Printed Electronics: What are Conductive Inks?

The applications of conductive inks

Additive electronics offer whole new dimensions of possibility in electronics and hardware design. Flexible Hybrid Electronics (FHE) represent a paradigm shift in how we think about product design – the sheer possibility can be overwhelming.

Whether you’re trying to keep up with the ever-shrinking requirements of consumer electronics (looking at you, bluetooth earbuds), cutting every milligram of fuel-wasting weight in automotive and aerospace, or chasing the next big opportunity in wearable sensors, structural electronics or soft robotics, the design options printed electronics open go far beyond what can be achieved with the rigid PCBs of yesteryear.

But what does it take to get from an idea in your head to a product in your hand? What tools do you need to add to your engineering toolbox? How can you prototype electronics with exotic substrates? What's even possible to achieve with FHE?

In this series, Voltera’s technology experts will be looking at what it takes to go from board-in-a-box electronics to additively printed and produced-at-scale FHE devices – and offer some helpful, practical insights along the way.

This week, we’ll start talking about one of the key components of any printed electronic device: conductive ink. Conductive ink is the lifeblood of our flagship product, the V-One, and Voltera has developed a rare expertise in both the materials science of the ink, and the practical applications of the devices it can create. By the end of this post, I hope you’ll see conductive inks as just another tool in your engineering toolbox: like your solder gun, voltmeter and tweezers!

While many innovations in conductive ink are working towards a distant fully-printed future, the fact is that you likely interact with conductive inks pretty regularly. A membrane keyboard in your laptop, RFID tags, glucose strips, solar panels, printed heaters... Conductive inks are a staple component in many mature industries. Depending on your definition, even SMT resistors fall under the ‘conductive ink’ family (fired thick film pastes, specifically).

So if inks are so ubiquitous, why is it then that most of us have never worked with them? Well, for a number of reasons, access to conductive inks (and printed electronics in general) is a bit of a murky situation. Getting from the lightbulb moment to an actual product in your hands can be an uphill battle.

Our journey with conductive inks

When we were in the trenches building what would eventually become the V-One, we went through this exact situation. When we realized at the very early stages that conductive inks were a key component of our product, we were immediately thrown off the deep end of datasheets with vague specifications and what felt like endless equally uncertain outcomes.

Luckily, after 9 years, hundreds of ink formulations, thousands of hours testing, and a lot of elbow grease, we’re here to help guide you through the same process – hopefully with fewer bumps and bruises along the way.

How do I choose a conductive ink?

If you’ve gotten here, I assume that, for whatever reason, you haven’t already seen Voltera’s Flex 2 or Conductor 2 inks. Our sales team would prefer that I stop writing here (If you’re on the Voltera sales team, assume the article is over).

For everyone else, let’s clear up the most important thing to get right off the bat – you can’t choose an ink in a vacuum. An ink is an engineering decision. It means starting from your requirements, looking at your available options, and then optimizing for your desired outcomes and available resources.

What are your performance criteria? How do you plan to make it? How much are you willing to spend? How does it have to interact with the user and the environment? Does it need to comply with any regulations? Etc.

In the same way, choosing an ink means making a number of important decisions, all of which stem from what you need your device to do. Once you know your application specifications, you then make an ink choice – which is really 3 decisions:

1. An ink
2. A substrate
3. A printing technology

I know it’s a lot to take in at once, but don’t worry – we’ll tackle this problem in bite-sized chunks, and soon you’ll be talking inks like a pro.

In this first article, we’ll be talking about fundamentals of inks, so you can get familiar with the language and key concepts.

In following articles, we’ll discuss what our options are in printing technologies and substrates, what decisions you might need to make, and end off with an example for how Voltera makes these decisions for our own needs – which will help illustrate what you need to care about, why, and how to test for it.

What are conductive inks?

Let’s start with a broad definition, and narrow down from there:

A conductive ink is a material which is printed, processed, and conducts electricity.

Let’s dig into that definition real quick:

Printed: Inks means additively patterning your surface in some way. Different inks exist for different printing technologies, which have their own pros and cons. We’ll dig into these in future articles!

Processed: Most inks aren’t actually conductive until they’re processed, which usually means applying heat. For our purposes, we’re only going to be talking about low-temperature inks (< 250 [C]).

Electrically conductive: Conductive inks are actually a subset of functional inks – meaning inks which are useful outside of purely aesthetic or structural needs. Conductive inks are by far the largest branch, whose job is to provide a path for electrons to get from A to B.

With that out of the way, let’s get into what these inks are made of!

What are Conductive Inks Made of?

Conductive inks have two major parts:

1. The filler. This is the conductive part – typically metal particles – which will give your ink its electrical properties.
2. The vehicle. This is everything else – the binders, dispersants, solvents, and additives – that suspend your particles, allow the ink to flow & dry, give it structural stability & flexibility, and all the other properties your ink needs to be its best self.

Filler Material

Filler material typically comes down to two options – silver, and everything else. I know that’s a broad generalization, but bear with me here – this is a practical guide, not a literature review.

When it comes to choosing a filler, silver often wins out for the factors most people care about:

1. Conductivity vs. Cost
2. Accessibility
3. Ease of use/stability

Silver typically gives you the best bang for your buck in terms of conductivity, but just as important as the performance is the fact that silver inks are ubiquitous. Silver grandfathered into the industry, and since most major ink suppliers offer silver-based inks, you have far more options for chemistries and configurations which may suit your specific needs.

As for ease of use, it’s best to illustrate by comparing with silver’s closest competitor: copper.

Copper

I know I said silver vs. everything else, but copper deserves a mention because it’s usually the first alternative people consider – typically because copper is technically a cheaper metal than silver, on paper. That, and if you come from traditional electronics, copper is everywhere.

The thing is, inks are NOT bulk metal. Just like a computer’s cost is more than the sum of its parts, an ink’s overall cost includes processing cost for raw materials, R&D cost, and any capital cost you might incur to use it.

The biggest detractor for copper is oxidation. Copper inks will oxidize when curing in air, which stops your copper particles from conducting. To prevent oxidation, you’d need to pump in a reducing agent in nitrogen gas, adding cost and complexity which usually outweighs the raw material cost.

There are some workarounds – coating copper particles in silver, for example – but the additional processing cost for these options, and their relative scarcity, means that silver still usually comes out on top in the conductivity/cost balance.

That said, there are some contenders out there who are working on ambient-curing copper inks, and for good reason – in concept, if processing cost was comparable, copper promises more robust solderability than silver, which can be attractive for compatibility with standard SMT processes.

Everything else

There are many other varieties of conductive ink fillers – carbon nanotubes, graphene, conductive polymer, or particle-free molecular inks – and each have their own particular kinks and caveats, which are outside the scope of an overview article. If you’re going down one of those routes, you probably have a really good reason to do so, or you’re planning on publishing a paper.

Particle sizing, shape, and loading (weight %) are major decisions when formulating an ink, and provide tradeoffs in terms of conductivity, processing temperature, cost, and flow properties.

That said, unless you’re getting into the nitty-gritty and elbows deep into formulation, it’s one of those things that is best left up to the manufacturer. For your own interest, here are some general trends to keep in mind: Metal loading Higher metal content generally means more conductive and higher viscosity. For example, low-viscosity, water-like inkjet inks will usually top out in the 10%-20% range, while thick screen printing pastes can get up to around 80% silver by weight. Particle size and shape In general, the smaller the particles, the higher the conductivity and the higher the cost. For convenience, I’ve grouped size ranges in a few rule-of-thumb categories:

Micrometer-sized (1-5 μm): Most common, cheapest, lowest conductivity due to contact resistance. Sub-micrometer-sized (~0.3-1 μm) Nano-sized (<0.3 nm)

When speaking about shape, you’re typically safe to assume that you’re working with flakes – but not always. Check out the scanning electron micrograph below, courtesy Alejandro Marangoni at the University of Guelph!

A quick note on conduction in inks I’ve said it before (a few paragraphs ago, in fact) and I’ll say it again – conductive inks are not the same thing as bulk metal.

When working with low-temperature inks, your conductive network forms by evaporating solvent during thermal processing, suspending your filler particles in your binder, and allowing the particles to come in contact in a percolated network – essentially, a random web of connections which allow electrons to get from one end to the other.

In the vast majority of cases, that’s all there is to it – your particles are just sitting there, touching each other unceremoniously while electrons find the shortest path through the network. That means that your conductivity is limited by contact resistance. If your particles get small enough (in the nanoparticle range), you begin to open up the possibility of necking and sintering, which blurs the line between individual particles. At that point, your conductivity can increase, but it’s usually a tradeoff between price and performance.

The vehicle

This is where a lot of the formulator’s magic comes into play. Choosing the right combinations & ratios of additives, binders, solvents, and dispersants to make sure that your ink is shelf-stable, mechanically robust, and printable is both an art and a science. Luckily, this isn’t an article on making an ink, so we’re not going to dive too deep into specifics – instead, let’s just get familiar with what each component does:

Solvent: The solvent dissolves your binder and particles and contributes to flow characteristics of the ink.

Binder: The binder matrix is a polymer or mix of polymers which provide the ink with structural properties – adhesion, flexibility, mechanical robustness, working temperature, and others.

Dispersant: If you want your ink to flow right, cure right, and stay shelf-stable, you have to make sure your particles stay separated and well-dispersed throughout your material. Clumped, agglomerated particles are a bad time.

Additives: All the other stuff – oftentimes, additives include some modifiers to tune the flow properties of your ink for optimal print properties.