A good method to clamp PCBs down for bed of nails fixture?

Question

I would like to build a bed of nails fixture with pogo pins.

Is there a good method that works to hold down a PCB that is releasable and easy to pop a PCB in and out of the fixture?

Answer 1

The tried and true method to do this is to contact a bed of nails fixture manufacturer that can make a vacuum fixture that uses vacuum to pull the PCBA down against the pogo pins. These fixtures are built in two layers with a thick lower plate (approx 1 cm thick) that holds the pogo pins with all connection wires below it. This layer is connected to an upper layer with a flexible gasket around the periphery. The upper layer is covered with a layer of neoprene foam rubber material upon which the PCBA will sit. An additional strip of the neoprene material is bonded around the periphery of the PCBA making a "nest" into which the PCBA can sit. This upper layer has holes and slots through it that allow the pogo pins to come up in contact with the bottom side of the PCBA. A vacuum line is ported into the space between the two layers. When the PCBA is in place and snug in its nest the vacuum being applied sucks the upper layer down to pull the PCBA in contact with the spring loaded pogo pins. Note that accurately machined tooling posts in the upper layer go through holes in the PCBA to accurately guide it into position.

This type of fixture works very well for high volume testing but does have some limitations. It works best for boards with no big components on the back side (or no components at all) and the boards via holes should be either a plated shut style or filled with solder in the board reflow process.

If you have a type of board with components and test points on both sides then you need to search out the style of fixture that is a more open construction with a hinged construction that engages the pogo pins against the board when the fixture is hinged closed.

Answer 2

Assuming this plays in the "a couple hundred boards to be tested by a reasonably skilled person" and not in the "needs to be operated thousands of times a day by a tipsy, bodybuilder teenager with anger management issues" league:

I'd go ahead and quick and dirtily design standoffs to be 3D-printed for cheap for your board. If it has mounting holes, these can simply be little "towers" with a slightly conical pin on top that happens to fit the mounting hole; otherwise, towers with a corner to match the corner of your PCB. I have made positive experiences (cyanoacrylate-) gluing such to a piece of sheet metal (but your favourite wooden plank would probably do). It usually gives you enough time to place the stands roughly and align them using an actual board before the glue cures. (For small boards, you might just print the whole stand + baseplate thing as one monolith; I've never tried that, though, being a stingy bastard.)

This way, you can design the height of your stands to exactly match the working range of your pogo pins without fiddling.

If the board side opposite to your test points is flat (or mechanically robust enough), a hardcover book as weight does the job of pressing the PCB against the pins quite well. If not, same idea of standoffs affixed to a plate as on the base side. I do agree with @Ralph, toggle clamps would work as well, substituting the clamp's force for weight.

Answer 3

Toggle clamps are a simple method for a low cost solution.

In an industrial setting, something more robust is required, especially for large PCBAs. Among others, Ingun provides platforms that have a similar mechanic that has a customizable sheet that can be drilled to have multiple pushfingers. Or you could modify a toggle clamp to have multiple pushfingers, if costs need to be brought down.