I'm currently designing some development boards that are quite large, around 100x180mm, with lots of through-hole components. The goal is to create an educational tool for students. Because of this, I want to isolate the bottom of the board where the component legs stick through.
I had the idea of using foam, attaching it to the bottom of the PCB so that it can sit flat on a desk and add a bit of grip.
Is this a safe and practical idea? If so, what type of foam would be best to use?
Unless there are heat producing or high power devices that are counting on airflow for heat rejection, a closed-cell flame-retardant, adhesive-backed foam would work.
Knowing students however, anything you put on there is going to get picked at, cut, gouged, and/or gnawed so adding some mounting holes in the corners and attaching little easily replaceable rubber feet to stand the board off the table might be a better choice.
PCB pillars are a purpose designed tool for the job:
While the foam that comes on them is self adhesive, if you don't remove the backing paper they're quite usable as a movable soft contact with the desk.
This particular example fits in a 4mm hole, but there are other sizes.
You might look at using something like hobby wood, thin wood sheets typically made of basswood.
You could have mounting holes in each corner of your circuit boards and maybe one or two in the middle, then use nylon bolts with nuts between the circuit board and wood to space them apart a bit and a acorn nuts on the ends of the bolts to act as feet.
The wood comes in various sized sheets and different thicknesses, and there are plywood versions for extra strength. You might have to experiment with different thicknesses to get the strength you need. If you can get a sheet size that can be cut to make bases for several units you can probably make them pretty inexpensively.
Unless the desk is perfectly flat with nothing on it, and you carve out bits of the foam where the through-hole component legs are, a layer of foam across the whole board is unlikely to sit flat on a desk without rocking around.
As already suggested, a hole in each corner to attach standoffs to is a common way of allowing development boards to sit flat on a desk.
Alternatively if the board is already designed, you can easily get hold of adhesive rubber feet that can be simply stuck in each corner. You can also use the little adhesive rubber bumpers like you would get for cupboard doors.
Maybe consider 3D printing a board holder that protects the bottom and keeps things from getting scratched. I do that sometimes when a board is going to be handled a lot (e.g. for firmware or HDL development).
If you want to use foam, probably a silicone foam at least 3-5mm thick with a non-conductive adhesive would work okay and be reasonably rugged. There are some foams with guaranteed electrical characteristics such as surface and volume resistivity and dielectric strength. You could look at those, or do your own testing.
My answer would be very much the one of GodJihyo, except it you get your PCB manufactured industrially, you might give a try at getting a 2nd "PCB", except with just holes for standoffs/nylon bolts. This gives you a "PCB" back.
