion to pure copper wire with only a limited amount of added work. Copper coating undercuts some of the best properties of the Galvorn yarn by increasing stiffness and mass, but this method could potentially be used to plate only the end segments of a length of Galvorn yarn or film with copper or nickel, allowing for a good electrical termination while leaving the rest of the yarn free of a metal coating.
We have experimented with several other options for connecting our materials to metal circuit terminals. These options include termination with crimping, screw terminals, conductive epoxy, carbon/silver loaded adhesives, and even a simple knot for the most basic operations.
CNT yarns have an advantage over many standard metal wires in that they are flexible, compressive, and durable enough to be tied into a tight knot without creating a mechanical weak point in the material. In the above example, we have tied knots through the open loops on a common sew-in led, forming a conductive connection. Copper wires and conductive metal threads such as stainless steel thread can break down over time and fail from sharp stress points and continuous flexing. A CNT yarn has far superior resistance to flex fatigue, and even many millions of bending cycles in this arrangement will not break down the thread. A similar technique was used to build the flexible LED wrist band shown below, and to and create our first electronic DexMat smart shirt seen in another past blog post.
A screw terminal fixture, like the one shown in the picture to the left, is another quick-connection method commonly used in circuit prototyping that can work well for CNT yarns. This option is easy and effective for a prototype or larger electronic device, but may yield some inefficiencies if used in smaller lightweight devices. A past blog post highlighting an auxiliary cable made from Galvorn yarn shows another example of using screw terminal fixtures to terminate a CNT yarn into an electrical component.
Another inexpensive option for terminating a CNT yarn to an electronic circuit board or another type of wire is a crimped connection like the one seen below on the right. For easy integration into existing systems, CNT yarn film could be crimped onto a standard metal plug or wire that can in turn soldered to a circuit or, as in the image below on the left, plugged into a breadboard for easy prototyping.
Crimping works for Galvorn CNT film as well as it does for Galvorn CNT yarn. The picture below shows a length of CNT film wrapped around a communication cable to form the outer EMI shielding, with a crimped connection to the cable termination. DexMat has built numerous Galvorn (CNT) tape shielded RG-316 cable prototypes that perform comparably to copper (Cu) double braid shielded RG-316 cables, such the one discussed in this article. This application highlighted all the properties the CNT film has to offer, improving the overall cable flexibility and strength while providing over a 50% weight reduction for the finished cable.
Connections for portable lightweight devices that are optimized for harsh weather conditions and environments are often assembled with connections that are more permanent than crimping or knot tying. With that in mind, solutions outside of crimping and screw connections are important to consider. In more demanding applications, typically a wire would be soldered into place, but with limited options for soldering CNT yarns, we can try a few more permanent methods.Our next two connection methods take advantage of silver particles to provide a conductive medium between CNT materials and other electrical components. The first product we tried is a silver conductive wire glue. This product is typically used to reset solder pads, broken traces, the occasional point-to-point "liquid" jumper, perfect for both rigid & flexible substrates. The glue is a one-part application that takes about 5 hours to set and 24 hours to fully cure. We prepared a sample with yarns glued directly to the same LED light assembly used in the other tests and also glued some yarns to a copper plate. The following video highlights that the glue was effective at holding the yarn in place but with a small amount of force it is easy to peel the yarn from the copper plate. Ultimately, this glue behaves more like a sticker holding the yarn down to the copper substrate.
Our second bonded method utilized silver-filled conductive epoxy. Conductive epoxies come in many varieties with different temperature ratings and levels of electrical resistance. For this test, we use a two-part conductive epoxy by MG chemicals with an advertised low resistance rating and cold bonding option. For the best results, you can heat the connection point to 65 degrees Celsius for two hours, but not all sensitive electronics are suitable for those extended temperature holds, so for this test we made the bond at room temperature and let it cure for 48 hrs. The material was relatively easy to mix together and apply to the samples but was a little higher in viscosity than anticipated, making it harder to achieve a smooth trace and contact point . A proper connection was made to the LED light as shown and the connections seem to be secure and have a high level of handling durability. The MG chemicals 8331 two part epoxy hardened very well and appeared to have a strong hold on the CNT yarn, but as the video below shows it is still able to be peeled away from the copper plate leaving minimal residue on the yarn itself.
Mechanically, the strongest connection proved to be the basic knot that we used in the first circuit example above. The knot primarily relies on the mecahnical strength of the CNT yarn itself, rather than relying on a crimp, glue, or epoxy to withstand force and tension. Understandably, however, a knot is not common when it comes to terminating connections to circuits, so it will not always be a suitable connection method.
We performed a side-by-side comparison of these two silver adhesive connections along with a simpler "crimped" knot connection in order to ensure that none of these methos produced significantly contact resistance between the CNT yarn and a sew-in LED. The picture below illustrates how we performed this measurement. To simulate a crimped connection, in this scenario we used the knot from the first example with insulated clamps to apply the pressure and make a strong contact with the LED contact strip. In each case, the resistance across the circuit was dominated in each case by the resistance of the LED itself, whcih was around 4.5 ohms. As a result, this test did not indicate that the contact resistances were necessarily equivalent to one another, but it demonstrated that none of them were so high as to be unsuitable.
We have only briefly touched on available methods for circuit termination, and many more exist beyond the scope of this article. There are numerous other companies developing secure and flexible connection methods for the wearables and e-textile industries. For example, companies like Ohmatex make connections that work well with conductive yarns and allow the user to wash smart clothing without worrying about corrosion issues. This technology is already being utilized in socks available from Palarum, which track and limit the frequency of patient falls in hospitals. T-ink is working on conductive ink compressed into plastic components to maintain a safe and strong connection for car parts; this method could easily be adapted for more flexible applications using CNT yarns and films between plastic layers. We have in the past performed a test of encasing Galvorn film between insulated plastic layers using a 3D printer to simulate the opportunity for rapid prototyping of flexible and corrosion-resistant circuits. This method could be used to join Galvorn CNT films to a flexible metal wire or strip, or to another length of Galvorn CNT film, in a permanent yet flexible crimp.