Conductive Adhesives
Will ECAs Finally Stick?
A new study reveals emerging applications for attaching very-fine-pitch parts using low-temp methods.
by Mike Buetow.
Electrically conductive adhesives (ECAs) have been touted for decades as a potential replacement for solder. Technology roadmaps by organizations ranging from IPC to the Surface Mount Council often listed ECAs as a “coming” technology, and scores of papers have been presented highlighting possible uses and likely end-products.

In early October, the international research firm IDTechEx released a new study called “Electrically Conductive Adhesives 2022-2032: Technologies, Markets, and Forecasts.” Matthew Dyson, Ph.D., a senior technology analyst at IDTechEx specializing in printed, organic and flexible electronics, spoke with Mike Buetow about the study’s findings.

MB: Tin-lead and lead-free solder alloys are a blessing and a curse. They generally wet well and offer robust mechanical strength and conductivity. They often are the least expensive method of component attachment, and we seem to know all the quirks of reflowing them. But, they aren’t environmentally friendly in today’s context. Lead-free alloys often require reflow temperatures that risk damage to sensitive components, and they aren’t readily applicable for some substrate materials, like PET. Per your research, ECAs have an opening to gain market share in certain applications. What can you tell us about that?

MD: ECAs are not a completely new technology. They’re already widely used in the display industry and quite a few other places. I think where they really shine is you can use them at significantly lower temperatures, and you don’t need to go through this whole reflow process as with solder. You can imagine an ECA as a kind of conductive glue. There are two types. It allows you to directly place that component and then apply a relatively small amount of heat to cure it, and that component, be it an LED resistor or a capacitor or so on, is immediately secured on the board, rather than having to go through this whole reflow process. Of course, the ECAs are more expensive than solder, which is a downside, and they don’t come with the kind of self-alignment benefits you get with solder. That can make using them a little more time-consuming or expensive because the components won’t get dragged into place the same way they would with solder. But they do have a lot of applications, particularly as we move toward a flexible, embedded electronic kind of approach, where the conventional reflow method and materials such as FR-4 substrates don’t really apply anymore.

MB: You mention two types of conductive adhesives. I assume you mean isotropic and anisotropic.

MD: Yes. Isotropic adhesives (ICAs) are really like conductive glue. Ultimately you’ve got a sort of loading of silver particles and some kind of conductive species with some kind of epoxy with sufficiently high loading that there is a conductive pathway between all those embedded particles, and you can just use them to stick your components on. Of course, the challenge with that is, if you want to attach to very small contact areas to achieve fine pitches, you’re going to dispense these isotropic conductive adhesives incredibly precisely. That can be quite difficult and/or expensive, so that’s where anisotropic adhesives (ACAs) come in. They only enable electrical or thermal conductivity in one direction. The main technology would be where you have a much lower loading of conductive particles, and then by squeezing your components onto the top, you trap this conductive particle between the conductive trace that you’re trying to attach it to and the conductive part of the component. It’s only in that vertical plane that conductivity occurs. That makes this sort of resolution requirement for placing components significantly less challenging because some of that pitch capability is coming from the material itself.

OLED Strip
OLED Car Brake Light
Figure 1. OLEDs are a common application for conductive adhesives.
MB: Could you talk about what some of the new technical innovations are in the ECA space?

MD: I think the most interesting one is the development of what I call field-aligned conductive adhesives. The basic idea is, rather than achieving anisotropy by trapping a conductive particle by applying heat and pressure, you can instead align the conductive particles within the anisotropic adhesive in advance by applying either an electric or magnetic field. There are a few different approaches here, but the basic idea is by applying a field, the particles will line up along those field lines in a similar way to iron filings on a piece of paper, and as a result, the anisotropy is already there when you come to mount your component. That means you can achieve finer pitches and don’t need to apply so much heat and pressure when mounting the component, which of course makes it faster and means you can use more thermally or physically fragile components.

MB: Your study looked at the different applications that have potential for ECAs, but did it also look at which types of components might be more likely to be used with ECAs?

MD: Yes, absolutely. You can certainly use them across a whole range of components, including BGAs, microprocessors and so on. The ACAs come into their own when you get to the smaller fine-pitch components. As this kind of resolution gets smaller, it becomes increasingly desirable to avoid those constraints around how exactly [the component] is positioned and whether these small amounts of adhesive can be deposited, versus can I just put on my components and the conductivity will be entirely vertical? There are some approaches now, particularly with anisotropic conductive films, which can get down to the tens of microns, which are certainly quite challenging to achieve with ICAs, for example.

MB: In the mid-1990s I worked on the IPC standard for electrically conductive adhesives, so obviously these have been around for a while. What, if anything, has changed that gives you reason to believe there is more potential for market penetration today?

MD: People are increasingly looking at electronics not as something that comes as a rigid component from [offshore], but as something that is incorporated in the device during the manufacturing process. That means this conventional mass production of PCBs is no longer applicable. If you move toward those kinds of embedded or flexible devices, you are going to need different techniques. That’s not to say regular PCB production is going to disappear by any stretch. I imagine the motherboard on your laptop will be a conventional PCB for a very long time, and probably very cheap, relatively commoditized circuits. But there’s a whole space in between those, such as making HMIs – human machine interfaces – center consoles in vehicles, in aircraft where weight is really important, even consumer devices, rather than designing your product around fitting a rigid PCB. Once you accept the idea you can put your electronics wherever you like, there’s a lot more freedom of form factors.

And then there’s all the applications for flexible electronics, particularly in the kind of wearable/healthcare/wellness-type space. This would be for things like continuous health monitoring, where you can attach a skin patch that would monitor, for example, your heart rate, temperature, or things like your ECG more precisely than a smartwatch and probably could monitor a wider array of things, and that would incorporate an antenna and some kind of digital processing to interpret that data and then send it to the cloud. You can imagine having multiple ones to enable an advanced kind of ECG of the type you might normally have to have in hospitals. There’s a lot of interest in wearables. There’s not that many of them, and those that do exist have a little plastic box with a PCB in it, which makes it significantly less comfortable to wear. Once those electronics can be mounted onto a flexible or potentially stretchable substrate, it becomes way more compelling because it’s conformal, much lighter, much smaller, doesn’t have a plastic box on top, and you can imagine them integrating electronics into your clothing with similar means. You have antennas in there for monitoring your health or sporting performance or whatever. You could have electronics also providing heating. Obviously, that would also need the control circuitry, but you don’t necessarily want to have a whole PCB in your jacket. There are all these applications for very flexible and embedded electronics.

A two-component, nickel-filled electrically and thermally conductive epoxy.
Figure 2. A two-component, nickel-filled electrically and thermally conductive epoxy.
MB: Your response reminds me of some reading I’ve done about electronic skin, or electronic patches.

MD: The way I see it is electronic skin is a kind of next step from a skin patch. At the moment, if you want some kind of electrical monitoring as a skin patch, you have some kind of sticky electrode, and then there might be a little bit of conductive ink using the wiring, and you’ll have a PCB and little plastic box [the size of] a matchbox, or maybe a bit smaller, that sits on top. The next step is to mount those electronics directly onto the flexible or stretchable substrate. That would often be determined to be flexible hybrid electronics in that you’re mounting some of these ICs, which have a little piece of silicon and may or may not be rigid. There are some examples of flexible but still inorganic ICs. That would be a kind of intermediate case. And what you’re mentioning with electronic skin – I think I’ve also heard it referred to as epidermal electronics – there’s some great work from Stanford, which uses some kind of OLED display put onto someone’s hand. I think there’s a little pressure sensor that’s completely conformal to the skin. In terms of commercialization, those sorts of skin electronics are quite a long way off. The intermediate stage of these skin patches you can certainly see becoming much more widespread over the next five to 10 years or so, possibly sooner.

We looked at a wide array of applications these ECAs can be used in. We describe the technology in terms of these materials, and look at some of technical innovations, such as field-aligned adhesives, and at a whole range of applications where they could be used, like in-mold electronics and automated HMIs and the skin patches you mentioned.

MB: Is your forecast for ECAs generally optimistic over the next 10 years, and will that be at the expense of solder? You’ve described a lot of new and emerging applications that simply weren’t around in the past for solder to take hold in.

MD: I think that’s true. I think most of the gains will be coming from these emerging applications, and I’m certainly not [thinking ECA] will completely replace solder. What I do think is there are emerging applications where solder isn’t particularly applicable, versus those new applications that will present a substantial opportunity for these different types of ECAs. I would say as well the solder industry is progressing, and there are innovations such as ultra-low temperature solder, either by choosing a different alloy or a rather nice example of an early-stage US firm that can encapsulate supercooled liquid solder and little nanospheres that explode during manufacturing, which enables solder to be used on these thermally fragile substrates such as PET, which will be needed for low-cost electronics.

Another application I think is beginning to emerge after quite a few years is smart packaging. You might be aware of RFID tags, as you get them on clothing labels, and those all utilize electrically conductive adhesives because they all have a tiny or very low-cost, very simple silicon chip that provides a bit of identification for that specific item. But as smart packaging becomes more common, it won’t just be about identification; it will be about sensing parameters over time, things like temperature, movement, those kinds of things, and then ultimately feeding that information back to the cloud by an antenna. Say companies contract their products throughout the supply chain and potentially even in your house. Those devices will need to be produced in very, very high volumes. To achieve that, you need to run your equipment at very, very high speeds because you are only mounting a very simple single little chip, and hence using electrically conductive adhesive is the way to go because you don’t need to do the whole reflow cycle. If you imagine all clothes have some kind of label on them or food packaging, pharmaceuticals … all have little computer chips in them, but they only need one, and it’s a pretty simple circuit, and certainly one of those significant players there has just received a fairly big investment from SoftBank. These are the kinds of things we discuss in the report, these emerging applications. Certainly the potential for smart packaging is huge, but people have been saying that for a while. However, it does seem like there’s another round of interest, another recent chunk of investment going into it. Similar technologies such as flexible ICs are also really promising and can potentially lower the cost.

Mike Buetow is editor in chief of PCD&F and CIRCUITS ASSEMBLY; mbuetow@upmediagroup.com.