We get it. You have this amazing idea and you think you know how to go about bringing it into reality — you’re excited to get started and you just want to do something. But before you launch out and boldly go where no person has gone before, there are a few things you should keep in mind.
Here at Voltera, we’re firm believers in DFX — Design For Everything. You need to know what you’re trying to accomplish! Your proof of concept needs to be able to translate to a final product. There’s nothing worse than a proof of concept that you can’t do anything with — either because the materials themselves are too expensive or hard to come by, or the machinery needed to produce the design is cost-prohibitive. But we’re getting a bit ahead of ourselves.
There are many functional materials that could come together to produce printed sensors, electronics, and other design features. Conductive ink is one of the most important ones, and so it’s going to be a major focus of this article. There are thousands — if not tens of thousands — of conductive inks to choose from, and which one you choose depends on a variety of factors. So how do you get started? How do you choose the materials that will work best for your brainchild?
Here are four major things to consider when choosing the materials for your new electronic product:
But here’s the thing — you can’t choose your ink without considering your substrate, and vice versa. And you can’t pick either your ink or your substrate without first considering what printing technology you’re going to use! Basically, it’s an intertwined web and we’re here to help you sort it all out.
Let’s get started.
In materials science, the term substrate refers to whatever material it is that you’re hoping to put your feature, or design on to. We’re going to start with the materials that you’re likely already familiar with in traditional electronics, and then branch out into some that we think you should consider — especially if you’re pushing the boundaries of what’s currently possible with electronics.
The most widely used PCB material — it’s a flame retardant (FR), glass-reinforced epoxy laminate, approximately 1.60mm thick. It uses eight layers of fiberglass material and has a maximum temperature threshold of 120 to 130o C for the glass transition temperature. It has good strength to weight ratios, nearly zero water absorption, and is commonly used as an electrical insulator.
The biggest thing to consider with this material is that if you’re doing any drilling of through-holes for your project (aka you need a two-sided board), you might want to go with an FR1 board due to the fiberglass dust that would be generated with an FR4 board that can be hazardous to your health.
Of course, with proper masking and ventilation, this is definitely something that you can work around — it’s just something to consider.
These boards are cousins to the FR4, but instead of containing fiberglass, they’re made of paper and phenolic resin which makes them cost-effective and easy to build. It does however affect their heat resistance — they can be prone to scorching or warping when heated during the reflow process, or with thermally cured conductive inks. The other thing to consider here is that you can’t do plated through holes on FR1 in traditional PCB manufacturing paradigms, so you can’t do vias for double-sided boards.
Spoiler alert: With the V-One you can drill through holes in an FR1 board with the drill attachment and, using rivets, can achieve functionally plated through holes for double-sided boards!
In the same way that we call facial tissues Kleenex, most folks refer to polyimide films as Kapton. These are a great substrate when your application is going to require the handling of extreme temperatures, vibration, radiation, or other demanding environments. It’s extremely versatile — it can be metalized, punched, or formed — and because it can be laminated on both sides, bonded to itself, metals, paper types, or other films, or even filled — it’s pretty much customizable for any situation. It’s also ideal for applications where size and weight play a role in the final look and feel of the product — such as microelectronics, or wearable electronics. Kapton flex circuits are already in lots of places where space savings matter — like inside your Airpods, your smart , smart watch, and laptop!
Polyethylene terephthalate (PET) — the chemical name for polyester — is a clear, strong, and lightweight plastic. It’s incredibly common in packaging food and beverages (hello, 2L bottle of Coke!). DuPont was the first to synthesize PET in North America, so if you’ve heard the term Dacron, you’ve heard about PET.
As an inert and fully recyclable material, PET stands to have the least environmental impact of any of the substrates. Given the toxicity and harm created by the traditional copper etching process for PCBs, that’s definitely something to consider. But what it comes down to most folks is the overall cost at scale. PET is way cheaper than polyimide! But there’s a tradeoff — it’s less thermally and dimensionally stable, with a temperature max of between 140-160o C.
Thermoplastic polyurethane (TPU) is a class of polyurethane plastics that have many properties that make them alluring in the world of flexible electronics: elasticity, transparency, resistance to oil, grease as well as abrasion resistance. Its stretchable nature also makes it an ideal choice for use in the textiles industry and in wearable electronics.
TPU powders are also used in processes like LASER Sintering and 3D inkjet printing — more on that later!
Polydimethylsiloxane (say that three times fast!) is a substrate that’s particularly useful for things that need to stretch. That being said, it’s not something that you can readily purchase — many researchers will cast their own PDMS because of its use in stretchable applications, in addition to its good thermal stability, transparency, and biological compatibility. The learning curve on the process of casting PDMS is steep, but once you know how to do it — it’s pretty easy!
When people think of printing something, even electronics, they think about printing on paper — but paper is porous! Generally speaking, you don’t want your substrate to absorb any of your conductive ink. To solve that problem, there are a bunch of coated and processed papers that are being developed to allow for applications in smart packaging, wearable technology, and textiles in general, for example.
Choosing 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.
Some questions you can ask yourself when choosing your conductive ink:
But first, some basics about what a conductive ink actually is.
A conductive ink is a material that can be:
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Conductive inks have two major parts:
As you can imagine, there are a number of different metals which might be able to imbue a conductive ink with electrical properties, but silver tends to be the main component of most conductive inks.
Why?
We’re glad you asked!
When it comes to choosing a filler, silver often wins out because of the factors most people care about:
Basically, silver gives you the best bang for your buck in terms of conductivity, but also — silver conductive inks are ubiquitous! Most major conductive ink suppliers offer silver-based inks, so you’ll have far more options for different chemistries and configurations to suit your specific needs.
Traditional PCBs created using subtractive methods like etching are typically made using copper, so it deserves a mention here. While copper is a cheaper metal than silver when purchased in bulk, and because they’ve been used for decades in traditional electronics, we tend to know all of its properties like the back of our hand, using copper in conductive inks has its drawbacks.
You can’t evaluate the cost of copper vs silver in the bulk market and come away with any kind of meaningful comparison for your materials. A conductive ink’s overall cost includes processing costs for the raw materials, research and development, and any capital costs you might incur to use it (think: printer!).
Copper conductive inks will oxidize when curing in air, which stops the copper particles from conducting electricity. In order to prevent this, you’d need to pump in a reducing agent, like nitrogen gas — which significantly increases the cost and the complexity of using it.
For these reasons — silver usually comes out on top in the conductivity/cost balance equation.
There are four major factors that you want to consider when choosing a conductive ink:
For conductivity, the main things that you want to keep in mind are your filler type, particle size and the percent composition of metal contained in the conductive ink itself. A higher metal content generally means an increase in conductivity, as does a smaller particle size — but see the cost section below why this can be a problem.
EXAMPLE: If you need a highly conductive ink — stay away from a carbon filler as it’s considered a resistive material and stick to something like silver. You’re also going to want something with a smaller particle size, and generally, a higher metal content. The best bet is to speak with your conductive ink supplier to go over your use case and they’ll be able to guide you to the best possible option. Don’t know who to go to? Contact us! We’ll put you in touch with some of our best contacts.
For this one, you really don’t care about the specific composition of your conductive ink, you just want to make sure that the viscosity value is within the acceptable range for the printing technology you’re hoping to use. Inkjet technology requires a less viscous conductive ink than, say, screen printing — this is why we say you can’t choose your conductive ink and your printing technology in the dark; you have to consider both together.
It’s all well and good if you’ve done this really cool thing using a platinum filler in a conductive ink, but if you’re looking to make a product out of it, you’re not likely going to see a return on your investment! Micrometer-sized flakes (1-5 µm in size) of some of these more expensive materials can be readily obtained and are less expensive than larger particle sizes — but still don’t compare to your more traditional filler materials like silver. There’s also an in-between size called “submicron” which spans the 300- µm range if you’re looking for a larger particle size, but looking to save a buck. In the end, you have to evaluate your idea with the practical applications of that idea in mind. Designing for design’s sake is a fool’s errand and no fun at all.
How does your ink process? In order for your conductive ink to become… conductive, you’re going to need to process, or cure, it. If your ink needs to be cured at high heat, but you’re looking to create something that’s flexible — you’re going to have a bad time. PET will melt at high temperatures! Generally speaking, you’re going to want to use thermally curable ink. It can be one of the easiest and most cost-effective methods of processing screen printable conductive inks. That’s not to say there aren’t some conductive inks out there that require some pretty precise processing, but thermally cured inks are by far the most available on the market.
In the end, if you’re designing with what you’re trying to accomplish in mind and make sure that your application and your material fit well together, you’re going to be on the right track to dispensing the right materials to bring your idea to fruition. Again this comes down to some of the more practical applications, so keep in mind how your ink needs to be cured and how that will affect the substrate the ink is printed on. Even FR4 boards will warp and/or scorch at sufficiently high temperatures! If high temperatures are a requirement, take a look into using ceramic substrates, or fired ceramic thick film.
PRO TIP: Choose your ink and the substrate at the same time.
Not doing this is like picking your t-shirt and your pants in isolation of one another — not the best way to have a party-ready ensemble! There’s no sense in picking an ink that requires a substrate that’s really hard to get your hands on or is prohibitively expensive. Of course, that doesn’t even take into consideration the more practical reasons for choosing a particular ink, like its resistivity, or the way the ink needs to be processed in order to lock the conductive particles in place. Stretchable inks are also often tuned for specific substrates, so you have to buy them together — unless you’re the brave type. As you can see, choosing your ink and your substrate at the same time can be an important piece of your particular product puzzle.
Now that you’ve chosen your substrate and your conductive ink, now it’s time to consider exactly how you’re going to print that beautiful, viscous conductive ink onto that carefully chosen material.
Let’s dive in!
Picking the wrong technology at the start of a project could lead to costly mistakes — capital investment upfront in terms of purchasing the equipment, and lost time when you have to pivot to make the design work with another printing technology.
Here’s a quick TL;DR of how these technologies stack up in one graphic:
If you want to learn more, please visit our website High Temperature Functional Materials Producer.