Bed of nail programmer - resources? - Page 1
Forum section: electronics / projects

author: jnz
author_rank: Frequent Contributor
timestamp: January 12, 2019, 12:35:15 am
content: I'm not sure, but I think there should be pogo pins that I can place into a 3D printed bed with some accuracy. Ideally I'd be able to find some pins that press in and I can run wires to, but I'm thinking it's more likely I'll have to roll a custom circuit board to hold pins and a 3D printed or machined fixture to then hold the board in the right place.Has anyone made fairly inexpensive bed of nails programming fixtures and liked some parts or ideas over others?

author: KL27x
author_rank: Super Contributor
timestamp: January 12, 2019, 12:52:43 am
content: Are you probing random test points splayed out over distance to verify the board/circuit or are you just probing an ICSP/JTAG header?

author: langwadt
author_rank: Super Contributor
timestamp: January 12, 2019, 01:30:37 am
content: Quote from: jnz on January 12, 2019, 12:35:15 ampcbs are so cheap it'll be hard to find something cheaper

author: sleemanj
author_rank: Super Contributor
timestamp: January 12, 2019, 01:54:09 am
content: I recently (well, 6 months ago) made a pogo test jig, home etched and 3d printed.Don't underestimate the force required to press them down if you have a number of them, my jig has 30 pins, 15 a side it takes considerable force. A toggle clamp ended up being the simplest way, I tried a number of different designs for a 3d-printed hold down method and couldn't get anything that was very reliable.Even though the point of a pogo is that they can take up small variances, getting them aligned to the same height is still pretty important in my experience, they can be really finicky.3d printing jigs - you'll probably go through a few designs, one problem I had is the very slight tolerance differences in my (admittedly old crusty and low quality) 3d printer meant that, for a 30 pin jig after tuning in in the hole diameter so it fit the pins snugly, some holes were too large and some were too small, so then you have to drill, which and getting the drill straight means a press really, and drilling PLA isn't great...In all, next time I make a similar jig I would have perhaps been better to have printed a pattern on paper, glued it to wood and drilled that as the pin-aligning part of my jig underneath the PCB (or, indeed, attached the etched PCB to the block of wood and drilled both in a press at the same time).

author: cdev
author_rank: Super Contributor
timestamp: January 12, 2019, 02:45:13 am
content: Quote from: jnz on January 12, 2019, 12:35:15 amThe ones I've seen have used big thick pieces of acrylic plastic to hold the pogo pins, not 3D printed holders.

author: jnz
author_rank: Frequent Contributor
timestamp: January 12, 2019, 04:17:09 am
content: Quote from: KL27x on January 12, 2019, 12:52:43 amA few JTAG pins, an a couple other points. Maybe 7 total.Yea, that's the question is a machined ren or acryllic block the pins are pressed in, or a 3D print (we have an SLA machine, holds tighter tolerances than filament). I haven't looked to see if someone makes press-fit pins I think I'd need.

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2019, 04:30:58 am
content: I helped build a test jig like that once. It used heavy fiberglass board about 1/6" thick with the pogo pins installed in it. The frame was made of wood with a cutout to locate the PCB and there was a hinge mounting the board that came down over the DUT.

author: KL27x
author_rank: Super Contributor
timestamp: January 12, 2019, 05:56:16 am
content: I've used a couple different kinds of pogo pins. The fancier one comes in two parts. The inner part is a complete unit, with contained spring and moving pin. The unit slides into the outer cylindrical housing, clicking into place. So they're replaceable.The cheaper ones were a single part. So if the pin goes bad, you have to desolder w/e the pin connects to in order to replace it.Ironically, the fancier pins are so well made, I have never worn out or damaged one of them. But it's still nice that you can change out individual pins and/or different spring forces and head shapes. The stiffer springs are quite stiff. You might need that to pierce through oxidation or flux residue. If working with clean ENIG pads, you can use the lowest force springs.The cheap pins wore out fairly quickly, and they had more wiggle to them, radially.Fancy pins:https://www.mouser.com/datasheet/2/193/55-11775.pdfThey're rated for 0.05" center to center. I have never tried to solder them into a pcb like a thru hole part. The housings are about 0.034-0.038" in OD, per the datasheet. I imagine it is just possible with a custom pad shape. What I do is just solder the housings flat on their side to the pcb for a SIL 0.05" pitched interface. If you use two short rectangular pads on either end of the housing, you can flow and adjust one end at a time to straighten the pins and get them parallel. But it takes a steady hand, for sure.Personally, I think the single point spear-head is the best for flat, clean pads. It has good piercing power and gives the most room for error in jig alignment. If you have any solder lumps on the pads, the multiple point serrated heads will register better on a lump in case of being slightly off but still have good piercing power. Rounded head pins are not that great on flat pads, IMO. I haven't found the point of them.On the PCB I purchase, I can often see a single sharp divot where the test probes were placed, which I think is from a high force spear point.

author: jnz
author_rank: Frequent Contributor
timestamp: January 14, 2019, 05:10:57 pm
content: Quote from: KL27x on January 12, 2019, 05:56:16 amYou don't happen to have a photo of the board you're mounting to do you? I think I need to see what you're talking about to really get it.

author: KL27x
author_rank: Super Contributor
timestamp: January 14, 2019, 07:29:04 pm
content: Umm, nothing to see, really. Just a row of pins soldered to a pcb. Then traced out to w/e you want to connect to it for programming or debugging.()The one on the right is what I was describing. The pads are too short for these pin housings, because I used the same PCB layout as for the earlier models using the not-recommended cheapskate pins.The one on the left is something that might interest you. I think the company is called TAG Connect. They make cables with a pogo pin interface on one end and a single row of female 0.1" header on the other end. I removed the pin cluster part out of the cable and attached it to a PCB. (The way the cable is, this part is enclosed in a small, rounded HDPE bead, so there's no good way to hold onto it or to attach it to a jig.) They use a DIL 0.05" pitch and the unique factor is the steel guide pins. This is really good for hand updating a long firmware in the field, or so I hear. But if you are using a jig of some type, it is not really needed.()Here's another pic to show how the pins look outside of the housing.They look fragile, but they are very sturdy. In the beginning, I built hard cases and always brought two when I had to leave the home office with them. I also had bought plenty of backup pins. Nowadays, I still use some of these interfaces fairly frequently, but I couldn't even tell you where I put the spare pins. And if I have to drive across town with one, I just throw one in a side pocket of a laptop bag.

author: IconicPCB
author_rank: Super Contributor
timestamp: January 14, 2019, 08:27:45 pm
content: in order to achieve resonable pointing accuracy You need to use G10 base material which is at least 8 to 10 mm thick for the fixture base plate.The probes do indeed come in two parts; a receptacle and the probe.These assemblies ( receptacle and probe ) come in a range of sizes down to 0.025"diameter and even smaller.The base plate is drilled using the prescribed drill size. Receptacles are then driven into the base plate flush with the top of the plate .Receptacles have a compression ring on their body whihc is used to hold the receptacle within the hole. A specific drift is neede to drive the receptacles into place.This ensures pointing accuracy of the receptacle.The probes also incorporate a number of features whihc ensure probe pointing accuracy and good electrical contact between probe sleeve and probe shaft.The receptacle can have a crimp connection or wirewrap pin or soldercup for the wire.Nothing majical just common sense.And Yes I have sold and manufactured test fixtures.

author: langwadt
author_rank: Super Contributor
timestamp: January 14, 2019, 10:00:12 pm
content: Quote from: IconicPCB on January 14, 2019, 08:27:45 pmI think stacking a number of PCBs could work, With the cheap protos you usually 5-10 identical PCBs anyway

author: IconicPCB
author_rank: Super Contributor
timestamp: January 14, 2019, 10:17:10 pm
content: Multiple bards would be a reasonable poor mans solution with reduced pointing accuracy.You must keep in mind the probe receptacle is a sitting in a precisely drilled hole which will act on the compression ring of the receptacle.That said 0.1" pitch probes will be quite tolerant of slight misalignments.I am using the probes alone without the receptacles in a single 0.062" thick PCB specifically made for the task without any dificulty.

author: cdev
author_rank: Super Contributor
timestamp: January 14, 2019, 10:37:34 pm
content: I've often wondered if there was some better way to build something like a temporary mini bed of nails, for holding contacts with some pressure onto multiple precise spots on a PCB.Ive seen some solutions which are sold commercially. They're fairly pricey, most of them.

author: langwadt
author_rank: Super Contributor
timestamp: January 14, 2019, 11:08:27 pm
content: Quote from: IconicPCB on January 14, 2019, 10:17:10 pmmaking the pcb holes the minor diameter of the receptable, using the ring as a depth stop and stacking the pcbs far apart as possibleshould make it at least as accurate as a pcb

author: IconicPCB
author_rank: Super Contributor
timestamp: January 14, 2019, 11:25:27 pm
content: If I didnt make it clear...the receptacle is driven into the plate with a special drift ( tool to drive the receptacle correctly) .The compression ring is compressed inside the hole to retain the receptacle in place.

author: IconicPCB
author_rank: Super Contributor
timestamp: January 14, 2019, 11:32:23 pm
content: Cdev,Yes there is. I used to run a subcontract bare board test laboratory. I had a universal grid single density machine with a cassette structure for its bed of nails function.Spring loaded probes ( some 30,000 odd thousand ) were interposed between test electronics points and user accessible 0.1" pitch universal grid which carried job specific fixture built with rigid probes.The spring probes were covered with a "striper" plate which exposed heads of spring loaded probes and engaged with the bottom plate of job specific fixture.The only thing that changed in this kind of setup would be job specific fixture.

author: KL27x
author_rank: Super Contributor
timestamp: January 14, 2019, 11:38:38 pm
content: QuoteI have used my sidewards interface at 0.05" pitch in a CNC mill. PCB misalignment is the major factor in the deal. If you can't get the pins straight enough in thick FR-4/G10 block, I imagine you might try using a plastic like acetal or hdpe. There's no grain in the plastic and it makes for straighter drill holes.

author: langwadt
author_rank: Super Contributor
timestamp: January 14, 2019, 11:44:22 pm
content: Quote from: IconicPCB on January 14, 2019, 11:25:27 pmI know that is the intended way, that doesn't mean you have to do like thathttps://www.pjrc.com/teensy/beta/testfixture_nails1.jpg

author: IconicPCB
author_rank: Super Contributor
timestamp: January 15, 2019, 03:58:28 am
content: Langwadt,So true. I have built a little programing fixture based on 6 pin AVR pattern. Pins had some programing issues so i drilled a small second layer PCB to straighten out the pins and simply used super glue and bicarbonate of soda to hold it all together.Bicarbonate of soda acts as an instant hardener of super glue.