# Techniques for building 'Bed of Nails' testing board

When testing a complete circuit board a 'Bed of Nails' can significantly reduce time and errors.

What are some techniques for making a DIY Bed of Nails?

Specifically, looking for what kinds of materials work best, how to trace the board properly to make the holes in the right place, in other words, it's the little things that experienced people know but for a beginner would be a lifesaver.

• Get a board. Stick "nails" in it. Wire them. Write a software to drive them. – Eugene Sh. Feb 27 '18 at 15:14
• The 'nails' used are usually pogo pins: en.wikipedia.org/wiki/Pogo_pin – Peter Smith Feb 27 '18 at 15:25
• – Brian Drummond Feb 27 '18 at 16:55
• Back in the 80s and 90s I would use for the base material/s a linen impregnated phenolic material. It tools very nice and is also mechanical stable. For the seal between the top/bottom plates (w/spring return) I would use duct-tape around the outside edges. – Steve Feb 27 '18 at 19:39
• i use PCB breadboard+pogos; stripboard w/o the strips. works will w/ standard spacing. – dandavis Feb 27 '18 at 21:02

Having the board artwork helps, but if you don't, you can use a sacrificial blank PCB as your template.

Fix the blank PCB the appropriate way up, usually bottom side up, to your bed of nails substrate material. The latter needs to be thick enough to provide the mechanical stability. You can even use two layers for the bed of nails substrate, later separated by a gap, and mechanically joined together by the pogo-pins.

You can use good double sided tape to securely adhere the blank board to the substrates so they do not move relative to one another while you drill.

Then, using a drill press, drill holes wherever you need a test pin, through the PCB and on through the substrates. Use the appropriate sized drill bit for the pogo-pins.

You should also drill alignment holes for larger pins in the bed of nails that will mate with whatever mounting holes you have on the PCB. These pins on the bed of nails should be conical at the tips and longer than the pogo-pins so the board lines up BEFORE it makes contact with the test pins.

• Your way was what I used, but two boards instead of just one. The taller guide pins were crucial for alignment. – Sparky256 Feb 27 '18 at 19:01

I believe pictures are worth a thousand words (?). I documented my DIY Bed-Of-Nails fixture here: https://piconomix.com/creating-a-good-programming-test-jig-is-not-that-hard/

In short, I create a base PCB with spring-loaded Pogo Pins and an upper deck (mezzanine) PCB with holes that the test PCB locates against. HEX Spacers are used to connect the two levels.

WARNING! Beware of tolerances. If your test pad is small (e.g. 1mm diameter) and one of the pogo pins is off-center (e.g. >0.5mm), then it will not make contact, or worse be intermittent and that will ruin your day.

• +1 Nice work. I like the way you commandeered the standoffs as a simple clamp, though the tolerance on the holes might be a tad sloppy. Over-tightening them might be an issue though. – Trevor_G Feb 28 '18 at 16:00

A more DIY-friendly option that I've recently discovered is a "plug of nails" interface (example). One end of the cable has pogo pins, alignment pins, and tabs to hold the connector in place. The other end of the cable has a standard header that you can connect to your test fixture. The datasheet for that particular part shows the pattern of holes and contacts that you'd need to add to your circuit board.

This sort of solution would require you to route test signals to a central location instead of being able to place them anywhere on the board, but would eliminate the difficulty/hassle associated with the physical construction of a traditional bed-of-nails test rig. For boards with a relatively small number of test points, I've found this to be worth the trade-off.

I have designed a bed of nails board a few years ago. I've used SHE-100 pogo pins with receptacles. The receptacles were mounted into a 12mm thick plate made out of Delrin plastic. Electrical connections were done with discrete wires point-to-point.

These design choices for the bed of nails allowed several useful properties.

• The holes for the pogo pin sockets had a high aspect ratio, so the pogo pins aligned themselves well in the vertical.
• The resulting bed of nails board was stiff.
• I could recover and reuse the pogo pins and their sockets easily.

edit
To put my DUT board into a perspective, here are a few statistics.
number of test points: approx 50
size: 100mm x 40mm
nature of the beast: 40W power converter, plus microcontroller
small production quantities: 3k units a year

edit
The design of the test fixture shown in this blog post is similar to what I've done, although there is a major difference: he made a PCB to connect the pogo pins, while I've used point-to-point wiring.

Take a plate of material that's stable enough e.g. acrylic glass could work and get some incurcuit test pins. Define the testpoints on your layout and get a 1:1 printout. Stick the printout on your plate and drill the holes accordingly. Probably a bigger challenge is then to fix the plate to your PCB so the pins end up in the right place.

Make sure the base is strong, thick, and stiff enough for the pins you are using. Avoid FR4, it's strong but can be a problem for repairs.