3D Printing For More Circuits

After several years of experimentation, and growing success in volume manufacturing for some use cases, technologies for 3D printing of electronic circuits are becoming more common. Some innovations in processes and materials are moving these technologies closer to mainstream electronics manufacturing.

Christopher Tuck, professor of material science at the University of Nottingham, observed that what’s particularly attractive among the many different processes and materials used for additive manufacturing (AM) is the ability to build up one layer at a time, which increases design flexibility. That results in improvements in performance and heat management, plus the ability to optimize component architectures.

It’s often said in 3D printing circles that with AM, complexity is free. Instead of being bound to multiple steps to create a planar structure in traditional IC or PCB manufacturing, the board and its components can be created together using multiple materials in fewer steps, sometimes in a single process. This allows the creation of a variety of shapes in boards and finished products, and complex circuitry like antennas and sensors can be produced easier and faster.

For electronics, AM’s ability to print 3D circuits means packaging can be done in new and different ways. Like several other providers, nScrypt sees 3D printing of circuits as the next generation of electronic packaging. “We can take advantage of the third dimension, strategically arranging circuits and embedding electronics inside a 3D structure so they can outperform standard PCB circuits,” said CEO Ken Church.

What kinds of circuits can be 3D printed?
Not everything can be manufactured using this approach, however.

“The industry today can print a 2D circuit, and we can sort of print a 3D circuit, meaning traces,” said Tuck. Others include sensing devices, FETs, microbatteries, inductors, capacitors, resistors, diodes, and RF circuits. The Holy Grail would be the ability to print active devices, but that’s not possible yet.

3D printing of electronics is still primarily lab-based. The most active components printed to date are FETs produced by several different university labs, including the University of Nottingham, but those are still relatively large-scale. LEDs have only been partially printed. The indium tin oxide layer has not been included because of its required processing conditions, said Tuck.

The most common methods for 3D printing electronics are derived from existing methods for 2D printing electronics. These include screen or ink printing, and extrusion or paste extrusion deposition. Extrusion processes are large-scale, with a minimum bead width of 150 microns, said Tuck. Inkjet printing generally achieves a line width of about 50 microns, while Optomec’s Aerosol Jet process prints line widths as low as 1 or 2 microns.

Fig. 1: Printed 35-micron wires/interconnects for stacked die in advanced semiconductor packaging applications. Source: Optomec

The Aerosol Jet process 3D-prints circuits, sensors, and devices on 3D structures, and it can build multiple layers of circuitry on or in a 3D shape, according to Bryan Germann, Optomec’s Aerosol Jet product manager. The technology can deposit a few microns of gold on a substrate that has complex topology, and it can do this with 10-micron resolution, in an open atmosphere, and with no other processes required.

In June, Optomec received new patents for using its Aerosol Jet technology to create 3D microstructures. These are miniature elements with feature resolution down to 15 microns. The technology has achieved layer thicknesses of 100nm, and aspect ratios greater than 100X for millimeter-high structures.

nScrypt’s Church noted the inkjet process scales down to sub-20 microns if needed. Most of the very small lines it prints are done in silver, but copper materials are improving, and the process also can dispense them. “Twenty years ago we were 10 to 20 times as resistant as bulk copper,” he said. “We’re still inferior in our conductivities, but now it’s only three to five times as resistant.”

The company began by 3D printing resistors and capacitors, then filters, using paste or ink, and printing conductor lines mostly in silver because that material doesn’t oxidize. Then it shifted to actives, starting with polymer FETS.

“The polymer made those FETs very slow and very large, so they weren’t competitive with silicon FETs,” said Church. “We printed antennas really well, too. These were conformal and tightly wrapped.”

ChemCubed’s ElectroJet multi-layer, multi-material process uses specialized materials to 3D print PCBs, packaging, hybrid electronics, antennas, photovoltaics, RFIDs, and passives.

In addition, HRL Laboratories has 3D-printed polymer interposers with what it calls “previously impossible” slanted and curved vias in ceramic and polymer materials. The vias are then metallized to electrically connect different devices and ICs. With 2-micron resolution and diameters of less than 10 microns, they allow complex routing. HRL developed its print process for polymer-derived ceramics on Boston Micro Fabrication’s projection micro stereolithography (PµSL) printer, and expects the new capability will help improve packaging.

Process and materials challenges
Most technologies for 3D printing circuits print on a flat or curved plane using an inkjet technology. Optomec’s Aerosol Jet technology, for example, prints with very high resolution onto complex, non-planar shapes.

With the goal of printing the entire object, nScrypt tries to eliminate through-holes and vias by printing ramps and using them to carry signals or power from one layer to another, on the outside of a curved substrate.

“The industry is still stuck on 2.5D square boards that are put in round objects, so all that space is wasted,” Church explained. “We say, print the board as a cylinder and put the electronics in [that cylinder’s] wall. The payload is no longer filled with electronics, and the number of connections you need is reduced.”

Fig. 2: Electronics are 3D printed on and in the walls of this 3D-printed cylinder. The circuit is a Bluetooth with MCU and five different sensors. Source: nScrypt

In inkjet printing, ink characteristics can be key. Last year, for example, sensor provider HENSOLDT AG 3D printed the world’s first 10-layer PCB. The board, which has electronic structures soldered to both of its outer sides, was achieved using Nano Dimension’s 3D printing technology and conductive ink, and a newly developed dielectric polymer ink.

In April, BotFactory began offering new resistive inks for its SV2 PCB printers. The inks enable speedy printing of resistors and sensors on flexible or rigid substrates. They also help resistors stay attached to flexible materials, and allow the fabrication of resistors with specific ohmic values.

The University of Nottingham, which uses inkjet printing techniques, would like to 3D print the entire electronic part “in one go,” said Tuck. That requires the elimination of support structures, as well as the right resolution and material properties. These must be as close as possible to those of the final device, and as close as possible to what the market actually needs. Challenges include the fact that many materials must be highly diluted or they won’t pass through the inkjet nozzle.

A long-term problem in 3D printing of functional electronic devices has held back the technology from use in more advanced applications. This problem is the loss of conductivity in materials and devices caused by functional anisotropy, and the different electrical conductivity between horizontal and vertical directions in layers. In this case, it’s produced by printing conductive inks containing metal nanoparticles. Recent experiments by the University of Nottingham’s Centre for Additive Manufacturing revealed that one ingredient in these inks — organic chemical stabilizer residue — is the culprit. The Centre is developing new ink formulations to overcome this problem.

Fig. 3: (Left) Drop-on-demand jetting of inks containing silver nanoparticles with organic stabilizer and in situ solvent evaporation. (Right) Optical image of a printed layer of silver and chemical maps showing the distribution of silver and organic residues at the surface of a printed layer on a silicon wafer. Source: University of Nottingham

Tuck noted that any process for printing electronics must be able to print multiple materials for the board, as well as the circuits. Anisotropy, which is a spatial difference in property between horizontal and vertical layers, is a problem in all of them. “Understanding what you’re printing is important: how does each layer interact with the next layer, or the diode material, or the semiconductor material?” he said.

Although several processes can print antennas — Optomec was one of the first to do so, and in volume — 3D printing of RF has both advantages and constraints to consider.

When 3D printed with nScrypt’s process, antennas can contain less resistive material without affecting performance, said Church. But RF circuits can be problematic due to conductivity and smoothness constraints. “Instead of being stuck in 2D on a plane surface because that’s what a board always has been, our technology is built for curved surfaces,” he said. For the U.S. Air Force Research Laboratory, the company’s Factory in a Tool system has been used to 3D print a doubly curved conformal phased-array antenna that actually gets better performance.

In RF, maintaining a certain circuit width to carry a certain frequency is also critical. With 3D printing, the line width on the ramp can be controlled to maintain that frequency, which is not possible on a traditional PCB, said Church.

Printing on bare die also has special requirements. That’s normally done with a wire bond, but nScrypt instead prints a ramp around the die to hold it in place. “We can then print from one pad up the ramp to the bare die pad,” said Church. “Going layer by layer this way lets us print more electronics inside, and we can put die anywhere in this object.”

Fig. 4: A 3D-printed, curved 4 x 4 active phased array antenna. Source: nScrypt

3D printing on chips depends on the ability to print transistors. But the problems are being able to print transistors with the necessary materials. Those may not be printable, or they may have other issues, such as not being able to print them small enough. “With the extrusion base type of 3D printing, to keep electronics small we’re limited on line width,” said Tuck.

Resolution versus throughput is a tradeoff that always must be made. And with solvent-based materials, those tradeoffs also include wasted material.

What’s being printed
AM often is used to augment an existing process for a specific application. “Why use a particular AM method? Because the other manufacturing methods available fall short or don’t work, or because there’s no other way to make that particular interconnect or to make that particular device,” said Germann. “Our Aerosol Jet technology is routinely one step in a massive process, wherever AM fits into the supply chain.”

Optomec focuses on production first, making very small features with functional materials, and depositing them onto complex geometry. “The scale of full production for AM varies hugely, from say 10 per month for military products up to 1 million per month for consumer electronics,” said Germann. AM companies must find the right applications for their technology to scale. The biggest users of AM in production environments currently are those with high value and low volume.

AM equipment providers that target specific applications or industries are more likely to be successful, because different processes are best suited to different types of circuits. Some processes are used for making antennas, others may be used for making parts of LED screens, and some providers focus only on prototyping PCBs. “At Optomec, we have four main market verticals. We don’t try to print everything and we want to sell equipment, software and process solutions for mainstream manufacturing,” said Germann.

Fig. 5: Top row: Molded plastic insert for smartphone with 3D-printed silver antennas for WiFi, Bluetooth, and 3G/4G phone signals. Bottom row: 3D-printed capacitive touch sensors. Source: Optomec

Since the industry has demanded it, nScrypt’s performance and speed have drastically improved, said Church. “We set up our printers in a line as semiconductor production and electronics manufacturing require, but we can print onto a 3D object. Our biggest printer is 8 feet by 12 feet.”

The military, aerospace, and aviation industries are major users of 3D-printed circuits. For example, NASA currently has installed an nScrypt bioprinter in the International Space Station (ISS), and will add the company’s electronic printer in 2024 or 2025, said Church. And Nano Dimension, one of the early companies to 3D print circuits, announced recently it is 3D printing RF circuits for use on the ISS.

For the military, frequent repairs on ground vehicles must be done in advance before something breaks. Because electronics can perform vehicle health monitoring, the same theoretically can be done in a car, and this is a trend in transportation, said Church. Another up-and-coming industry is energy, where circuits are printed for solar cells and wind towers.

Medical devices aren’t far behind, and some small AM electronics processes also are being reviewed for use in consumer and automotive applications. “These processes are now permeating into many manufacturing industries,” said Germann.