When I write about printed electronics, I tend to focus on the “electronics” a lot more than the “printed.” For this blog entry, I’m going to try to do just the opposite.
One of the reasons printing and electronics are a great match is their technical maturity, driven by continuous innovation, that allows them to be so ubiquitous in our daily lives. For example, no matter where or how you are reading this post right now, the device you are using is a product of both technologies working together to achieve functionality and aesthetics.
When I talk to our customers and technology partners, the initial conversation usually revolves around an end user application. From there, the conversation moves towards the technical path. And one of the first things we need to nail down is the printing process – centered on printing technique and the conductive ink to be used. And we all know there is a wide variety of printing techniques and conductive inks to choose from – each with their own advantages and limitations.
To narrow down possible choices, I learn from the customer their key requirements for the end application. Some of the main points that dictate their printing choice (roughly in order of importance) are substrate type, conductivity requirements, minimum feature size, printing speed, and post-processing limitations.
Let’s look at some commonly used substrates and a few of their applications – taking into consideration their printed electronics requirements:
Fabric – wearables, biomedical applications, automotive interiors
Plastics – flex circuits, automotive interiors and exteriors, smart packaging
Paper – disposable sensors, smart packaging
Glass – photovoltaics, touch screens, lighting
Now, if we think about the printing surfaces of these substrates, it becomes clear that they represent a wide range of surface roughness, surface energy, porosity, flexibility, etc. Even among each substrate type (such as fabric), there are numbers of sub-types (woven, knitted, non-woven, etc.) that have their own challenges and opportunities.
The customer can work around these numerous variabilities by choosing an easily transferable printing surface (such as TPU) that can be used as an overlay on their substrate of choice. If that’s not an option, various surface modification treatments can be employed – or the chemistry of the ink can be tailored to match the substrate’s requirement. Questions on tailored inks? Check in with our Inks Group for more info.
Once we know the substrate, the next considerations are the conductivity requirements of the customer’s application(s). They are going to vary. Think about the conductivity requirement for a large lighting array vs. the heating elements for a car seat. They are very different. In certain applications (such as capacitive touch sensors), it might be that different printed areas for the same application require different conductivity ranges. The upside to these challenges? Conductivity requirements can be met by tweaking a few variables:
Ink material – Not all materials converge to the same bulk conductivity. Bulk silver, for example, has a 10% higher conductivity than copper... and around 50% higher than gold. On the other hand, carbon can have a wide range of conductivity, depending on its allotropic form.
Conductive material loading – Conductive inks will usually be made up of the following main parts:
- conductive particles
- a solvent in which the particles are suspended
- a component that aids in uniformly dispersing the particles in the solvent
- a binder (typically polymeric) that enhances adhesion to substrates
Generally, a higher loading of conductive particles translates to a higher conductivity of the ink. But this higher loading could come at the expense of the binder – which itself might be very important for other properties like adhesion, stretchability, solubility, etc., of the ink.
Print thickness – Ultimately, the more conductive ink you put down, the more pathways you are providing for the current to flow through. For the same print design and same conductive ink, a 10 micron thick print will provide 10 times the conductivity as a 1 micron thick print. In broader terms, the thicknesses from commonly used printing techniques follow this trend:
Dispensing > Screen printing > Ink-jet printing > flexographic > gravure
While feature size isn’t usually a dominating factor, keep in mind certain printing techniques are far better suited for high resolution printing than others. Usually, finer features correspond to lower quantities of ink deposition; which in turn corresponds to thinner lines (so feature size limitations also follow the same trend shown above). Similarly, printing speed isn’t always a first order determinant of the printing technique, but can quickly become a major factor as technology transfers from R&D to production scale.
No one wants speed bottlenecks. When thinking of production capabilities, constant innovations across the board, irrespective of the printing technique, means the printing step is rarely the culprit. However, printing technique usually dictates the nature of the inks used, and that, in turn, determines post-deposition steps, such as drying and annealing, that might slow down the entire production line.
It’s all about end application requirements. And your choice of printing techniques and inks to be used should be carefully considered to meet those requirements. For certain applications, off the shelf conductive inks might work very well. But novel applications (whether for form or functionality) very often need the printing technique and ink to be exactly matched for optimal performance.
Decades of combined experience in designing conductive inks for novel applications has made the ink’s team at NovaCentrix a highly sought out group (we’re lucky to have them here with us in our Austin office – we confer with them a lot). You can learn more about our conductive inks here. Or feel free to drop the team a line.
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