Electronics additive manufacturing can output the same result as conventional PCB methods, but getting there is a much different process.
When considering the benefits of additive manufacturing processes for PCBs, several factors need to be considered. Each printing method has its strengths and weaknesses, so it’s important to understand the nuances of each. Items like circuit layout or total resistance of traces carrying power or signal may be different from traditional PCB processes. Some printed electronics technologies like inkjet have hundreds, or even thousands, of nozzles, which enables them to swipe over the area and recreate the image of the pattern in seconds, typically printing a layer thickness less than 1μm. Screen printing, another common method, can create layers with a minimum thickness of 25μm with very flexible feature size. Screen printing is not ideal for prototyping, however, as stencils for each pattern to be printed must be used. Aerosol jet can print digital referenced-CAD or imagery, but the cycle time may take longer. The way it works is like drawing with a pencil. If there is an area to fill, you would have to move your hand back and forth, which can affect the cycle time to make the circuit. Designers must weigh the commonality with traditional PCB layouts vs. the time it takes to make the circuit. These are three prime examples of the different applicable technologies and nuances of printed electronics.
One of the misnomers of printed electronics is that it’s a 1:1 comparison to PCB fabrication. For all printed electronics technologies, the cross-section of the trace needs to be optimized in order to hit the electrical targets. In traditional PCB design, the designer doesn’t really think about that. Because a copper-clad sheet is full bulk metal, one might think, “How much copper is needed to carry my current or signal?” much like wire gauge standards. For printed electronics, however, whether screen printing, inkjet or aerosol jet, almost every case involves printing nanoparticles sintered at a lower temperature than the full melt temperature, resulting in traces that are higher resistance than their copper-clad counterparts. To add more complication, not all materials sinter to the same degree due to sintering mechanics of powders used, so every ink recipe from different manufacturers will result in differing electrical performance using the same process.
This is a fundamental design rule to consider for electronics additive manufacturing. In almost all cases, additive is performed with some type of slurry, paste or ink comprised of nanoparticles suspended in some form of organic binder or solvent mixture. To make those materials conductive, they are sintered or fired and melted together. There is some inherent porosity as a result. This porosity, along with the polymers and organic binders included in the inks to promote adhesion to various substrates, results in a slightly higher resistivity of the material on the substrate compared with a full piece of bulk copper.
If a specific impedance of a trace or a total resistance of a circuit is needed, or a significant amount of power must be dumped through a circuit, this decreased conductivity will affect that. As such, the designer must think about how thick or wide to make the traces to compensate for that increased resistivity. Designing the circuit to the temperature the part is going to reach and matching the thickness of the cross-sections are important design considerations.
If you were to take a scanning electron microscopy (SEM) cross-section of a printed feature, it would be different from solid copper clad, as you would see thousands of melted nanoparticles and some resulting porosity, as well as remnant organic binders or polymers. As a result, printed features are normally higher resistivity by some factor than any bulk metal; for example, a printed silver might be anywhere from two to 10 times higher, or more, than bulk silver. It’s the harsh reality of additive processes.
The general rule of thumb is the conductivity of the material drives everything, and so the resistivity measurement must be known for a given situation. Say, for example, a circuit is being printed onto Kapton. Kapton is a fairly high-temperature material. Silver nanoparticle ink can be sintered at 200°C or higher without much issue, so a highly conductive trace is possible. But still, it’s not going to be like bulk silver. The conductivity must be measured, or the measure of what that conductivity is going to be obtained from the ink manufacturer. Then the geometry is scaled according to that conductivity measurement. When planning boards, electrical designers don’t normally think about the inherent property or resistivity of copper or silver. That’s because those material properties are known.
Likewise, manufacturers should understand the print fidelity because not all inks print the same. They should characterize the electrical performance and calculate the resistivity of the materials. Adhesion and environmental studies should be performed to see what the ink will stick to. Will it stick to glass? Will it stick to polycarbonate? Or, they might be able to contact ink suppliers such as Lord, Henkel or DuPont and ask which inks stick to which materials.
For prototypes or production, printed additive electronics have some significant advantages in flexibility and speed. Tradeoffs in material performance and the types of geometries that can be created should be considered, however. Designers should consider the inherent material property differences between traditional PCBs and printed circuits before attempting to leverage this technology. This is not so much a barrier to entry as it is a set of design rules to follow to ensure a successful application.