Aug 1, 2019

A Guide to Additive Manufacturing Material Selection for PCBs and Electronics

nano-particlesIf you are new to additive manufacturing, particularly for electronics and PCBs, you’ll quickly find yourself learning about a new set of manufacturing processes, different materials, and device architectures for 3D printing. With all this information fragmented in different locations, how does one go about choosing the right materials for use in additively manufactured PCBs?

The additive manufacturing material selection process needs to consider more than just the mechanical, thermal, or electrical requirements of your device. When designing a new PCB, you’ll need to choose materials and processes that complement each other. Not all processes can be used with any material, and not all materials can be used in a single process. For PCBs, you’ll need to choose a manufacturing process that can actually produce the architecture you need for your device, and then select materials that are adaptable to that process.

3D printing head for additive manufacturing.

3D printing on a planar substrate.

Additive Manufacturing Material Selection vs. Process Selection

For PCBs, you’ll need to determine which materials to use for the substrate and conductive elements in your board. A given set of materials is not universally adaptable to any process, so you must balance the capabilities of your system with the performance requirements of your materials. Therefore, one of the first points to consider before selecting materials is the printing process and architecture of your device, as this will limit the range of appropriate materials.

Certain processes (FDM, powder bed fusion, or SLS) are difficult or impossible to adapt to multilayer and non-planar PCBs and are best used for devices that will appear on the surface layer of a planar substrate. In addition, these higher temperature processes require a substrate that will not degrade when depositing conductors at high temperature, or they simply require conductors that melt at a lower temperature. These processes, as well as laser direct structuring (LDS), can be used to fabricate PCBs on injection-molded substrates or other rigid substrates.

Other PCBs, such as multilayer PCBs, require a process that can deposit conductive and insulating elements simultaneously to maintain high throughput. Aerosol jet deposition and LDS are lower-temperature processes that can be used to deposit a substrate and conductor from different starting materials. However, they suffer from low throughput when used for multilayer PCB fabrication in that the substrate and conductors cannot be co-deposited. Therefore, from a manufacturing efficiency perspective, inkjet printing is a better process for multilayer PCBs as both materials can be deposited simultaneously in a layer-by-layer printing process.

Substrate Selection

Once you have determined the deposition process that will yield the architecture you need, this will limit the range of available substrate materials. The material you choose for your substrate will also have several effects on the performance of your finished board. There are a few performance aspects to consider:

  • Dielectric constant and loss tangent: The substrate determines the effective dielectric constant and loss tangent seen by signals propagating along conductive traces on the board. This, then, affects whether a given trace will act as a transmission line. These points are quite important for RF devices.
  • Thermal conductivity: This will determine how quickly heat generated by active components and components with a high current will dissipate away from the board. This will affect the operating temperature and whether the board’s temperature is uniform.
  • Electrical conductivity: This determines the level of losses for signals traveling along transmission lines and the leakage current between circuit elements held at different potentials.
  • Chromatic dispersion: This is quite important with higher speed digital boards as it affects whether digital pulses stretch as they travel along traces. In other words, the pulse acquires a chirp if chromatic dispersion is severe.

It is often difficult to select a material that has low losses, flat dispersion, high thermal conductivity, and low electrical conductivity simultaneously. Good thermal conductors also tend to be good electrical conductors, thus there is a tradeoff between these two material properties. It is best to select a substrate material with low electrical conductivity and implement some thermal management practices. This involves the use of active or passive cooling measures to dissipate heat and proper dispersion of active component around the board.

Regarding the optical properties of the substrate, you will rarely find a material with low loss and flat dispersion over a broad frequency range. Rather than printing directly on rigid substrates, using an additive manufacturing system that can deposit insulating polymers broadens your range of useful materials. Polymers have a broad range of material properties and are highly adaptable to several 3D printing processes. This gives designers greater freedom to adapt the geometry and material properties of their substrate to meet their needs

Conductor Selection

Obviously, you want to use a conductor with the largest possible electrical conductivity as this minimizes DC losses in the board. However, you will need to take into account the process used to deposit it. With a process like FDM, PBF, or SLS, the metal needs to be heated to its melting temperature to fuse into the desired shape. However, there are alternatives to working with these high-temperature processes.

Processes like inkjet printing and aerosol deposition allow conductive elements to be deposited from nanoparticle inks rather than being melted and extruded through a nozzle, as is the case in FDM or a similar process. This also eliminates the use of a laser (used in SLS and LDS) or another mechanism to bring the metal stock to high temperature, which can reduce the initial costs and ongoing maintenance for the printing system. Conductors deposited from nanoparticle inks have lower conductivity than  copper foil on rigid PCBs, but is nevertheless sufficient for most applications, including high frequency antennas, making inkjet printing and aerosol printing attractive processes for 3D-printed PCBs.

TEM image of metal nanoparticles; the size and shape of metal nanoparticles in conductive printing inks is one point to consider during additive manufacturing material selection.

The size and shape of metal nanoparticles in conductive printing inks is one point to consider during additive manufacturing material selection.

Additive Manufacturing of Non-planar and Multilayer PCBs

The planar processes listed earlier and aerosol jet printing can be adapted to multilayer PCBs under certain conditions, but you lose the primary advantage of higher throughput provided by 3D printing. This is because, in those processes, you cannot co-deposit the substrate and conductors simultaneously. After a substrate layer is deposited, you must deposit and cure a layer of conductors, followed by filling in the empty space with dielectric. This mimics the already inefficient traditional processes used in rigid PCBs.

This is where inkjet printing really shows its advantages for 3D printing PCBs. In this process, a dielectric ink and a conductive nanoparticle ink can be deposited simultaneously. With the right selection of polymers and conductors, the two materials can be selectively cured using infrared or UV lamps. This expedites the process for fabricating multilayer PCBs, as separate printing processes are not required for different materials. An inkjet printer is also useful for printing non-planar and rigid-flex PCBs.

The additive manufacturing space for electronics is still maturing, and standards governing the performance and required properties of dielectric and conductive inks are still being developed. In the meantime, designers and engineers still have plenty of freedom to experiment with new materials and processes. Expect more clarity from the industry in the future as the range of materials available for use in additive manufacturing systems for electronic devices expands.

If you have a great idea for a new electronic device with unique geometry and architecture, then you need the right additive manufacturing system to bring it to life. The DragonFly LDM additive manufacturing system from Nano Dimension is uniquely adapted for 3D printing of planar and non-planar electronics. The additive manufacturing material selection guidelines provided here are directly applicable to this system. Read a case study or contact us today if you’re interested in learning more about the DragonFly system.

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