Sep 3, 2019

3D Printing Tolerances: Considerations for High-Frequency Electronics

additive-manufacturingIf you’ve paid attention to the additive manufacturing space, then you’re aware of the interesting mechanical designs you can create with the right 3D printing system. The capabilities of 3D printing systems are expanding to include aircraft engine parts, medical implants, organic electronics, and even fully functional PCBs. The ability to create fully functional devices of great complexity, with a flat price structure, is changing the way manufacturers are thinking about their processes.

These changes in manufacturing processes and the availability of unique electronic materials in additive manufacturing processes are starting to affect the way designers think about creating new products. In high-speed and high-frequency electronics, 3D printing tolerances should be addressed at the PCB level to ensure printed PCBs and the devices they contain function as intended and maintain signal integrity.

Checking 3D printing tolerances with calipers.

3D printing tolerances span beyond mechanical dimensions.

Any product that is printed with an additive manufacturing system will have some limitations on printing accuracy. For very large products that are post-processed (such as metal mechanical products), these tolerances become less of a problem as they can be much smaller than critical feature sizes. However, as additive manufacturing is used to create smaller functional electronics that operate at higher speeds and higher frequencies, limitations on 3D printing accuracy may start to have effects on performance in different applications.

Which 3D Printing Tolerances Become Important at High Frequency?

The answer to this question is somewhat complex and really depends on what one means when using the term “high frequency.” With analog signals in the kHz to low MHz range, the wavelength of a signal will generally be much longer than any of the traces in a 3D-printed PCB. This means that even long traces will not behave like transmission lines and analog signals need not be considered to act as propagating waves. Conductors need to have very large geometry to have a chance to exhibit any resonance effects, such as antenna-like behavior.

The critical feature size required to produce resonance and the critical trace length that produces a transition to transmission line behavior both decrease as frequency increases. With very high-frequency circuits (that is, reaching tens or hundreds of GHz), printed antennas, and RF devices, roughness on conductor edges, variations in trace cross-sections and bends, and plane spacing can lead to a number of linear and nonlinear electromagnetic effects that degrade signal integrity.

It is important to note that signal integrity problems will always exist—the best you can do is reduce their effects to the point where they are unnoticeable. Nonlinear effects that can arise in PCBs due to 3D printing tolerances are intermodulation, signal distortion, harmonic generation, signal reflection, and radiation due to antenna-like behavior. Simple design and manufacturing process changes can have a positive impact in any of these areas.

As high-speed signals can be deconstructed into an infinite series of high-frequency signals, some of this signal content in the frequency domain will produce the effects mentioned above. In particular, signal distortion and reflection can lead to higher bit error rates in a digital system. In extreme cases, trace and passive component roughness lead to high localized heating due to resistive losses, causing damage to the trace or substrate and even delamination at high input power in high-speed and high-frequency circuits.

Designing High-Frequency PCBs Around 3D Printing Tolerances

When designing a PCB for 3D printing, three important parameters to consider include:

  1. The roughness and continuity of the interface between printed conductors and the substrate.
  2. Geometric variations in traces and vias.
  3. The printer’s resolution. 

The first two parameters depend on the printing resolution, as well as the process and materials used during deposition. The second parameter also depends on the printer’s resolution, as well as temperature variations that arise during the deposition process.

When considered in isolation, the printing resolution determines the smallest feature size that can be deposited during the printing process. A higher print resolution generally translates into tighter 3D printing tolerances, straighter conductors, and more precise trace bends in PCBs. Higher resolution also translates into lower roughness at the interface between a conductor and the substrate. The materials being deposited in the product will also affect interfacial continuity. Conductors and insulators deposited from nanoparticle inks tend to form a continuous interface thanks to the surface tension of the host liquid.

Iron nanoparticles suspended in water.

Using conductive nanoparticles provides better quality printing products.

The flatness or straightness of conductive planes, traces, and vias depends on the temperature gradient and cooling rate during the printing process. Working with a high-temperature process for PCB fabrication, such as FDM or a similar process, creates a larger temperature gradient as the material is deposited. As the deposited material cools, its volume will decrease due to thermal contraction. 

At a high cooling rate, contraction may not be uniform throughout, creating waves in the deposited conductors. If the printer resolution is smaller (that is, larger features are deposited in each layer), then this waviness will be exacerbated, leading to lower quality conductive elements in the finished product.

The Benefits of an Inkjet System for High-Frequency PCBs

Using an inkjet system with nanoparticle inks for PCB fabrication tends to produce PCBs with higher quality than other printing methods because it offers competitive resolution and uses a low-temperature process. The layer-by-layer deposition process also allows designers to fabricate boards with complex shapes and embedded conductive and RF components. Furthermore, the cost structure is independent of device complexity, making it ideal for low-volume fabrication runs of highly complex PCBs.

The great aspect of using an additive manufacturing system to fabricate PCBs is that the same measures used to improve digital and analog signal integrity can be directly translated to a 3D-printed PCB. This can be done without changing the cost structure or lead time involved in the printing process. Simple changes, like modifying the layer stack in a multilayer PCB, trace dimensions, and via geometry, can have a major impact on signal integrity while having a negligible impact (positive or negative) on costs.

This allows unique components like RF amplifiers and antennas that can operate reliably in the GHz range to be co-deposited alongside a substrate material. Compared to other 3D printing methods, using an inkjet system that is uniquely adapted to PCB fabrication offers much higher throughput with competitive or lower costs. Designers also have much greater freedom to implement unique interconnect architecture compared to PCBs designed for traditional subtractive fabrication methods.

If you are designing high-speed or high-frequency devices for use in unique applications, then you need an additive manufacturing system with precise 3D printing tolerances in PCBs. The DragonFly LDM additive manufacturing system from Nano Dimension is ideal for low-volume manufacturing of complex electronic devices with planar or non-planar architecture. The substrate material available with this system is also ideal for working with high-frequency devices. Read a case study or contact us today if you’re interested in learning more about the DragonFly LDM system.

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