What is Printed Electronics Design?

August 6, 2019 Tuomas Heikkila

What is printed electronics design? The answer is simple: it is electronics design. You utilize circuit theories, mathematical calculations and computer-based simulations for doing electronics design. You design electrical functionality and performance for a product using printed electronics materials. The materials are the key point as the materials used for printed electronics have different electrical performance characteristics than those used in traditional PCBs. In addition, electronics are built in different ways using printed electronics materials. It is well known how PCB traces are done. At first, an electronics engineer designs it, defines dimensions based on the electrical requirements, and after the design is ready, the manufacturing files are released. 

In manufacturing, the PCB is manufactured according to design files by copying electrical circuits on the copper of a PCB, for example, by exposing masked UV-sensitive photoresist film to UV-light. Then, copper which hasn’t been exposed to UV light is etched away. The result is a trace, as designed. Its dimensions are correct and it fulfills electrical requirements. In printed electronics, we need to achieve the same results, but with new design rules, materials, and manufacturing methods.

Inputs and outputs of printed electronics design are basically the same as they are in PCB design. The trick between input and output is also the same: electronics design. You need to take materials information and design rules into your design process and output is manufacturing files. The same physical laws are valid in the electronics design of PCB and printed electronics and these set the boundaries of what can be done. Two circuits, one made by PCB and the other made by printed electronics, can have exactly the same functionality, but circuit designs look and actually are different. This is is because of the physical capabilities and limitations of the material used in electrical circuits. In both circuits, you need voltage differences applied over impedance to get current flowing. To get the same current flowing in both circuits requires adjusting impedances to same level or setting circuit-specific voltage levels. These parameters we typically need to play in printed electronics design. We are looking for optimal solutions by fine-tuning impedances and setting correct voltage levels.

In electronics design, it is essential to know the material characteristics of the final product. From PCBs, you know copper thickness, sheet resistance, its thermal properties, the dielectric constant of PCB material etc. Exactly the same parameters you need to know from printed electronics. What is final thickness of silver ink conductor, what is its square resistance, what is dielectric constant of substrate material? You execute electronics design for these new materials. Ohm law, Kirchhoff circuit theory laws and Maxwell equations are applicable in printed electronics as well. There are hundreds of different conductive inks in the market, each having unique square resistivity. Some inks have high conductivity (which typically is still much more than pure copper), but after curing these cannot elongate at all. Other inks can be stretched after curing but conductivity is even worse. In electronics design, it is essential to understand what the square resistance of used ink is after final curing.

Another design challenge is material parameters used in printed electronics depend on the used production. Method how you print conductive inks, how you cure these, how are other printed inks beneath of conductor, as an example, affect the final square resistance. You change the production you may need to change your layout design. Or then production must be set according to the electrical circuit requirements of your design. It is extremely important you know how manufacturing of printed electronics is. This does not make any difference in PCBs, you need to know how these are built and what are limitations of this particular production, but in PCBs, manufacturing methods are more standardized and each manufacturing are basically similar with slight capability differences. In printed electronics, we are not at this level yet.

Conductive inks can be printed by several methods. The most used methods are screen printing and inkjet printing, and with googling you can find many others as well. The key thing relating the printing process is to understand manufacturing capabilities and its limitations. What is minimum clearance you need to have between traces? How many conductive layers you can use? What are the minimum and maximum widths for traces? Familiarize yourself with the design rules of the production you are going to use and check the design against these design rules. Quite many design rules available in PCB design tools can be used as such in printed electronics design with correct rule definitions. If manufacturing includes design rules which are not supported by electronics design tool it means you must do manual design rule check. For example, if you can use multiple conductive layers which are isolated by printed dielectrics, it means you have exactly same design rule between 1st and 2nd conductive layer traces than traces printed on the same layer. And this is not supported standard PCB design tools.

Also, printed electronics need components to get functionality and assembling components on printed electronics circuit is not a standard soldering process. Typical materials used in printed electronics are plastics which mean these thermal characteristics are different compared to PCB or FPC. This means adhesive materials are different as well. Low-temperature solders, conductive glues or other adhesive materials are typical for printed electronics SMA and these may require special footprint for components. Can you put traces going beneath of components? Do you need special keep out areas?  What kind of components you can put on printed electronics? These are questions which need to think about from different perspective compared to SMA of PCBs. In addition to this, manufacturing files for surface mount assembly may be different. Can you use paste stencil file or should you provide dispensing glue map instead? Check beforehand what SMA requires. 

Because printed electronics is quite new technology area, information of material characteristics and manufacturing methods are not available on the same scale as for PCBs. In addition to this, there are huge amount of printed conductive inks which have different electrical characteristics, and it depends on manufacturing equipment and methods how characteristics are in ready part. Design depends on implementing theories of electronics for new materials and manufacturing methods. For me, electronics design means utilizing theories, physics and mathematics ensuring electrical functionality and performance. These methods need knowledge of materials as an input. I have seen in printed electronics there is lack of materials information and sometimes decisions were done without calculations behind it. Then it is not design, but it is guessing. And this is not electronics design then.

Would you like to find out more about how Altium can help you with your next PCB design? Talk to an expert at Altium, read about printed electronics in Altium Designer documentation, or listen to a podcast about Printed Electronics materials or Printed Electronics at Tactotek to learn more about printing electronic circuits directly onto a substrate.

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