Today’s rugged embedded systems and mission computers are almost always multi-circuit board assemblies, where multiple PCBs are linked together with cables and connectors. Cabling and connectors can be a failure point in some designs, yet they are critical elements that provide signals and power around the group of PCBs. They require a certain level of electrical performance from signal and power perspectives, as well as mechanical ruggedness for deployment in harsh environments commonly found in the field.
Rugged embedded systems and mission computers most commonly use two options for board-to-board interconnects whenever a direct board connector or mezzanine connector will not satisfy the requirements. These are:
- A flex or rigid-flex cable assembly with a board-to-board connector
- A custom cable assembly or wiring harness design
Which of these is best in rugged embedded systems? Of course, the answer is: it depends, mainly because the two options represent a balance between mechanical ruggedness and overall footprint. We have used both options in the same system for different purposes, and we find that one or two specifications are usually enough to motivate the selection of one of these types of interconnects.
Flex or Rigid-Flex Cabling vs. Custom Cable Assemblies
First, we can offer a brief comparison of these options, which will illustrate some use cases where we have found both types of interconnects useful. At a high level, cable assemblies tend to be better for power handling, while flex PCB cabling can achieve high density. However, depending on exactly what your cable assembly needs to do, these characteristics are not the only important factors.
| Cable Assembly | Flex PCB Cabling |
|---|---|
| Larger conductors are allowed | Can use larger or smaller conductors |
| Lower wire resistance and higher power handling | Resistance will be limited by copper foil thickness |
| Lower density | Can reach very high density |
| Larger vertical footprints | Quickly lower profile |
| Low flexibility | Can be statically bent or dynamically flexed |
| Bulky connectors with locking and retention | Smaller, low-profile connectors may not have locking and retention |
| Can have very high current and voltage limitations | Current and voltage limited by the flex dielectric materials |
| Assembled manually, which may use crimp contacts | Assembled with standard PCB soldering processes |
Looking through the list, we can see use cases where flex PCB cables can be used for power and where wired cables can be used only for signal, and vice versa. In some use cases in aerospace, for example, flex PCB cabling is an excellent option due to very low outgassing and the ability to use leaded tinning on components and SMD pads. In some systems where much greater mechanical shocks or vibration are expected, clients have found that wired cabling is a much better option as it may include the required retention and locking mechanisms needed for such mechanically harsh deployments.
Flex PCB Interconnect Example
We have found that in rugged embedded systems and mission computers, flex PCBs are the best option when an interconnect must carry high-speed digital signals between PCBs. They offer a low-profile interconnect, and there are many off-the-shelf connector options that provide the required number of signal pins. Some of these off-the-shelf connector options have locking and retention mechanisms, which help fixate the flex PCB interconnect at each end.
An example flex PCB cable with a high-density connector for high-speed I/O is shown below. These connectors are normally mixed SMD and through-hole components; they have SMD pads for the signal pin connections, and they will include through-hole pins for stronger bonding to the PCB as well as ensuring correct connector alignment.
Rugged flex cable with Samtec connector. (Image source: Multi-Teknik AB)
These types of board connectors are excellent options for rugged embedded systems because they provide vibration-resistant mating and high channel bandwidth. In addition, these connectors come in a variety of profiles, so they can be used for add-on modules, flex cables, or low-profile connections to an SoM.
Custom Cable Assembly Example
Custom cable assemblies offer many features that make a board-to-board interconnect much more rugged than a flex cable. There are several conditions in which a cable assembly will be a much better option than a flex ribbon or a board-to-board interconnect:
- Large currents or large power are being used
- There is a risk of tugging or pulling on the cabling
- Large diameter wires are needed for thermal or mechanical reasons
- Part of the cabling will exit the enclosure
- A panel mount or chassis mount connection is needed
A moderate-density board-to-wire connector used for custom cable assembly is shown below. These connector solutions can include a locking mechanism, and they are very inexpensive, making them very useful for low-volume rugged embedded systems. They can also be hand-assembled as needed, whereas a flex cable with a board-to-board connector will typically go through reflow.
Custom cable assembly example.
In designs intended for low-rate initial production (LRIP), we normally provide a logical schematic and a cable fabrication drawing intended for hand assembly. This will include any crimp contact part numbers, cooling part numbers, etc. For higher volume builds, we prefer to provide a full mechanical drawing showing integration of off-the-shelf components and materials required to build the cable assembly.
Other Options for Rugged Board-to-Board Interconnects
There will always be situations where a flex cable or custom cable assembly will not be suitable for a rugged embedded system. There are other options for connecting boards together, which can take a multi-circuit board assembly beyond simple stacking of circuit boards. Some examples include:
- Straight or right-angle pogo pins
- Edge connectors or slot connectors
- Castellated pads for direct SMD bonding
- Simple pin header mating
- Floating wires
These options are less common than the standard board-to-wire connector options or flexible printed circuit ribbons, but they can still provide rugged performance in harsh environments. As more features and modularity come into rugged embedded systems design, we expect more focus on board-to-board interconnects to appear in design processes.
Whether you’re designing an ultra-rugged aerospace system or feature-rich embedded computing products, make sure your design firm understands how to coordinate with electronics manufacutring services and contract manufacturers to help you produce and scale with maximum quality. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We’ve also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we’ll make sure your next high speed digital system is fully manufacturable at scale. Contact NWES for a consultation.
