PCB Design Tips No.1 : Stake It !

I'm taking a break from my CAN project today to post the first in series of PCB Design Tips.

As electronics junkies we've all encountered some pretty dreadful circuit boards:

Maybe it’s the unsafe mess you discover when you take the lid off that bargain-basement battery charger. Or maybe you're the one slopping solder around on a quick-and-dirty proto-board.

Every now and again however, you need things done right. Whether it's a PCB you're putting into production or your high school battlebot's motherboard, sometimes you need a board that's built-to-last. 

If you want to discover how to put together a top-notch PCB, then quality doesn't come higher than Space Grade.

 PCB's for space missions must withstand extremes of vibration, temperature and radiation. And they must not fail: Getting the soldering iron out to re-attach a loose wire isn't an option when your board's half way to Jupiter! 

Space agencies like NASA and ESA publish strict rules on PCB build-quality.  Example document numbers to google are NASA-STD-8739.1B and ECSS-Q-ST-70-08C. Of course, no one's saying your one-dollar widget needs ultra-precision, but a quick skim through rules written by the experts can be a great source of tips for improving your PCB designs.

Take flying-leads (wires from your board to a connector, say). The rookie approach might be to dab a bit of solder onto the lead ends, and finish there.

That's not the way the professionals do it (see figure). In addition to being soldered to the board, space engineers make sure the lead goes a second hole where its staked ('glued'). 

The space-quality rules also tell you that bends in the wire mustn't have a radius of less than 2x the diameter of the wire; that soldered ends of wires must protrude 1.5mm through the board; and that insulation must be stripped back 1-2mm from the point where wires meet the board. 

Staking glues are carefully chosen also. A blob from a hot-glue-gun may be o.k. for a hobby-board, but for PCB's to stand up to serious vibration, you need a glue with enough elasticity (the technical term is a low modulus) to 'soak up' stresses. Specialist epoxy resins like Scotchweld 2216 (I've no connection to the company - this isn't a plug!) are common in the industry. At $20'ish-a-tube, it isn't cheap, but it could be a lot cheaper than putting that failed board in the bin.

So there you have it - next time you need a board that's built-to-last, take a tip from the experts: Stake It! 

I'm always delighted to get any feedback on these posts, so do leave a comment...