bed of nails with some component pins only 1.75 mm apart - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: June 01, 2021, 02:10:00 am
content: Hi,I need to make a simple bed of nails with a total of 12 test pins for an old board for which I only have gerbers.The pins are of through hole components so I have to use concave bed-of-nails spring probes. And the pitch pair is only 1.75mm apart. So I have to find very fine spring probes.Any practical advise?I think 3D printing is out of question given the fine pitch and the fact that it is going to be used to test up to a couple of thousands of boards across a year. I was thinking perhaps of making a DXF with the probes position for the holes and then having it drilled by a professional CNC centre...Also given the relatively thin probes, I am wondering:1) how to avoid bending of the pins because they have to protrude more than 12mm when compressed in order to clear components very close nearby.2) Given that they have to work for a couple of thousands cycles, I cannot make the holes too loose... so wondering how easy it is to insert them manuallyAny advise is welcomed!Thank you

author: sleemanj
author_rank: Super Contributor
timestamp: June 01, 2021, 05:21:15 am
content: Use pogo pin sockets to hold the pogo pins - probably the P50 series pins, R50 series sockets.Constructing the plate holding those sockets, could be done by CNC drill or probably resin 3d printing.FDM 3d printing maaaybe with a narrow nozzle and well calibrated printer at least sufficient to have undersized holes to come back with a non-cnc press to finish.As for not bending pins, ensure the side-load is small. But don't underestimate how much force a whole lot of pins takes to push anyway, one pin is weak but 20 pins is not, there is probably a chinese proverb in that somewhere.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 01, 2021, 07:30:39 am
content: Have you thought about designing a PCB with THT pins for your Pogo pins?Then simply use two of those PCB's with a spacer in between and solder the pogo pins to the PCB's. I think it's also possible to have sleeves for the pogo pins and solder the sleeves to make the pogo pins easily replaceable.And of course once you're designing a PCB, you can also add a connector and route the the pogo pins to that, or even integrate test electronics on the PCB (and leave the other unpopulated)Some related Idea's:Once you've designed the PCB, ordering a few more is not so big of a deal. You can even make a stack of those PCB's and then clamp the pogo pins by adding shear forces to some of the PCB's in different directions. This also centers the pogo pins if the holes are too big.If you keep the PCB's simple, (just route to a connector) then the whole sandwich construction is easily replaceable for moderate cost, so instead of replacing worn out pogo pins, just grab a few new PCB's and solder new pogo pins.If your PCB has mounting holes, then you can also add them to the test jig PCB's and mount round rods in them to align everything, and add extra mounting holes to mount the spacers and mount the whole jig to whatever you want to mount it to.You write you "only" have gerbers of the original project.With KiCad you can create a PCB from a set of Gerber files. This very easily gives you all coordinates for your pogo pins, mounting holes & PCB outline.Don't rely on solder for mechanically stressed connections. Use some kind of back-plate for the pogo pins

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 01, 2021, 09:19:00 am
content: http://www.coda-systems.co.uk/catalog/index.htmlwill have all the probes sizes you need.For mounting, use PCB material - 2.4 or 3.2mm thick if you need extra stiffness.Use a PCB house rather than a CNC shop to drill it as they will have the right tools and likely cheaper for a 1-off like this.If you're doing this a lot, a cheap Chinese CNC engraver is perfect for this sort of job

author: ricko_uk
author_rank: Super Contributor
timestamp: June 01, 2021, 12:10:58 pm
content: Thank you Mike, Doctorandus and sleemanj. Much appreciated!

author: phil from seattle
author_rank: Super Contributor
timestamp: June 01, 2021, 04:32:26 pm
content: Quote from: Doctorandus_P on June 01, 2021, 07:30:39 amThis!Somewhere I saw a KiCad add-on that takes a design and allows you make a test board for it. Probably requires a complete design so gerber only doesn't work.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 01, 2021, 06:16:10 pm
content: Thank you Phil,I have been using Altium and not familiar with KiCad. But between Altium and Solidworks I can make it easily.But I am always interested in suggestions to make it mechanically simple and also reliable for being used for thousands of times. I am thinking of integrating most of the suggestions made above from various contributors and have some vertical guides and a "lid" holding the PCB which also slides up and down so it is always contacting the pins straight on when lowered.Perhaps dowel pins at the bottom (multiple thick PCBs at the bottom - or 2 spaced with some spacers) with some dowel pins and ABS block with holes at the top that slides up and down.And because the test is only few seconds it would take more time to clamp it than to do the test (almost) so I am thinking that the operator just holds it down with their hand.I will post the 3D CAD when done but in the meantime if anybody has any other suggestions, they are always welcome!!Thank you all

author: fcb
author_rank: Super Contributor
timestamp: June 01, 2021, 06:26:36 pm
content: The operator holding down a board is fine except if they slip and bend your pins, this may not be a big deal if it's only a few K operations (where repairing the jig a couple of times isn't a big deal).Bit more complex, but I'd consider 3D printing a jig that operator places the PCB on AND THEN bring-up the test pin plate/carrier/whatever onto the board (assuming from the underside). Basically seperate the action of the operator placing the board and the action of the pins making contact.

author: Hydron
author_rank: Super Contributor
timestamp: June 02, 2021, 10:39:37 am
content: For low volume fixtures I've used these in the past:https://www.merifix.com/They take standard size pogo pins ("S-075"), so I assume you can buy cup ones - low spring force ones ideally.Minimum pitch is specified as 0.075" though, which is close to 2mm. Might get away with it (with a bit of deflection of the cup probes) if you only have pairs of probes so close, rather than say a row or grid?

author: ajb
author_rank: Super Contributor
timestamp: June 02, 2021, 06:42:03 pm
content: Quote from: Doctorandus_P on June 01, 2021, 07:30:39 amSeconding this. I've done a few where I just design PCB with the pogo pins and whatever connectors I need to marshal out all of the test signals, the bottom PCB gets all of the connectors and the top one is just to align/reinforce the pogo pins so doesn't get any other components. If you have high speed signals where the traces of the top PCB hanging off of the pogo pin would be an issue then you can design the PCB so those tracks are easily cut near the pogo pin pad--and do put them on the TOP of the PCB if possible, where you can see them, and not on the side that will be facing the other board and impossible to get to once the fixture is assembled. I would recommend against putting any more than the bare minimum of electronics on the pogo PCB though, if it will be used for any sort of volume. Eventually the pogo pins will need to be replaced, and even if you have the pins socketed there will be many opportunities for the board to get damaged otherwise, so best make it as easy as possible to replace whenever necessary.Socketing the pins at 1.75mm spacing may be tricky, I've done 1.5mm spacing but 2mm is more comfortable, even without sockets. 12 pins isn't that many, I would probably just plan for the whole pogo board to be replaced and solder the pins in directly unless the fixture PCB needs to be very complex otherwise.If you're having pogo pins hit both PTH pins and the bare board or are using multiple kinds of pins it may be worth 3D printing (or whatever) a fixture that sets the relative heights of the pins correctly, especially if you anticipate building more than one. Or you can assembly the fixture the top PCB spaced the correct height above the bench for each set of pins and do the shortest ones first.One quick-and-dirty technique I've used for designing fixtures like this is to copy the Altium project, delete all of the components in the sch except for the ones that will be used for test points (keep net labels for them!!!), and then replace the components with their test pin counterparts. Go to the PCB, mark out any board geometry you want to be aware of when designing the test PCB (like tall parts on the fixture-facing side or locating features or whatever), then update from sch and clear the rest of the routing. Voila, you now have pogo pins in all the right places. Go back to the sch and wire them up to connectors and add whatever other circuitry you needed, and add whatever locating features you need to the PCB. In this case if you only have the gerbers you may be able to import them to create reference geometry. This technique is easiest when each test point is its own single-pin component, but you can also create test pin array dummy components that have however many holes you need in the right places to hit the part you're trying to probe. Just make sure the origin matches the mating part!

author: ricko_uk
author_rank: Super Contributor
timestamp: June 02, 2021, 08:43:37 pm
content: Thank you all!