3D print more circuits

2021-11-10 03:44:54 By : Mr. Sky Zeng

How additive manufacturing changes packaging and PCB design.

After several years of experimentation and more and more successes in the mass manufacturing of some use cases, 3D printing technology for electronic circuits is becoming more and more common. Some innovations in processes and materials are bringing these technologies closer to mainstream electronics manufacturing.

Christopher Tuck, a professor of materials science at the University of Nottingham, observed that among the many different processes and materials used in additive manufacturing (AM), what is particularly attractive is the ability to build one layer at a time, which increases design flexibility. This leads to improvements in performance and thermal management, as well as the ability to optimize component architecture.

It is often said in the 3D printing industry that with AM, complexity is free. In traditional IC or PCB manufacturing, there is no need to go through multiple steps to create a planar structure. Circuit boards and their components can be created together using multiple materials in fewer steps, sometimes in a single process. This allows various shapes to be created in boards and finished products, and complex circuits such as antennas and sensors can be produced easier and faster.

For electronic products, AM's ability to print 3D circuits means that it can be packaged in new and different ways. Like several other vendors, nScrypt views 3D printing of circuits as the next generation of electronic packaging. CEO Ken Church said: "We can use the third dimension to strategically arrange circuits and embed electronic devices in 3D structures to make their performance better than standard PCB circuits."

What kind of circuit can be 3D printed? However, not everything can be manufactured using this method.

"Today's industry can print 2D circuits, and we can also print 3D circuits, that is, traces," Tucker said. Others include sensing equipment, FETs, micro batteries, inductors, capacitors, resistors, diodes, and radio frequency circuits. The Holy Grail will be the ability to print active devices, but this is not yet possible.

3D printing of electronic products is still mainly based in laboratories. The most active components printed to date are FETs produced by several different university laboratories, including the University of Nottingham, but these are still relatively large. The LEDs are only partially printed. Tuck said that because of its required processing conditions, it did not include an indium tin oxide layer.

The most common method of 3D printing electronic equipment is derived from the existing method of 2D printing electronic equipment. These include screen or ink printing, as well as extrusion or paste extrusion deposition. Tuck said the extrusion process is large-scale, with a minimum bead width of 150 microns. The line width of inkjet printing is usually about 50 microns, while the line width of Optomec's Aerosol Jet process is as low as 1 or 2 microns.

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

According to Bryan Germann, Aerosol Jet product manager at Optomec, Aerosol Jet can 3D print circuits, sensors, and devices on 3D structures, and build multilayer circuits on or in 3D shapes. This technology can deposit several micrometers of gold on a substrate with a complex topology, and can achieve this with a resolution of 10 micrometers in an open atmosphere, and no other processes are required.

In June, Optomec received a new patent for using its aerosol jet technology to create 3D microstructures. These are micro-elements with feature resolutions as low as 15 microns. This technology has achieved a layer thickness of 100nm and an aspect ratio of millimeter-level structures greater than 100X.

Church of nScrypt pointed out that the inkjet process can be reduced to less than 20 microns if needed. Most of the very small lines it prints are done with silver, but copper materials are improving, and the process can also distribute them. "Twenty years ago, our resistance was 10 to 20 times that of bulk copper," he said. "Our conductivity is still poor, but now its resistance is only three to five times."

The company started with 3D printing resistors and capacitors, followed by filters, using pastes or inks, and printing wires mainly with silver because this material does not oxidize. Then it turned to active materials, starting with polymer FETS.

"Polymers make these FETs very slow and very large, so they can't compete with silicon FETs," Church said. "We also printed the antennas nicely. These are conformal and tightly wrapped."

ChemCubed's ElectroJet multilayer, multi-material process uses special materials to 3D print PCBs, packaging, hybrid electronics, antennas, photovoltaics, RFID, and passive components.

In addition, HRL Laboratories has 3D printed polymer interposers with so-called "previously impossible" inclined and curved through holes in ceramic and polymer materials. The vias are then metalized to electrically connect different devices and ICs. With a resolution of 2 microns and a diameter of less than 10 microns, they can realize complex wiring. HRL has developed a printing process for polymer-derived ceramics on Boston Micro Fabrication's projection micro-stereolithography (PµSL) printer, and expects the new features to help improve packaging.

Process and material challenges Most 3D printing circuit technologies use inkjet technology to print on flat or curved surfaces. For example, Optomec's Aerosol Jet technology can print at very high resolution on complex non-planar shapes.

In order to print the entire object, nScrypt tries to eliminate vias and vias by printing ramps, and uses them to transmit signals or power from one layer to another on the outside of the curved substrate.

“The industry is still stuck on 2.5D square plates where round objects are placed, so all the space is wasted,” Church explained. "We said, print the circuit board as a cylinder, and then put the electronic device on the wall of the [cylinder]. The payload is no longer full of electronic devices, and the number of connections you need is reduced."

Figure 2: 3D printed electronic components on and on the wall of this 3D printed cylinder. The circuit is a Bluetooth with MCU and five different sensors. Source: nScrypt

In inkjet printing, ink characteristics may be the key. For example, last year, sensor supplier HENSOLDT AG 3D printed the world's first 10-layer PCB. The two outer sides of the circuit board are soldered with electronic structures, which are made using Nano Dimension's 3D printing technology and conductive ink and a newly developed dielectric polymer ink.

In April, BotFactory began supplying new resistive inks for its SV2 PCB printer. These inks can quickly print resistors and sensors on flexible or rigid substrates. They also help the resistor stay connected with the flexible material and allow the manufacture of resistors with specific ohmic values.

The University of Nottingham uses inkjet printing technology and hopes to 3D print the entire electronic part in a “one-shot”, Tuck said. This requires the elimination of supporting structures, as well as the correct resolution and material properties. These must be as close as possible to those of the final equipment and as close as possible to the actual needs of the market. Challenges include that many materials must be highly diluted or they will not pass through the inkjet nozzle.

A long-term problem with 3D printing of functional electronic devices has hindered the use of this technology in more advanced applications. This problem is due to the loss of conductivity of materials and devices due to functional anisotropy, as well as the different conductivity between the horizontal and vertical directions within the layer. In this case, it is produced by printing conductive ink containing metal nanoparticles. Recent experiments by the Additive Manufacturing Center of the University of Nottingham have shown that one of the ingredients in these inks-organic chemical stabilizer residues-is the culprit. The center is developing new ink formulations to overcome this problem.

Figure 3: (Left) On-demand jetting of ink containing silver nanoparticles with organic stabilizer and in-situ solvent evaporation. (Right) Optical image and chemical map of the silver printing layer, showing the distribution of silver and organic residues on the surface of the printing layer on a silicon wafer. Source: University of Nottingham

Tuck pointed out that any process for printing electronic products must be able to print multiple materials for circuit boards and circuits. Anisotropy is the spatial difference in attributes between the horizontal and vertical layers, and is the problem of all these layers. "It is important to understand what you are printing: how does each layer interact with the next layer, diode material or semiconductor material?" he said.

Although there are multiple processes to print antennas-Optomec was the first to do so, and in terms of volume-RF 3D printing has both advantages and limitations to consider.

Church said that when using nScrypt's process for 3D printing, the antenna can contain less resistive material without compromising performance. However, due to the limitations of conductivity and smoothness, RF circuits may have problems. "Our technology is built for curved surfaces, not trapped in 2D on a flat surface, because that's what the board has always been like," he said. For the US Air Force Research Laboratory, the company's tool system factory has been used to 3D print the hyperboloid conformal phased array antenna, which actually achieved better performance.

In RF, it is also critical to maintain a certain circuit width to carry a certain frequency. Church said that with 3D printing, the line width on the slope can be controlled to maintain this frequency, which is impossible on traditional PCBs.

There are also special requirements for printing on bare molds. This is usually done by wire bonding, but nScrypt will print a ramp around the chip to hold it in place. "Then we can print from a pad along a ramp to the die pad," Church said. "By doing it layer by layer in this way, we can print more electronic devices inside, and we can put the chip anywhere on this object."

Figure 4: 3D printed curved 4 x 4 active phased array antenna. Source: nScrypt

3D printing on a chip depends on the ability to print transistors. But the problem is being able to print transistors with the necessary materials. Those may not be printed, or they may have other problems, such as not being able to print them small enough. "By squeezing basic types of 3D printing, in order to keep the size of electronic products small, we limited the line width," Tucker said.

Resolution and throughput are trade-offs that must always be made. For solvent-based materials, these trade-offs also include wasted material.

The printed content AM is usually used to enhance the existing process of a specific application. "Why use a specific AM method? Because other available manufacturing methods are insufficient or ineffective, or because there is no other way to make specific interconnects or make specific devices," Germann said. "Our aerosol jet technology is usually a step in a large-scale process, wherever additive manufacturing fits into the supply chain."

Optomec first focused on production, using functional materials to make very small features, and then depositing them on complex geometries. "AM's overall production scale varies greatly, from 10 pieces per month for military products to 1 million pieces per month for consumer electronics," Germann said. AM companies must find suitable applications for their technology to expand. Currently, the largest users of AM in the production environment are those with high value and low capacity.

AM equipment suppliers for specific applications or industries are more likely to succeed because different processes are best for different types of circuits. Some processes are used to manufacture antennas, others may be used to manufacture LED screen components, and some suppliers only focus on PCB prototyping. "At Optomec, we have four main vertical markets. We don't try to print everything, we want to sell equipment, software, and process solutions for mainstream manufacturing," Germann said.

Figure 5: Top row: Molded plastic inserts for smartphones with 3D printed silver antennas for WiFi, Bluetooth and 3G/4G phone signals. Bottom row: 3D printed capacitive touch sensor. Source: Optomec

Church stated that the performance and speed of nScrypt has been significantly improved due to industry needs. "We set up printers in a line according to the needs of semiconductor production and electronics manufacturing, but we can print to 3D objects. Our largest printer is 8 feet x 12 feet."

The military, aerospace and aviation industries are the main users of 3D printed circuits. For example, NASA has installed the nScrypt bioprinter on the International Space Station (ISS) and will add the company's electronic printer in 2024 or 2025, Church said. Nano Dimension is one of the early companies to 3D printed circuits and recently announced that it is a 3D printed radio frequency circuit for the International Space Station.

For the military, frequent maintenance of ground vehicles must be completed in advance before failure. Church said that because electronic equipment can perform vehicle health monitoring, it can theoretically also be done on a car, which is also a trend in the transportation field. Another emerging industry is energy, printing circuits for solar cells and wind towers.

Medical equipment is not far behind, and some small AM electronic processes are also being reviewed for consumer and automotive applications. "These processes are now permeating many manufacturing industries," Germann said.

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