Cheapest fastest method of mounting PCBs, low volume production ??? - Page 1
Forum section: electronics / projects

author: dazz1
author_rank: Frequent Contributor
timestamp: May 25, 2018, 08:55:04 pm
content: HiI am working on a product that requires PCBs to be mounted on a 1.6mm thick aluminium chassis plate. This product is to be manufactured in low volume (batches of 10 or so) One of the PCBs is a Rasp Pi.At present I am using a 2.5mm screw, spacer and nut. These are proving to be fiddly and time consuming to fit. Specifically getting the nut onto the end of the screw is the most difficult part. The nuts are non-magnetic so they fall out of a nut driver. There isn't enough space to start the nuts by hand. For one or two products it doesn't really matter but each product has 8 screws and these screws are taking too much time to install.In addition the plastic spacers are compressing over time causing the screw/nut to loosen.I am looking for a better option.I need a conductive path between the pcb and chassis for emc so this rules out plastic clip on pillarsIt is highly desirable for the underside side of the chassis to be flush or nearly flush. This rules out having the nut and thread protruding out the underside.I can't change the design of the pcbs to make easier access.I have looked at threaded brass stand-offs but they still need a nut/screw from the PCB side.Clinch nuts look promising but I am not sure if the added time/cost of pressing the nuts into the chassis will be offset by the reduced time/cost of assembly. I'd also need to buy/make a press.Blind clinching stand-offs look like good candidates but I don't have any experience with these. They still need a screw but this can be fitted with the chassis flat on the bench.This must be a common problem so I am looking for answers from people that have found solutions.Dazz

author: ajb
author_rank: Super Contributor
timestamp: May 25, 2018, 09:02:42 pm
content: Sounds like you've covered the options. Press-in standoffs generally require a rather hefty press to insert, and may require specific dies as well, so it's worth talking to a sheet metal fabricator if you're in a position to order a reasonable quantity of the chassis at a time.But for batches of 10, I expect the setup cost of any other solution is going to overshadow any savings in assembly time (unless there are a LOT of mounting points per unit). The easiest method is probably to use threaded standoffs and install all of the standoffs on the chassis first, then all of the components can be mounted with the chassis flat and right-side-up.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 25, 2018, 09:03:09 pm
content: My first thought would be the brass standoffs - what exactly is the reason why they won't work? Is there an access/clearance issue on one side or another?If you need the bolt to be flush on the bottom of the metal plate, you use a countersunk bolt on that side so the screw side of the spacer is facing up then a nut to hold the PCB in place (get magnetic ones...?) Otherwise, use the threaded spacer in the opposite direction, needing just a screw from the PCB side and a bolt on the back of the plate. Sill another option would just be using a spacer without a screw end - a bolt facing in both directions - and just use two short screws to screw into it from either side.I'm sure there are other options, but this one came to mind and requires basically no redesign of anything - appropriately sized screw holes are all that are required.

author: jmelson
author_rank: Super Contributor
timestamp: May 25, 2018, 09:08:53 pm
content: Quote from: dazz1 on May 25, 2018, 08:55:04 pmCountersink the holes from the bottom, and use flat-head screws. Use threaded standoffs, and a screw from each end.That will leave the bottom flush. You may need to use 82 or 100 degree screws with that thin a panel.Jon

author: tautech
author_rank: Super Contributor
timestamp: May 25, 2018, 09:12:40 pm
content: Quote from: dazz1 on May 25, 2018, 08:55:04 pmWhat external plugs/sockets are in use and couldn't you use their enclosure mounting to provide that conductive path ?

author: ChunkyPastaSauce
author_rank: Unknown
timestamp: May 25, 2018, 09:42:18 pm
content: Quote from: dazz1 on May 25, 2018, 08:55:04 pmAny reason they're non magnetic?

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: May 25, 2018, 11:07:11 pm
content: thread forming screw?less investment than clinch fastenerseven the pcb will creep over time, so beleville or other spring washer may be a good idea too

author: nctnico
author_rank: Super Contributor
timestamp: May 25, 2018, 11:20:21 pm
content: Quote from: f5r5e5d on May 25, 2018, 11:07:11 pmOr create a thread in the aluminium plate. When using a forming tap you don't get any swarf from tapping the holes.

author: Gregg
author_rank: Super Contributor
timestamp: May 26, 2018, 04:13:21 am
content: Clean your nut driver with suitable solvent like IPA so masking tape will stick. Cut some short strips of masking tape to about the width of one flat of the nuts you are using and wrap one strip of tape from one inside flat of the nut driver over the edge to the outside. The tape will wedge the nut into the socket of the nut driver long enough to install it and the strip of tape on the outside will show the orientation of one flat to make it easy to pick up the nuts form a smooth surface, If one strip of tape isn’t enough try two adjacent flats.

author: nctnico
author_rank: Super Contributor
timestamp: May 26, 2018, 09:31:28 am
content: Perhap the easiest solution is a nut which gets soldered into the PCB.pemnet.com has lots of these. This way you have a nut and spacer as one.In general you should avoid (mechanical) designs where you need seperate spacers and/or a screw+nut. It just takes too long to mount these.

author: mjkuwp
author_rank: Unknown
timestamp: May 26, 2018, 11:00:27 am
content: One idea would be to design a 3D printed frame to either be the spacer or hold the spacers. however, you'd have to be careful to find a flame rated plastic. If it is possible to use the 3D printed part as a fixture and remove it after assembly, that's an option that can reduce some assembly difficulties.I use 3mm screws in Raspberry Pi boards. The drilled holes are just ever so slightly too small for 3mm so I find the appropriate drill bit and clean the hole. It is such little material that it can be done with my fingers but for speed I normally use a powered drill.

author: max_torque
author_rank: Super Contributor
timestamp: May 26, 2018, 11:06:16 am
content: What environment does the finished product live in, in terms of temp, humidity and vibration?"snap fit" nylon stand offs may be sufficient if you have a low mechanical stress during operation?iehttps://www.rapidonline.com/Affix-CBS-12-Nylon-PCB-Pillars-12-5mm-Pack-Of-50-33-2214?IncVat=1&pdg=pla-301834660990:kwd-301834660990:cmp-757438067:adg-44804851896:crv-207912323492:pid-33-2214:dev-c&gclid=CjwKCAjwi6TYBRAYEiwAOeH7GYnOoSBN0Id80Ut8uih46MHpUD21JLjJFqhR06M-4cImqDnojpM7axoCRpsQAvD_BwE

author: bob225
author_rank: Frequent Contributor
timestamp: May 26, 2018, 11:15:30 am
content: 2 machine screws and a threaded spacers, I tend to find hex button heads work well, fit the stand offs to the plate, screw the board to the stand offstainless are a good compromise

author: chris_leyson
author_rank: Super Contributor
timestamp: May 26, 2018, 01:00:35 pm
content: QuoteThat's neat, I like that one at least you don't need two pairs of hands to hold board, nut, spacer and screwdriverWurth do some SMT spacers as well as pressfit connectors for high power applications.http://katalog.we-online.com/en/em/search/ASSEMBLY/SPACER_STUDS_ROUND_SPACERS_1?sparam%5Bfamily%5D%5Beq%5D=SMD_SPACER

author: dazz1
author_rank: Frequent Contributor
timestamp: May 26, 2018, 11:22:57 pm
content: HiBrass stand-offs will work but the main time consumer is getting the nut started on the thread. Components around the holes make access difficult.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 26, 2018, 11:24:09 pm
content: Quote from: tautech on May 25, 2018, 09:12:40 pmHiThe connectors on the boards are all internal to the enclosure.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 26, 2018, 11:24:54 pm
content: Quote from: ChunkyPastaSauce on May 25, 2018, 09:42:18 pmHiI am using stainless steel fasteners.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 26, 2018, 11:25:08 pm
content: Quote from: dazz1 on May 26, 2018, 11:22:57 pmAs has been mentioned by several now, you can either orient the standoff to have the nut on the facing with better clearance or you can just get a spacer that is just threaded through, requiring just a machine screw on each side.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 26, 2018, 11:27:03 pm
content: Quote from: ajb on May 25, 2018, 09:02:42 pmI am planning on building a custom press to cut connector holes in a custom plastic moulded enclosure. It wouldn't be difficult to make that press suitable for press-in fittings as well.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 26, 2018, 11:29:47 pm
content: Quote from: nctnico on May 26, 2018, 09:31:28 amHiThese would be good to include in the manufacture of a custom board. I wouldn't want to hand solder them after production. Too time consuming.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 26, 2018, 11:31:13 pm
content: Quote from: max_torque on May 26, 2018, 11:06:16 amInside a custom waterproof enclosure, exposed to the elements and transport bumps/vibration. That is why I have used SS fasteners.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 26, 2018, 11:32:15 pm
content: Quote from: bob225 on May 26, 2018, 11:15:30 amI started with hex button heads but I find Phillips are faster and easier.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 26, 2018, 11:36:28 pm
content: Quote from: DaJMasta on May 26, 2018, 11:25:08 pmI forgot to mention one of the easiest options I have tried so far is using a threaded stand-off as a nut on top of the PCB. The extra length makes access much easier for tightening.

author: ChunkyPastaSauce
author_rank: Unknown
timestamp: May 26, 2018, 11:55:17 pm
content: Quote from: dazz1 on May 26, 2018, 11:24:54 pmSome of the other options look pretty good, but fyi you can buy stainless steel nuts with high magnetic permeability (many types of stainless steels).

author: tautech
author_rank: Super Contributor
timestamp: May 27, 2018, 12:00:19 am
content: Quote from: dazz1 on May 26, 2018, 11:36:28 pmLike these used for PC Mobo mounting ?https://www.banggood.com/Suleve-M3BH4-100Pcs-M3-Male-Female-Brass-Hex-Standoffs-Support-Spacer-Pillar-Screw-for-PCB-Board-p-1284142.html?gmcCountry=NZ&currency=NZD&createTmp=1&utm_source=googleshopping&utm_medium=cpc_elc&utm_content=frank&utm_campaign=pla-brand-elc2-nz-pc-0517&gclid=CjwKCAjwi6TYBRAYEiwAOeH7GbaUnftkvFQbnt_eDp8viRUR0S36iAYK-FyxcZUYbOYge0fAE_q9tBoC6hYQAvD_BwE&cur_warehouse=CN

author: SL4P
author_rank: Super Contributor
timestamp: May 27, 2018, 09:03:19 am
content: Zero assembly effort, unless you choose one of the threaded variants...EDIT:Corrected linkshttp://www.hardware-andfasteners.com/sale-8550936-pa66-spp-pcb-standoff-hardware-4mm-16-mm-white-nylon-pcb-spacers.htmlhttps://www.alibaba.com/product-detail/Nylon-Spacer-Support-Fastening-Solutiongs-M3_1901810458.htmlhttps://www.ebay.com/itm/100x-Nylon-Plastic-4mm-PCB-Boards-Locking-Snap-In-Posts-Spacer-Standoff-3-5-22mm-/221870699492http://www.keystone-europe.com/products/spacers-standoffs-3/nylon-pcb-supports-spacers/

author: dazz1
author_rank: Frequent Contributor
timestamp: May 27, 2018, 11:27:11 am
content: Quote from: tautech on May 27, 2018, 12:00:19 amYes. They work quite well as nuts, especially the fully threaded type. You can apply a nut driver socket without risk of damaging nearby surface mounted devices. Cheaper than clinch nuts and they don't need a press.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 27, 2018, 11:30:56 am
content: Quote from: David Chamberlain on May 27, 2018, 09:52:11 amLet me think about using a glue gun for a femto second.No.I am certain there is no danger of breaking out the glue gun.

author: SL4P
author_rank: Super Contributor
timestamp: May 27, 2018, 11:42:08 am
content: Oops - in post #25 (top of this page)...I meant to link to the nylon press-through standoffs.Drill a hole, bolt or push it in, then snap your boards on top as needed.No tools needed to assemble or release for repairhttp://www.hardware-andfasteners.com/sale-8550936-pa66-spp-pcb-standoff-hardware-4mm-16-mm-white-nylon-pcb-spacers.htmlhttps://www.alibaba.com/product-detail/Nylon-Spacer-Support-Fastening-Solutiongs-M3_1901810458.htmlhttps://www.ebay.com/itm/100x-Nylon-Plastic-4mm-PCB-Boards-Locking-Snap-In-Posts-Spacer-Standoff-3-5-22mm-/221870699492http://www.keystone-europe.com/products/spacers-standoffs-3/nylon-pcb-supports-spacers/

author: tautech
author_rank: Super Contributor
timestamp: May 27, 2018, 09:31:01 pm
content: Quote from: dazz1 on May 27, 2018, 11:27:11 amOk. So a screw through the PCB mounting plate into a F-F threaded brass standoff with a M-F standoff as the nut could look and be the best solution.http://nz.element14.com/c/fasteners-mechanical/fasteners-fixings/spacers-feet/standoffs?standoff-material=brassNow just to find some that are affordable.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 28, 2018, 08:05:52 am
content: Quote from: SL4P on May 27, 2018, 11:42:08 amHiUnfortunately nylon standoffs are good insulators and not good for EMC. Everything needs to be well bonded to the chassis.Dazz

author: dazz1
author_rank: Frequent Contributor
timestamp: May 28, 2018, 08:15:42 am
content: Quote from: tautech on May 27, 2018, 12:00:19 amYes, I think so. I can use those on the top and bottom of the PCB. In the long term I will aim to build a press (which I will need for other purposes) to use for clinched threaded standoffs, then use these brass threaded spacers from the top.Dazz

author: Lee Leduc
author_rank: Regular Contributor
timestamp: May 28, 2018, 09:26:58 am
content: Keystone makes PCB standoffs you may find useful.http://www.keyelco.com/category.cfm/Spacers-Standoffs/Supports-Spacers-Nylon-PCB/p/495/id/507I've used these with good results.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 03, 2018, 09:29:39 pm
content: HiThe problem with plastic standoffs is that they won't electrically connect the PCBs to the aluminium chassis. Making a good electrical connection through metal standoffs will help reduce EMC emissions.Dazz

author: kony
author_rank: Regular Contributor
timestamp: June 03, 2018, 11:19:28 pm
content: I suggest shopping around for sheet metal manufacturing companies. If you can handle the design yourself and make their life sensibly easy to work from your data, there won't be problem to get economical offers even for singles qty. If lead times in 3-6 weeks range are an issue, you might consider investing into simple sheet metal bender and handpress, laser cutouts are usually doable in one week turnaround with no price extra.Clinched fasteners win almost every time - just for the love of god please never use loose nuts anywhere - if totally desperate to have it removeable, use caged nut insert instead. For rework there is option of rivet nut.