Mounting a PCB using only surface mount  - Page 1
Forum section: electronics / general

author: charliehorse55
author_rank: Contributor
timestamp: March 22, 2024, 06:46:56 am
content: I'm working on a product that requires one side of the PCB to be completely exposed/flat. I can't grab it from the sides, or use any through holes. So, that leaves me with some way of "grabbing" it by the inside surface. I've looked for off the shelf solutions, but haven't found much. All of the "surface mount" fasteners I've found actually require a through hole to sit in.So far, my best idea is to create a custom SMD footprint for a "welding nut", something like this:Some rough calculations show it should be more than strong enough, at least initially. Not sure how reliable lead-free solder is long term when exposed to repeated mechanical stress.Ever seen something like this done before? Is there a better way to do it?

author: selcuk
author_rank: Frequent Contributor
timestamp: March 22, 2024, 07:04:46 am
content: You can check these as alternatives:https://www.we-online.com/en/components/products/em/redcube_terminals/redcube_smd

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 22, 2024, 07:08:21 am
content: You're looking for PEM nuts:https://www.pemnet.com/products-overview/products-nuts/Which, mind, is a trademark, and there are of course equivalent products elsewhere.Weld nuts would never have enough solder joint -- they might also not be readily solderable (preferably, tin plated). We've been going over this (sort of) here, as it happens,https://www.eevblog.com/forum/mechanical-engineering/mechanically-strongest-solder/so you may resolve some ideas there (or, have further questions).Mind, soldered nuts are at best a compromise. Even if the solder joint itself doesn't fail, the bond to the PCB laminate is generally not very good either. Actually, depending on direction (shear vs. tension) and history (fatigue, creep), either one may fail, with laminate more likely in tension or peeling, and solder more likely in shear over long time periods or many cycles of loading. (I think?)But these numbers pale in comparison to hardware that engages the laminate itself well, like a press-in (in compression, maybe not so much in tension), swage (gets expanded into the laminate), or simple compression (screw into a normal nut or spacer or other flange) joint. The whole gamut is covered by PEM's catalog, so you should find many ideas there.Tim

author: Smokey
author_rank: Super Contributor
timestamp: March 22, 2024, 07:58:02 am
content: https://www.digikey.com/en/products/detail/w%C3%BCrth-elektronik/7466005R/6643996QUANTITY UNIT PRICE EXT PRICE1 $2.54000 $2.5410 $2.12100 $21.2150 $1.85700 $92.85100 $1.69100 $169.10M5 screw.... And it's rated for 70A!

author: Ian.M
author_rank: Super Contributor
timestamp: March 22, 2024, 09:09:20 am
content: 3M VHB tape?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 22, 2024, 09:24:54 am
content: There are quite a few SMD stud/bold products, MAC8 do many variants, also several cheaper options available from LCSCStuds tend to need a small (1mm ) hole for an alignment pip but this can be nonplated and the pip doesn't go all the way through the PCBhttps://www.lcsc.com/products/SMD-round-nut_11790.html?keyword=nuthttps://www.lcsc.com/products/SMT-hex-nut-post_11820.htmlhttps://www.mac8japan.com/category/80https://www.mac8japan.com/products/219

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 22, 2024, 09:29:36 am
content: Alsohttps://www.mac8japan.com/products/https://www.mac8japan.com/products/265

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 22, 2024, 10:46:26 am
content: The reasons you are having trouble finding stuff is probably for the reasons T3sl4co1l already mentioned. Solder is not a mechanically strong material. Delamination may be a secondary / smaller issue. When the nut is place on the "other" side, then the solder is only used to keep the nut in place and it's mechanical strength is not relevant.If you want to keep it all on one side, then the bigger the surface area with the PCB, the stronger the joint will be. Brass flange nuts may be an interesting option.https://www.kaizenmetals.com/products/fasteners-and-fixing/brass-nuts/brass-flange-nuts/Or nuts such as these which have a bigger diameter:https://www.nutty.com/brass-fasteners/brass-hex-machine-screw-nuts/There are also a lot of different high current terminals which provide some kind of bolt or screw connection. A simple search at Digikey (With an extra filter for SMT types) gives a bunch of examples, though most are duplicates from terminals mentioned earlier in this thread.https://www.digikey.com/en/products/filter/terminals/screw-connectors/396They are not designed for PCB's / solder ability (the brass may be oxidized) but that could be fixed in some cleaning process, for example in a tumbler.Most types of nuts are not very high, and as a result you have a very limited range for the length of the bolts. As an variant, you can also solder the head of brass bolts directly to the PCB. (The countersunk types have a large flat surface) Or use the nuts (bigger surface area because of the flange) and put a piece of brass all thread in it before soldering.Instead of solder, you can also use glue / epoxy, which may be stronger then solder.It's also possible to glue or solder some sub frame to the PCB, and then use that with some kind of fastener.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 22, 2024, 05:42:40 pm
content: Oh also, if some secondary operations are acceptable, and it's not so much the one-sided-ness as the low profile, you might consider countersinking the holes, and using cone screws. Works very well even in sheet metal, though you have to be more and more careful with just the right countersink depth and angle, and screw dimensions as the panel thickness goes down, obviously.Secondary, merely because, if you ask the PCB fab to do it themselves, it's probably going to be a pricey custom operation for prototyping, and I'm assuming this is a few-off case by default. But you would of course want them to do it in production, and the cost will be reasonable then.It will be difficult to impossible to get any kind of connection to the countersink itself, but a pad on the facing/mating side can still make a connection.Or the countersink is made early in fab, before plating, and becomes a plated pocket, and you could get vias in and around it as well (drill small holes, then the large hole, then the countersink). (Done carefully, you could even solder in a cone-head screw as a threaded stud; you'd probably want to use solder paste, and a jig to hold it perfectly perpendicular.) This is generally even more expensive, but again might be a production option. Probably the solder nuts or other bracketry would be preferable at this point.Oh speaking of bracketry; you could just do something like, get a slab of brass, machined, formed or cut with whatever holes or threads you want, and make an enormous lap joint soldering it to the PCB, spreading the load out over as much area as you can afford. This is a good way to get tabs, flanges, etc. onto a board, though do be careful about bending stresses, as for example a simple L-bracket sees exponentially more stress right under the bend, right into the solder fillet, whereas a reinforced (say the bend is embossed to put a stiffening ridge into it) or gusseted or tee shaped joint spreads it out further through the base metal, sharing the load into the solder joint and laminate more evenly.(Brass can easily be tin plated, the best case for soldering, but can also be wetted by itself, though at some difficulty compared to the ideal case; hence you might "butter it up" beforehand. Which can include acid fluxes if needed, and then wash it off before soldering to electronics.)And the above mentions of glue and VHB tape, a more permanent solution obviously, but if once is all you need, they're quite good for this.Tim

author: floobydust
author_rank: Super Contributor
timestamp: March 22, 2024, 06:04:20 pm
content: Quote from: mikeselectricstuff on March 22, 2024, 09:24:54 amNote PEM copied MAC8 after I griped to them at a trade show about not offering anything lol. MAC8 is hard to procure in North America.SMT standoffs and nuts. They can be mounted by pick'n'place machine and use solder to hold them in place on a PCB.https://catalog.pemnet.com/category/uts-and-spacers-standoffs-surface-mount-type-smtsohttps://www.mac8japan.com/category/80

author: Smokey
author_rank: Super Contributor
timestamp: March 22, 2024, 06:30:00 pm
content: I had pcbway countersink some holes on a PCB recently. It didn't increase the price much.

author: charliehorse55
author_rank: Contributor
timestamp: March 23, 2024, 01:38:49 am
content: Thanks for all the ideas. I've changed course, it seems like the best idea is to glue a frame to the PCB. The backing can then be bolted to this frame, so the unit can still be disassembled. There's plenty of free space available on the PCB surface to form a strong bond.