Pogo pin test jig questions - Page 1
Forum section: electronics / projects

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 17, 2022, 11:35:11 pm
content: I'm thinking of adapting the design of one of my boards so that if I have to produce a large batch of them in future, I can automate the testing more. For this I'm thinking about making a bed-of-nails pogo-pin jig. I've never done this before, so I have a few questions:The test jig will need to supply power to the board under test using the pins, but the board has potential peak current consumption of about 0.6A, so I'm concerned about whether pins will actually be suitable for carrying this much current, and if so what type of pin I should use for power and ground. I was thinking that I should use a larger pin, a 0.1" pitch size (versus 0.05"), with a serrated tip, with the idea being that multiple contact points and bigger tip diameter (1.5mm) will adequately convey the power without introducing too much contact resistance. Does this sound okay?How good are concave/cupped pins at interfacing with the soldered side of pin headers or connectors? I have a set of test points that will be the same as some of the pins on a header, so I'm wondering if I should forget adding discrete test pads for those and just use the backside of the header with concave/cupped test pins instead.I gather there are two ways to mount pogo pins on a tester PCB. First is to simply solder the pin directly in a through-hole. The other is apparently some kind of "holder" that is soldered, and the pin itself is placed in the holder. The latter benefits by making worn out pins easily swappable. I don't really know what these holders are supposed to look like, though, as all guides I can find just use the former method. What's the best way?Finally, does anyone know if LCSC sells pogo pins? I've tried searching under all the words I could think of that could possibly describe them, but came up with naught.

author: DeanA
author_rank: Regular Contributor
timestamp: July 18, 2022, 06:08:45 am
content: I normally use these type (or whatever shape you think will make the best connection):https://au.element14.com/multicomp/p100-h-250-g/plunger-serrated/dp/1568268?st=p100-h-250-ghttps://au.element14.com/multicomp/p100-b-250-g/plunger-point-3a/dp/1568266?ost=156-8266The serrated ones seem to make contact with header pins fine, never tried the concave ones, they may be better.They are rated at 3A so I guess that will take your 600mA just fine.I would definitely always use the Receptaclehttps://www.newark.com/multicomp/r100-sc/probe-pointed-contact-3a-29-59mm/dp/96M8342?ost=1568269Main thing is making sure everything lines up. I always use 3mm-4mm dowels that align with tooling holes on the PCB before the test pins make contact.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 18, 2022, 06:30:22 pm
content: Thanks for the link to the receptacles. I had been looking at pins on Farnell, but somehow didn't see them in my searching.I still don't quite understand how these receptacles work though - it's not obvious just looking at crappy pictures. The thin end of it goes in the PCB? But it has a cut-away part to solder a wire to? So what's the point of that if it's PCB-mounted?

author: fcb
author_rank: Super Contributor
timestamp: July 18, 2022, 07:25:59 pm
content: Talk to someone (or Howard) at CODA Systems.http://www.coda-systems.co.uk

author: Benta
author_rank: Super Contributor
timestamp: July 18, 2022, 07:32:47 pm
content: Pogo pin receptables are normally pressed into a precision-drilled mounting plate, say, 10...15 mm thick Delrin. That's what the 1.8 mm thicker ring is for. Also attached to this plate are suitable guides for positioning the PCB correctly.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 18, 2022, 09:02:06 pm
content: Ah, I see, so the common receptacles are not designed to be PCB mounted. And the little flange is for secure friction-fit retention when pressed into the carrier plate.The site that fcb linked had a handy graphic explaining:Seems you need a special tool to press the receptacles in, though.But if I were wanting to mount pogo pins on a PCB, then these kinds of receptacles are not suitable? So I would just solder the pins themselves to the PCB? Or are there different kinds of receptacle?

author: tszaboo
author_rank: Super Contributor
timestamp: July 18, 2022, 09:09:21 pm
content: Quote from: HwAoRrDk on July 18, 2022, 09:02:06 pmYou can mount them on a PCB. Or rather, two PCB that are separated by spacers 10-15mm from each other.It works quite OK, if you plan to use a lot of them, double up the thickness to 3.2mm for the PCB.If you are concerned about the current, put two or more pins in parallel.

author: Benta
author_rank: Super Contributor
timestamp: July 18, 2022, 09:23:29 pm
content: No one says that the "press ring" has to be buried in the carrier plate. It can also be done as a surface recess in the mounting hole top. It's just for holding the receptable securely when the PCB is pressed down on the pogo pins. Don't underestimate the force needed BTW.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 18, 2022, 09:48:22 pm
content: Quote from: tszaboo on July 18, 2022, 09:09:21 pmJust now seen some examples of that!Yeah, that seems like a good idea. I imagine it would be quite a challenge otherwise getting proper alignment and trueness with only a single PCB. I guess it also adds rigidity if you solder each pin on both boards.Downside being that without receptacles you can't swap pins without desoldering. Is that really much of an issue when you're not talking about thousands and thousands of cycles? I wouldn't be doing anywhere near that. The datasheets for those previously linked Farnell pins imply they're pretty robust, good for 10^5 cycles.

author: sleemanj
author_rank: Super Contributor
timestamp: July 19, 2022, 12:46:02 am
content: As well as making the pins replaceable, the receptacles make it easy to assemble so that they all are at the same height, since the ring sits flush.The more consistent height you achieve the better, especially if there are lots of pins.

author: tszaboo
author_rank: Super Contributor
timestamp: July 19, 2022, 06:39:57 am
content: Quote from: HwAoRrDk on July 18, 2022, 09:48:22 pmUse receptacles. They are not expensive, plus, you might want to replace the pins, if the default ones don't make a good contact due to solderpaste or something. And I think if those pins are even slightly deformed because of the soldering, they don't work properly.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: July 19, 2022, 08:56:11 am
content: Respect the head shape to get a certain contact. And design for unit testing. Which for example means not having a test pad right next to 20mm high electrolytic capacitors. Your test pads can be hasl/gold plated flat pads or be solder domes - domes require solder pasting through the stencil.For preference, keep 'pin points' on the back of the PCB. If your fab house offers electrical testing of the PCB, budget that in - trap bad boards before they get placed and baked.FYI common head shapes for P100 pins attached. B heads are points that stab into solder. D heads are domes that ride against plated pads. A heads hug solder domes and PTH legs.I should add that most PCB mounting pogo pins just have dome heads.

author: ajb
author_rank: Super Contributor
timestamp: July 19, 2022, 08:22:01 pm
content: Quote from: HwAoRrDk on July 17, 2022, 11:35:11 pmPretty well in my experience, but it will depend on the size and shape of the connector pin and the pogo pin and the solder fillet. You want the front end of the pogo pin to be wide enough to reliably accept the end of a protruding pin, and you can't have big blobs of solder in the way. I have one fixture that uses three different pin styles at different heights to hit flat round pads, empty PTH holes, the contacts in the top side of micromatch-style female connectors, and protruding pins from male headers. All of them work fine, it's mostly just down to picking the right size/shape of pogo pin.Depending on the assembly and handling process, you may want sharper pins where possible to punch through crud/oxide on flat or soldered pads.Another critical factor is the jig that holds the board. It must ensure that the board is correctly aligned to the pin field before pressing down (otherwise at best you will not make contact in the correct location, at worse you may damage pogo pins, at worst you may destroy the fixture or the DUT). It must also apply sufficient force against all of those pogo pins. Sure, each one might only be a few gram-force, but multiply that by a hundred or so pins and you will need some substantial force. The DUT board itself must be well supported against these forces, to avoid bending in the fixture. Toggle clamps, like the one in the attached photo, are good for applying clamping force, but will need some way to mechanically interface with the DUT.Something to consider in designing this setup is a few extra pins specifically to detect that the DUT is properly seated in the fixture. These could be 2-4 extra pogo pins set slightly lower than all of the others and near the corners of the board, with the corresponding test points just connected to ground within the DUT. That way when the fixture detects that all of those pins are in contact with the DUT it knows that the device is fully seated in the fixture. You could even connect these detection pins to hardware in the fixture to immediately disconnect power from the DUT and terminate any testing so that premature removal or loosening of the DUT from the fixture won't blow anything up.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 19, 2022, 09:23:40 pm
content: Quote from: AndyBeez on July 19, 2022, 08:56:11 amAll my test points will be on the bottom side of the board, which only has a smattering of small SMD components, so no worries at all about clearances. The board is ENIG, and test pads won't have solder on them.Quote from: AndyBeez on July 19, 2022, 08:56:11 amHmm, all the advice I've seen so far is to use pointed pins for flat pads. What's the advantage of using domed pins?

author: Benta
author_rank: Super Contributor
timestamp: July 19, 2022, 09:42:33 pm
content: Quote from: HwAoRrDk on July 19, 2022, 09:23:40 pmThat they don't punch holes in your pads.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 19, 2022, 10:00:21 pm
content: Quote from: Benta on July 19, 2022, 09:42:33 pmIs that a real, potential problem for multi-layer boards?I could imagine an extreme scenario where high board clamping pressure and very sharp pin pointsmightpierce a thin dialectric and go through to the copper layer underneath, such that you're then shorting that pad to a ground/power plane (or whatever is underneath).My boards are a 4-layer stackup with 0.1mm thickness dialectric between outer and inner layers. Should I be concerned about this and use non-pointed pogo pins?

author: Benta
author_rank: Super Contributor
timestamp: July 19, 2022, 10:04:33 pm
content: I said pads. I didn't say "punch through your board".Yes, it can be. If the pad is perforated, you may not have a good connection if you need to test the board a second time.But do as you like. I'm sure the pogo manufacturer made the rounded ones out of a whim.