Once confined mainly to home-brew tinkering, circuit boards created via 3D printing are now practical for some manufactured products.
J.F. Brandon, BotFactory Inc.
In the past ten years, 3D Printing has gone from a niche prototyping tool to a process acceptable for mass production. Most of the recent hubbub has been about monolithic plastic and metals. But new materials and processes have appeared to help create 3D-printed PCBs that meet long-standing engineering problems.
If the history of electronics manufacturing could be summarized in one phase, it would be, “Shrinking everything to nothing to squeeze out something faster.” The push towards miniaturization has been driven by the inviolable laws of nature – faster devices that consume less power require shorter electrical paths.
However, the printed circuit board is an outlier in the electronics world. PCBs still use basic drilling and plating processes perfected 50 years ago. That is not to say that PCB manufacturing is trivial or antiquated. But the investment in new PCB manufacturing methods is a pittance compared to the hundreds of billions put into chip fabs by IC makers such as TMSC and AMD.
It is worth looking at the details of PCBs and their construction. The word ‘printed’ in printed-circuit board only describes half of the process – the silkscreen masks are the only part that is printed. A PCB is originally copper foil on a rigid fiberglass laminate which is selectively etched, drilled, and plated using a set of silkscreens and chemical baths to produce the final product.
The sole purpose of the PCB is to reliably connect passive and active components and provide a reliable platform for integration or interactions with the rest of a product. For example, the PCB in the average computer keyboard connects electronic elements together, but it also must manage human interactions and provide a sound mechanical connection to the body of the product. In addition, PCBs must be designed so they can easily be stenciled with reflow solder paste and integrated into industrial surface-mount pick-and-place lines. Optical inspection and flying-probe systems require PCBs that can be easily analyzed and binned for repair or discard automatically. All in all, modern PCBs can play a variety of roles within the end products in which they are found. So it is worth considering new manufacturing processes that can expand the capabilities of PCBs.
PCBs have thermal, electrical, geometric and mechanical requirements that go beyond what most materials for 3D printing can offer. For example, the average $500 3D printer that uses Fused Deposition Modeling (FDM) uses PLA, ABS and PETG which melt under the harsh gaze of any standard soldering station. Metal 3D printing techniques are designed to handle one material at one time. Yet PCBs require, at a bare minimum, a dense and conductive metal for conductors.
Three technical paths have appeared for PCB printing: inkjets, extrusion, and additive manufacturing (AM)-electroless plating. First consider ink-jetting. New nanoparticle and particle-free inks have allowed inkjet printing to go beyond CMYK inks and graphics. Inkjets can now lay down metal (overwhelmingly silver) inks in fine patterns on flexible materials. In combination with a polymer ink, it is possible to create PCBs with complex multilayer circuitry (blind and buried vias are trivial items) in only a few steps on a single machine.
The inherent advantage of creating PCBs layer-by-layer this way is that each layer can be tested and validated. The minimal level of processing simplifies the dispensing solder paste, part assembly, and testing for every layer. The disadvantages are that material dispensing via inkjet printing is slow relative to all other additive manufacturing processes– deposition speeds can be in the millimeters-per-hour range. It’s possible to create precise traces with inkjet printing (metal traces with 100-micron widths are commonly attainable). But the smaller droplets limit deposition speeds.
And there are problems with metal inks: Applying too much can cause bleeding and cracking during drying, thus limiting PCB fabrication speeds. Solderability is a particular blind spot – silver can wilt under standard pastes like SAC305, suffering from tin ‘scavenging’ silver during reflow. In addition, inkjet polymers melt at temperatures that standard PCBs easily manage. Fortunately, industry-accepted low-temperature tin-bismuth and indium-based solder pastes are compatible with inkjet-printed PCBs.
Today there are two PCB printers that use ink jetting – the BotFactory SV2 and the Nano Dimension DragonFly. Each printer uses the same process to create multilayer circuitry, although the BotFactory SV2 utilizes inexpensive thermal inkjet heads instead of the piezo heads found in the DragonFly. Nano Dimension has focused on printing for production, whereas BotFactory has emphasized integration of pasting and PCB assembly into a small unit, working on projects with the USAF to automate the entire process. In this regard, BotFactory is unique in the electronics industry and is the only commercial product below $20,000.
Single nozzle jetting
Inkjetting isn’t the only way that nanoparticles can be deposited to create circuitry. An alternative method extrudes lines onto flat surfaces and uses fused-deposition modeling to provide a polymer structure for the traces to inhabit. Pastes are notoriously difficult to control when creating fine traces and spaces, requiring precise control and extremely close contact with the substrate surface.
Here the target surface must be covered twice – first for mapping, then for pasting. The two-step procedure handicaps the scalability of the process for production. Pastes must be devoid of air pockets lest each bubble act as a kind of ‘spring’ and impede the extrusion process. The flip side of using viscous pastes is that metal-loading is higher and it’s possible to deposit metal in thicker layers, boosting conductivity and solderability right off the bat. However, at this time, silver is the overwhelming favorite material and thus suffers from the same constraints as inkjet-printed PCBs in regards to silver scavenging.
The first example of 3D printed electronics was demonstrated by Voxel8 in 2015. The printer used FDM and paste extrusion to create basic circuit traces. After delivering early beta systems, Voxel8 switched to industrial-scale fabrication with a broader focus on multi-material printing rather than just electronics. nScrypt has taken a similar tack, creating a more general tool that includes pasting as well as polymer extrusion to create three-dimensional objects with traces within the object.
AM + electroless plating (also abbreviated as AMEP, or 3D-Print-and-Plate) is a completely new category of AM that combines existing additive manufacturing processes and well-understood electroless plating techniques. An object is printed via stereolithography (SLA) or fused-deposition modeling (FDM) using a distinct metal-loaded material that can be electroless-plated afterward. AMEP continues to be a topic of academic research. Last year, researchers at UCLA published results on using SLA to create multi-material prints that could be selectively plated. Using two vats of pure and metal-loaded polymers, the process enhances the existing premise of AM with no extra constraint on fabrication speed.
Palladium, a metal that is traditionally extremely expensive, normally serves as a seed material in this process. But on the other side of the world, UK researchers devised a way of printing less expensive metal on a new polyimide material. Polyimide (also known as Kapton) is highly prized by electrical engineers in flexible and printed electronics for its low thermal expansion and dielectric constant. UK researchers found UV energy can be used to chemically bond silver particles and the polymer chains, providing seeds for plating afterward.
The technology described above has not been commercialized, but the overall concept has been utilized for creating unique antennae at firms like Swissto12. There a high-resolution SLA print is made, coated, and then electro-plated (not electroless plated). Electroplating requires a current to initiate and control the plating process, whereas a PCB often has unconnected traces and vias that require plating.
Overall, the greatest challenge to AMEP is that it cannot create conductors within an object unless there are exposed holes or the PCB undergoes multiple dips into the plating baths. As it stands, the technology has the ability to meet all the technical requirements for high-performance PCBs, including ease-of-solderability and high-thermal tolerances.
What it’s not
There is some confusion about what is and isn’t ‘AM electronics,’ and certain lines have been drawn. In the AM industry overall, any process that uses subtractive processes is not additive manufacturing. So it is fair to argue that any process that builds circuitry on pre-existing substrates or augments AM with subtractive processes is not 3D-printed electronics. Consider the traditional PCB: UV-curable polymers mask copper foils, utilizing a process and materials seen in AM technologies like inkjet printing and stereolithography. By conveniently ignoring the drilling process for vias and shaping, one could argue that PCBs are made with AM when they clearly are not.
When an entire model is fabricated using AM, it typically has characteristics and form factors that go beyond what would be possible if subtractive fabrication is included. In other words, use of subtractive processes detracts from the entire point of adopting AM.
An example of a semi-additive process that is commonly cited as 3D-printed electronics is Laser Directed Structuring, a technique developed by LPKF. Essentially, an object consists of an injection-molded plastic that has been filled or coated by an organometallic compound. When a laser applies a circuit pattern to the surface, metallic seeds form and create an electroless nickel or copper plating. The technology is limited by the reach of the laser, inhibiting the possibility of allowing conductors to pass thru the object. Thus LDS parts are not true 3D PCBs by any means.
The same limitation also applies to aerosol jetting, a concept commercialized by Optomec. Here a carrier gas (nitrogen typically) jets out of a fine nozzle at high speed and carries a fine suspension of materials such as nanoparticle metal inks. The wide variety of viscosity, metal-loading and material choice makes aerosol jetting a candidate for creating sensors on objects, overcoming the limited choice of materials for LDS. As both processes utilize non-AM elements, they arguably do not create 3D-printed electronic devices.
Credible advances in materials, metals and polymers have made it possible to 3D-print PCBs that are useful in many applications today. However, the 3D-printed circuit board made in a few hours which is a perfect replica of a traditional PCB is the Mount Everest of AM. The most mature technique is inkjet printing; it comes closest to reaching the necessary geometric and electric properties, with materials advancing quickly to meet the thermal and mechanical needs. Extrusion is well-understood but hard to scale, and its fundamental capabilities are uneven. AM-EP is the dark horse in the race, combining old and new techniques to provide another path to the 3D PCB.
Ultimately, all technologies can be viable paths to reducing PCB size and shortening traces, yielding lighter devices in forms that would have been unthinkable just ten years ago.
BotFactory Inc.
www.botfactory.co/
