I'm looking into designing a board for a robotics application that is going to be spinning very quickly, and am wondering if there is anything special to take into account. It's going to be in the ballpark of 6000rpm, with the edge of the board 50mm from the center. That's a constant acceleration of around 2000g, with shocks of a couple times that. If I shock mount the board, that would reduce the sudden acceleration but not the rotational one.
What should I do differently from normal PCB design? Would through hole be better than surface mount? Which way should I orient the board, so that the acceleration is parallel to the board (preferable) or perpendicular? Which way should I orient components? Would conformal coating or potting be a good idea? I'd prefer not to, both for repairability and because of the weight. At what kind of accelerations would the component internals themselves start to break?
Also, maybe this is the wrong place to ask this, but what's the best orientation for a lithium polymer pouch battery in this environment?
Best Answer
The best advice I can give you is keep heavy stuff along the centreline. This also applies to xtal oscillators you may be using. You might also want to ensure that the mass of elements you use are balanced about the centreline of spin or you might get some heavy-duty vibration.
For small/low mass SMT components surface mount is fine but it might be worth considering glue to add extra bonding - not too much of course because the glue has mass.
Over about 10,000 g we tend to solid pot the electronics and we've not had a problem with jobs shipped over many years up to g forces over 20,000. Connectors (inter-board PCB stuff) can be a bit flaky so use those with turned pins and multi-leaf contacts (at least three).
There is a company in the US called Statek that make hi-G xtals - not cheap but if you want peace of mind I'd consider using them. Alternatively, look at Linear Technology's array of silicon oscillators - they not bad (and no moving parts other than the electrons) - I've been testing one across temperature recently and it stayed within 10 ppm/degC across 60degC.
I've never known the internal wires on a chip to fail. We always try to make sure the acceleration pushes the components onto the PCB but sometimes this is not feasible and then we pot stuff. I believe, at 6000g you should be OK not potting.
Also, It might be worth doing a spin-test on the finished article to check for vibration and anything going wrong. We do that with all our kit. Vibration we fix just like a tyre fitter does - small weights aren't preferred so we usually drill small amounts of metal off the outer casings.