Aerospace PCB Design Considerations

[askingforafriend]

I'm interviewing for some aerospace PCB Designer positions but have no experience designing PCBs for aerospace. I'm trying to fill in my knowledge gap as best as I can beforehand and was hoping to solicit some advice from you nice folks.

What are some design considerations I should be aware of? "Typical" stack-ups?
Interview questions that you've been asked or would ask?
Any resources you could point me to? Even pictures of example PCBs would be helpful.
Any interesting aerospace stories you would like to share?

I've been looking up what I can on the various SAE, RTCA, and IPC standards but without access to many of them, it's mostly surface stuff. Any standards I should be particularly focused on?

If you want to be specific to my situation in most of the positions I would be dealing with digital designs in airplanes. I'm also in the USA.

[Rerouter]

main thing would be, thin or absent atmosphere, so you need to be well aware of trace seperation to prevent voltages jumping between traces,

A little bit of knowledge on radiation hardening probably won't hurt,

Other than that, not too sure other than designing for reliability,

[Psi]

Note: I am not an aerospace engineer but this is what i can think of.


* Vibration damage
- Board flex can damage components, ceramics etc..
- Heavy parts will pull on their solder joints with high force under high G and may rip themselves off the PCB under their own weight.
- Trimpots could move their value.
- Some capacitors are microphonic, they produce voltage if you shake them or subject them to vibration.
- You can put two caps in series rotated 90deg from each other so any board flex in one direction might crack one but not the other.  Usually a cracked cap goes short, so two caps in series can provide some redundancy. But you might also not use ceramic caps at all to prevent the problem in the first place.
The solution for vibration might be a 2 factor approach. Harden the design to handle vibrations and also mount the PCB with some sort of damper system.

* Changes in component value at extreme temperatures.
- Use parts with a very wide temp range and be aware of what their value will be at each extreme. The circuit must be able to work in either case.
- Insulation to reduce temp swing and make temp changes more gradual.
- PCB could include a resistive heating system to maintain a set temp range.

* thermal cycling
- joints get weaker if you keep thermally cycling them. The bigger the extremes the worse it gets.

* Longevity
- Anything in aerospace might be required to last years unattended. So reliability is a big thing.
- Leaded solder instead of lead free.
- Maybe aerospace rated conformal coatings.
- Mil spec parts

* Radiation
- Parts must be able to withstand the radiation at high altitudes.
- There are various options for radiation hardened parts.  etc you can get CPUs rated for space.
- Memory bits get flipped by radiation so the system must be able to detect and correct memory/bit errors. You might have multiple identical systems that check themselves and compare outputs.

[dzseki]

main thing would be, thin or absent atmosphere, so you need to be well aware of trace seperation to prevent voltages jumping between traces,

Does vacuum have less dielectric strength than air?

Also, while in space things are usualy very cold, but if something gets hot there is no convection to help to dissipate the heat, only heat radiation works, but that is only effective at higher temperatures, what semiconductors do not like much. Thermal management is a big deal.

[daqq]

Does vacuum have less dielectric strength than air?

No, but low pressure gas has a much lower strength and acts silly when exposed even to relatively low voltages (see glow discharge).

[Rerouter]

You rarely get a perfect vacuum, It decreases with atmospheric pressure, then when you get up towards the thermosphere it starts increasing again,

[mengfei]

Good topic.
Hoping to pick up some ideas here too - Not that I can work for an aerospace agency, far from it 
I guess some youtube topics about space & electronics could help.

[Pseudobyte]

Per IPC your spacing requirements increase dramatically for assemblies designed to go above 3050m.

[Kasper]

I haven't designed aerospace PCBs but I've modified a bunch.  Certification can take years so respins are rare. It was cheaper to solder about 10 mods on boards for years than to respin them.  I guess that means good practice would be to make the boards easy to modify.

Also good to understand requirements to pass vibration testing for example when to apply RTV. And how much room to leave for RTV.

Also, conformal coat shadows are a good thing to know about.