Cheap contact pin between two boards - Page 1
Forum section: electronics / projects

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 10, 2023, 06:01:45 am
content: Hey People,Help me with ideas. I have two boards 1 to 3mm (40-120 mil) apart (tbd) separated by a copper block. I want ~50 contacts between them that are small (let’s say 1mm OD including insulation or clearance) easy to assemble and cheap. Even for a small batch I planning this means thousands of those and I’m yet to recover from the sticker shock of DigiKey prices for Harwin or Milli-Max single pins. $0.3 when buying 10k just won’t cut it.I could use a piece of wire but then cutting aligning trimming each one individually sounds horrible to me. Ideally I would use something as a pre cut solid core wire with a short insulator section in the middle that also aids with alignment. The pcb contact points can still change, I have lined up castellated holes in mind.Another idea I had is to have a very narrow pcb also with castellated holes stood vertically, covering the edges and making connection between the two horizontal boards that way. Problem is getting such a narrow board made with extra edge processing.Any tips?

author: Dr_Neoo
author_rank: Contributor
timestamp: December 10, 2023, 09:22:32 am
content: Any reason that you can't use a high pin count board to board connector? Take a look through what Molex, JST and Hirose has to offer.Or use a suitable pin header if you want a soldered connection.

author: Psi
author_rank: Super Contributor
timestamp: December 10, 2023, 09:29:21 am
content: A fine pitch double row pin header seems like the best plan

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 10, 2023, 05:21:45 pm
content: Quote from: Psi on December 10, 2023, 09:29:21 amQuote from: Dr_Neoo on December 10, 2023, 09:22:32 amYeah, sorry, I should've started with the picture. My post was written late in desperation.So I really don't have the space for larger blocks.This render shows 1.2mm od spring pins. These are still thicker what I'd like, but the spring action at least frees me to solder the two boards together. (Which is not a requirement, but nice.) The biggest problem is that these are also not cheap. I just can't believe there is not an appropriately sized stamped pin or something along those lines.If I knew of a convenient JLC-like service that does photo etched copper parts (without the FR4) I would make my own pins.

author: zapta
author_rank: Super Contributor
timestamp: December 10, 2023, 07:11:37 pm
content: Doe shit help?

author: ajb
author_rank: Super Contributor
timestamp: December 10, 2023, 08:46:14 pm
content: Quote from: Marsupilami on December 10, 2023, 06:01:45 amIf a suitably cut and stripped piece of insulated wire will work, there are robots to do the cutting and stripping for you. Any wiring harness CM should have them. Depending on dimensions you might want PTFE insulation to better withstand soldering, which is a bit harder to process, but otherwise ought to be straightforward and fast to produce once setup. Although this method is probably better suited 3mm spacing than 1mm; the shortest insulation segment you can reliably achieve will depend on the material and the machine (drive method and blades).

author: nali
author_rank: Frequent Contributor
timestamp: December 10, 2023, 08:55:14 pm
content: Something like good old Vero pins perhaps?https://verotl.com/pcb-pins/pinsFailing that, Samtec do some multi-pin sprung connectors. Not cheap, but cheaper than individual pogo pins.

author: Psi
author_rank: Super Contributor
timestamp: December 10, 2023, 09:08:36 pm
content: Isotropic tape or zebra strip between PCBs?Probably not going to work but just thinking outside the box incase it gives you ideas

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 10, 2023, 09:24:49 pm
content: It’s unclear what your limits are exactly, but the high volume pin headers will be cheapest.Smallest before the price goes silly, looks to be 1.27mm, which come in dual row and 1.5mm plastic height.

author: Whales
author_rank: Super Contributor
timestamp: December 10, 2023, 10:21:50 pm
content: Current requirements?Flat flex ribbon cables are cheap, standardised and don't need perfect board alignment. Put a locking connector on each board, don't solder them directly unless you absolutely have to.

author: Someone
author_rank: Super Contributor
timestamp: December 11, 2023, 12:18:46 am
content: Quote from: nali on December 10, 2023, 08:55:14 pmThere are ranges of press/friction fit pins from several manufacturers (and likely many if you look hard enough). But even down at cents per unit for the pin, assembly comes back and true cost goes back up.Quote from: Marsupilami on December 10, 2023, 06:01:45 am.. then you add a tight mechanical constraint (freedom for other parts). Time to take a step back and stop trying to force a solution on finding a mythical ideal cheap pin.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 11, 2023, 03:03:55 am
content: Quote from: nali on December 10, 2023, 08:55:14 pmHuh, ok, not the exact dimensions I imagined, but this is along the lines I was thinking. Even if I have to clip them short this might work considering the price. (~$35/1k)Thank you!

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 11, 2023, 03:11:04 am
content: Quote from: ajb on December 10, 2023, 08:46:14 pmI'm worried what would be the break even point considering NRE or tooling cost or whatever, at least for any US supplier.I'm sure I could have this set up if I needed millions. I'll look around tho. Thanks!

author: twospoons
author_rank: Frequent Contributor
timestamp: December 11, 2023, 03:14:55 am
content: For 50+ contacts using spring contacts raises another problem: the total force required to clamp the two boards together becomes substantial. As has been said above, surely you can find dual-row header pins, maybe on 2mm pitch, for the price you are after. Mounting 50 individual pins is going to be a PITA.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 11, 2023, 03:29:43 am
content: Quote from: Someone on December 11, 2023, 12:18:46 amI'm factoring in the assembly cost already, that's why I want the part itself to be as cheap as possible. For example I would be perfectly happy with pins used in some pin headers, if only they were not embedded in plastic. Yes, I could strip them out, but that adds yet an extra step to worry about.Quote from: Someone on December 11, 2023, 12:18:46 amBelieve me I took so many steps back in this project already that I could walk around the globe backwards twice.It is challenging. I'm trying every avenue until it turns out impossible or I find a better alternative going some other direction. I have very specific reasons for not having more space and why it looks as it looks. It is also totally possible that there is no combination of choices that I could make that makes the basic concept viable. Unfortunately that's a common problem with innovation, to know if it is possible at all. I still feel I haven't exhausted every possibility regarding the mythical pin, which is only mythical until I find one that happens to fit.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 11, 2023, 03:41:31 am
content: Quote from: twospoons on December 11, 2023, 03:14:55 amThat worries me to, especially considering that the top board cannot be screwed down. It will be glued (epoxied/whatever) to the copper base and the upwards pressure from the pins wouldn't help. I need to look into the spring forces closely, if I happen to find a pin that's appropriately priced, which I haven't yet.There are some pins out there that are surface mount and come with a pick and place cap, and that would be pretty straightforward to assemble.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 11, 2023, 03:58:04 am
content: Ok, here's an example why I'm reluctant to believe that it's difficult to do what I'm looking for:Spring pin $0.273 @ 10,000It has a base, a tip, a spring and it has to be put together.https://mou.sr/3NjHv84This is the same manufacturer, but it's the dumbest pin in the history of dumb pins$0.316 @ 10,000https://mou.sr/48cn7xWI just want a dumb pin with spring pin pricing but scaled down according to functionality(It's a rant, not a question. I understand that a million other things go into pricing. Thank you for listening, I feel better now.)

author: GigaJoe
author_rank: Frequent Contributor
timestamp: December 11, 2023, 06:08:22 am
content: https://www.lcsc.com/products/FFC-FPC-Connectors_545.htmland cablehttps://www.lcsc.com/products/FFC-FPC-Connect-Cables_920.html

author: Psi
author_rank: Super Contributor
timestamp: December 11, 2023, 06:26:32 am
content: Plenty of china copies of Mill-Max pins/pogos if you like the product but not the price.Mill-Max parts tend to be for high cycle count test jigs, rather than end user products.It's why they're so expensive, a lot of their customers are people making production jigs and want the best quality/reliability at any cost.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 11, 2023, 09:54:36 am
content: Quote from: Marsupilami on December 11, 2023, 03:29:43 amThe 1.5mm plastic is inside part of your given spacing,(1-3mm) and one thing we have done for programming adaptors, is to use the nearest low cost header, that we then move the plastic location on, using a clean vice.Ie if you want < 1.5mm spacing, the plastic can go outside the gap.

author: nali
author_rank: Frequent Contributor
timestamp: December 11, 2023, 11:18:55 am
content: So, your total pin length needs to be 3mm + 2xPCB + soldering i.e. up to ~7mm?You might get lucky and be able to (mis)use a bootlace ferrule. If you get the length right you may just be able to use a PCB rivet tool to swage the end and add some mechanical strength before soldering.

author: tszaboo
author_rank: Super Contributor
timestamp: December 11, 2023, 02:34:45 pm
content: The reason they are not doing it, as you found out, because it is expensive.If you want a lot of connections, in 1-3mm distance, probably the easiest and cheapest is standard pin headers, and bottom entry sockets.

author: Dr_Neoo
author_rank: Contributor
timestamp: December 12, 2023, 06:25:08 pm
content: Quote from: PCB.Wiz on December 11, 2023, 09:54:36 amWould it be an option to use a small pitch header with long pins and then reverse mount it? This would move the plastic part of the header to the other side of the board. Soldering might be a challenge but with a high temp plastic it could survive reflow using a pin in paste process.

author: tooki
author_rank: Super Contributor
timestamp: December 12, 2023, 09:03:16 pm
content: Quote from: Marsupilami on December 11, 2023, 03:41:31 amThe force for 50 pins will be enormous: the lowest-force pogo pins are around 60g of force, so you’re looking at 3kg of force needed!!The very last PCB layout I did at the physics department was a 16-channel version of a board (original was 4 channels). The 4-channel version uses 7 SMD pogo pins mounted as through-hole so that they protrude only about 0.5mm. This already requires a surprising amount of force to mate securely. We quickly established that quadrupling that (and ideally even more, to add extra grounds) would require lots of clamping hardware we didn’t have room for. So instead I found some spring-contact board-to-board connectors from Samtec. I chose ones that have built-in threads, but they exist without them too, and in thinner versions than we used, too. (One single pogo pin was used for the high-voltage (up to 700V DC) supply, to keep it physically isolated from the fine-pitch connectors. The end result is super elegant and compact, and the customer is thrilled with it.4-channel version:https://sengerm.github.io/Chubut_2/doc/testing/index.html16-channel version:https://sengerm.github.io/Chubut_2_16CH/doc/230810_characterization/main.html(In both cases, I only did the main board; the customer did the mating (“carrier”) boards himself.)

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 29, 2023, 07:26:59 pm
content: Quote from: tooki on December 12, 2023, 09:03:16 pmI ended up with some custom made spring pins from a random Chinese company. I don't have my boards yet, but the look and feel of the pieces are all right. Got my 1k samples for $70. At $10k they're charging $0.035 per piece.They are rated for 30g at working height.The adhesive I'm eyeing now has a shear strength of 1.4MPa after curing. Let's be generous and assume that tensile strength is half of that so 0.7MPa. I have about 600mm2 area for the adhesive and the pins are distributed more or less evenly. This tells me I have 40kg to work with, yet I only need about 1.4 to compress the springs. Unless of course I'm a decimal or two off, which happened before. I'll do some experiments.

author: tszaboo
author_rank: Super Contributor
timestamp: December 29, 2023, 07:54:14 pm
content: Care to share which brand?

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 29, 2023, 11:20:03 pm
content: Quote from: tszaboo on December 29, 2023, 07:54:14 pmYou mean the pins? I don't think "brand" is an applicable word here. There are a bunch of companies on alibaba.com doing basically the exact same thing and I picked one kinda random.https://dghonshi.com/