My project Super OSD fits on a PCB with dimensions of approximately 40mm x 70mm. It's very space constrained and I can't expand the PCB too much. How should I approach this problem? I realise it's not strictly electronics, but it is a problem many have presumably solved before. I first considered screws but I didn't find any ones small enough - I need to go as small, or smaller than 4mm diameter. I was thinking of using heatshrink to keep the module secure if screws were not an option. Any opinions?
I'd probably go for M3 or M4 stainless steel socket head cap screws. They're strong, corrosion resistant, and readily available around the world. If you google DIN 912 A4, (DIN 912 meaning a metric socket head cap screw and A4 meaning stainless steel), you'll find lots of suppliers.
One such supplier: http://www.mcmaster.com/#din-912-a4-socket-head-cap-screws/
I'd have those screws pass through nylon unthreaded standoffs and thread into whatever you're mounting the board to. Here's what I'm talking about: http://www.mcmaster.com/#catalog/116/3240/
You might also try snap-on plastic standoffs like these: http://www.mcmaster.com/#catalog/116/3245/
They're designed to snap into a 5/32" (3.97 mm) hole, so they should just fit.
Is there no open space anywhere on the board? Remember that mounting holes don't have to go on the outside edges: you can put them in the middle if need be.
Is there open space around the edges of the PCB? Bud, etc., make enclosures with slots that a board can be placed into, but you'd have to have designed the board to fit an existing enclosure. Alternately you can purchase supports that look like square columns with slots in them that will work as board standoffs. I have even used double-sided tape to get a PC board to fit into a small area when I couldn't add space for mounting holes.
You can get threaded spacers down to M2 - these are about 4mm dia
Is this unit going to be provided for people to fit themselves or are you going to fit the unit into a specific model plane and sell it as a whole, or fit it for people in their planes?
Is this a fit-and-forget project or will it be fitted and taken out regularly?
I am wondering whether standoffs are a no-go because you'd have to fix the pillars into the plane and that will mean drilling holes - possibly on a non-flat surface or where the screws will show on the outside? I also doubt self adhesive standoffs will survive the vibration and moisture/dust for very long.
If the unit is pretty much a fit-and-forget thing then I wonder whether it could be secured with some expanding polyurethane foam (PU foam) around the base and edges - or even across the whole unit with masking (eg adhesive tape) left in place to allow slots to be formed for the connectors and switches - this would 'pot' the unit against environmental issues and avoid the need for a case.
The foam would give a vibration-tolerant mount and the unit could be cut out later with a knife if really necessary. The only issues are whether the foam is OK on the plane's plastics and the customers accept this form of mounting.
Bonus: No PCB land required for the mounting holes so you can make it even smaller!
I would propose designing your enclosure so that the board is seated in a groove on one end, and has a plastic leaf spring-like catch on the opposite side. Gently pull back on the catch and then lift out the board when you need to remove it.
I'm coming from this standpoint since I have a 3D printer and can print pretty much whatever I need to fit around PCBs, but even if you don't have access to one, services like Ponoko and Shapeways can help you out.