Design Philosophy in Printed Electronics
In printed electronics, I think about design philosophy from a new perspective. My question about design philosophy is related to printed electronics manufacturing: do I optimize my layout for electrical performance or manufacturability, which may then mean compromises for electrical characteristics?
In printed electronics design I need to make trade-offs between trace resistance, voltage levels, power consumption, and layout area. With proper balance of all of these factors, I have been able to define the topology for an electronics solution which implements the required functionality. Resistance plays a significant role in this process, and typically doesn’t enable new features, but instead sets limitations. The key objective regarding resistance is always to minimize it. Two tactics for doing so are using highly conductive inks and maximizing trace thickness. This may sound easy, but these tactics may expose other challenges related to design for manufacturing. In printed electronics, it’s not always obvious, and your manufacturing capabilities might not always be clear enough. You must assess manufacturing capabilities and limitations and design printed electronics layout accordingly.
Printing method defines trace characteristics
Traces in printed electronics can be manufactured by many different methods, and all have own parameters for how to optimize electrical characteristics of the trace. One of these methods is screen printing, which is fast, cost-effective, and easily scalable for high volume production. In screen printing the selected screen is a key factor defining the dimensions of the printed and cured trace. Typically, reusable screens are prepared for production so that when a photoresist capillary is added to the screen, the layout image is exposed on the capillary, and then the layout image is revealed by washing the non-exposed capillary away. Together, the selected screen and added capillary make up one of the parameters defining how much ink is transferred to the film during printing. Additionally, the thickness of the printed trace directly defines what its resistance is. The trace gets different thicknesses if either the screen, capillary, or both are changed to a different type. In addition, the selection of screen and capillary defines limitations for the minimum trace width and minimum clearance between traces. If the production has a limited range of screens and limited capillary options, it limits the capability of manufacturing different electrical characteristics as well. Regardless of the method by which the trace is printed, it’s important to understand what kind of parameters affect electrical characteristics, and what possible limitations printing imposes on layout design.
Processing liquid materials
In printed electronics, traces are manufactured by processing liquid materials, and traces become solid during curing processes. This liquid raw material is the most important parameter for determining trace resistance, and by printing we define only how much material there will be in the trace. The “quality factor” in this case is the cross sectional area of the trace. The bigger the area, the more raw material used and the lower the resistance. However, it is good to notice that the raw material is what matters the most.
Typically it doesn’t make sense to compensate for resistance of a low conductive ink by increasing cross sectional area, because that solution requires significantly more layout room. Instead, from a resistance point-of-view, changing to a more conductive ink is more effective than increasing thickness or widening the trace. Sometimes, selection of ink based on the application is important. For example, translucent conductive ink may be used for illuminated capacitive sensor applications, where light should come through the electrode. Another case may require high conductive ink because higher current is required for driving LEDs. In screen printing, ink selection typically affects how the screen must be prepared and how liquid ink needs to be processed during manufacturing, including setting parameters which this technique sets for the printing machine (for example, selection of squeegee, printing speed, printing pressure, etc.). The conductive ink selected for the product affects both screen selection and the parameters for getting target trace dimensions. Together, trace dimensions and ink define the final resistance values of the trace.
Design for manufacturing or manufacturing for design?
Three critical parameters determine the resistance; selected ink, thickness of trace, and width of trace. If we have a critical limit for resistance, and freeze one parameter, it typically affects the other parameters as well. If we fix the conductive ink and want to minimize resistance, it means affecting the maximum thickness and maximum width of the trace. If we have especially limited room for the layout and we need to minimize trace width, our ability to reduce trace resistance is limited to selecting high conductivity ink or/and maximizing trace thickness. Regardless of our choices in printed electronics design, these factors always affect manufacturing printing setup and parameters.
Design for manufacturing in printed electronics is more complicated than designing a PCB for manufacturing. In printed electronics, there are a variety of methods to print certain resistance. For instance, you can achieve the same resistance with a thin layer of high conductivity ink or thicker layer of lower conductivity ink. However, you’re allowed to drive different amounts of current through these traces. Using the same ink, you will get different square resistances just by changing the thicknesses of the capillary used in the screen, which will, in turn, affect ink deposition and trace thickness. There are many individual parameters during manufacturing that directly affect the square resistance value of a printed trace, and knowing every detail of these requires lots of experience and knowledge of printing processes and conductive inks. Poor quality control, lack of information about ink processing, or lack of printing experience may result in incorrect square resistance values and significant variation between parts.
One additional dimension when compared to the PCB world is conductive ink selection. Conductive ink selection is a critical element for printed electronics performance and thus is one part of printed electronics DFM. Processing any functional ink during manufacturing requires know-how of the ink for achieving target results. In the best cases, the ink has been validated by the printing manufacturing and they have the knowledge to select screens, tools, and parameters based on the electrical requirements. For me it adds huge uncertainty if manufacturing does not have validation data to show they can process this particular ink to get proper results.
All of these bring out many questions relating trade-offs between electrical characteristics and DFM. What is reasonable/acceptable level of increased resistance, and thus power consumption for allowing simplified manufacturing setup? Or should we instead push manufacturing to adjust their equipment and methods according to electrical requirements?
Requirements for manufacturing
There are two extreme options for designing printed electronics from the manufacturing point-of-view. The first option is to follow manufacturing limitations and design layout according to these rules. This option necessitates tradeoffs between electrical performance, and manufacturability of certain production abilities. The second option, is to design your layout based only on the electrical performance, and have manufacturing adjust printing equipment and parameters accordingly. This option does not make any compromises for electrical performance, but the tools and methods are adjusted so that the desired result will be realized. All other options exist between these extremes. It is clear there is no answer to which option is optimal, or how things are done correctly, and because of this challenge, my answer is to specify critical requirements for manufacturing.
Naturally, it is clear that the application and requirements it sets for electronics determines what is important in printed electronics manufacturing. Is it some specific ink we must use? Is it trace thickness? Or it is just resistance, no matter what the ink or trace thickness? Make sure you understand clearly which are the most important characteristics, and specify them. You must also take into account the power budget and consider how much you can compensate for higher resistance by increasing supply voltage levels. Once these are clear, unambiguously specify critical characteristics, and define controls for these to be followed during manufacturing. I have had cases where certain resistance ranges using certain ink was very critical because of relatively high current requirements and a limited power budget. Another case involved the driver voltage being too high compared to the current, so that it didn’t matter which ink we used or what the resistance was as long as it was silver-based. In all possible cases, the voltage loss in the trace was so small—even with the thin layer of low conductive silver inks—that it was negligible. Of course manufacturing requirements were different for these example cases. The first case required strict manufacturing requirements, whereas the second one was more open.
Printed electronics design philosophy relates mainly to trade-offs between manufacturing capability and electrical characteristics. In this game, it is important to understand the critical characteristics set by the application, or requirement specification, and know which of these two cannot be compromised. For printed electronics manufacturing, it may be challenging to precisely specify everything, because production may not have similar capabilities, or because following these specifications may require significant resources, leading to extra costs. And if you specify one parameter, there may be several different options to fulfill it, but it might mean another critical item is not getting fulfilled. Printed electronics is very sensitive for manufacturing, and all printed electronics designers should be aware of this.