Hi
I 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 pillars
It 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
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.
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.
I need a conductive path between the pcb and chassis for emc so this rules out plastic clip on pillars
It 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.
Countersink 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
I need a conductive path between the pcb and chassis for emc so this rules out plastic clip on pillars
What external plugs/sockets are in use and couldn't you use their enclosure mounting to provide that conductive path ?
The nuts are non-magnetic so they fall out of a nut driver.
Any reason they're non magnetic?
thread forming screw?
less investment than clinch fasteners
even the pcb will creep over time, so beleville or other spring washer may be a good idea too
thread forming screw?
Or create a thread in the aluminium plate. When using a forming tap you don't get any swarf from tapping the holes.
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.
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.
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.
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 off
stainless are a good compromise
Hi
Brass stand-offs will work but the main time consumer is getting the nut started on the thread. Components around the holes make access difficult.
I need a conductive path between the pcb and chassis for emc so this rules out plastic clip on pillars
What external plugs/sockets are in use and couldn't you use their enclosure mounting to provide that conductive path ?
Hi
The connectors on the boards are all internal to the enclosure.
The nuts are non-magnetic so they fall out of a nut driver.
Any reason they're non magnetic?
Hi
I am using stainless steel fasteners.
Hi
Brass stand-offs will work but the main time consumer is getting the nut started on the thread. Components around the holes make access difficult.
As 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.
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.
I 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.
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.
Hi
These 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.
What environment does the finished product live in, in terms of temp, humidity and vibration?
Inside a custom waterproof enclosure, exposed to the elements and transport bumps/vibration. That is why I have used SS fasteners.
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 off
I started with hex button heads but I find Phillips are faster and easier.
Hi
Brass stand-offs will work but the main time consumer is getting the nut started on the thread. Components around the holes make access difficult.
As 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.
I 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.
The nuts are non-magnetic so they fall out of a nut driver.
Any reason they're non magnetic?
Hi
I am using stainless steel fasteners.
Some of the other options look pretty good, but fyi you can buy stainless steel nuts with high magnetic permeability (many types of stainless steels).
Let 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.
Hi
The 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
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.
Wurth 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
