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Questions relating to designing PCBs
By smdFan
#50402
Hi there,

PCBs are normally fastened to a package. For this reason there are usually holes in the four corners of a PCB for a screw.

1. What is the standard size of this screw?

2. What is the smallest possible size of screws used?

3. If you do not want to make a hole on your PCB, is there any alternative? Something like a glue?

Regards,

smdFan


.
User avatar
By leon_heller
#50404
I use 3 mm clearance holes for 3 mm screws and pillars.

Leon
By JT42
#50409
But normally I use four (or more if it's a big board) holes and use screws and threaded aluminum spacers.
Last edited by JT42 on Mon Oct 27, 2008 8:19 pm, edited 2 times in total.
By Philba
#50417
you can use any size bolt/screw you want but be aware of your PCB vendor's size limits. Some PCB design SW has size limits too. I know Eagle has a max size hole though I don't recall what it is.
By smdFan
#50422
Philba wrote:you can use any size bolt/screw you want but be aware of your PCB vendor's size limits. Some PCB design SW has size limits too. I know Eagle has a max size hole though I don't recall what it is.

What about glue? for very small size PCBs (40mmx40mm)?
By Philba
#50438
sure, but make sure the glue isn't conductive. I personally prefer to use screws but if it's not going to be subject to a lot of stress, almost anything will do.
By khearn
#50725
It also depends on your enclosure. Many enclosures have pillars with holes for screws to mount boards. You can usually find drawings for enclosures that will tell you what size screws they are made for. Most small enclosures with mounting points are made for 4-40 screws.

I don't remember off hand what size holes you need for 4-40 screws. Every time I make a board I either do a quick Google search for "4-40 hole size" or else I check one of my other boards.

Some people use hot glue for mounting boards where screws aren't convenient. I'm not sure how conductive hot glue is, so either do some testing to make sure it's non-conductive, or be careful about where you put it. I'd guess it's not very conductive, because I've seen people use it in spots where it would cause problems if it was conductive. But I'm not sure, so don't blame me if it is conductive.

Keith
By Andrew02E
#50821
Throwing in my two cents for the questions...

1. It depends, really. I have not heard of a 'standard' screw size for mounting PCBs. For the stuff I do, I've used 2-56 screws for smaller devices, 4-40 screws for typical size/use devices, and 6-32 screws for the stuff that gets dropped/thrown/harassed.

2. Again, it depends. You can find very small screws, some so small that they're probably not practical for PCB applications.

3. There are parts that will hold your PCB by the edges. I believe they're called 'card edge' connectors, but I could be wrong. I have one on my desk that says 'Richco'. It's a T-shaped piece of plastic with a groove down the center for the PCB edge. Certain enclosures are designed with grooves already on the inside, but you have to base your PCB outline on the enclosure then.
I've also used hot glue for smaller PCBs (for hobby use only). The trouble with hot glue is it gets brittle in colder temperatures and malleable in warmer temperatures (and doesn't always hold well). I've also used silicone rubber adhesive, which usually works pretty well. I don't know the dielectric properties of either, though, but it wasn't an issue for what I used it on.
By RC
#51725
Can the four screw holes be added to a board in Eagle Light outside its 100 x 80mm board area limit? I'm just getting started with Eagle and the docs say you can't place packages and signals outside this area. Can pcb mounting holes be outside this area?
User avatar
By bigglez
#51752
Greetings (No First Name Supplied),
RC wrote:Can the four screw holes be added to a board in Eagle Light outside its 100 x 80mm board area limit? Can pcb mounting holes be outside this area?
No. To create larger boards you must upgrade.

Comments Welcome!
By RC
#51767
Peter,

Thanks for the reply. Using Eagle Light, is there any way to have the board sent out with its size larger then 100mm in one direction and I drill the mounting holes manually? Thanks.
User avatar
By leon_heller
#51768
You could edit the Gerber files to get a larger board outline.

Leon
User avatar
By bigglez
#51777
Greetings (Still No First Name Supplied),
RC wrote:Thanks for the reply. Using Eagle Light, is there any way to have the board sent out with its size larger then 100mm in one direction and I drill the mounting holes manually?
I take it that you are a "newbie"? Editing Gerber files
is probably not for you.

If you plan to use BatchPCB you can request (by email)
that your board is made to a different (larger) size,
and add the holes yourself after fabrication by GP.

What is your first name?

Comments Welcome!
By RC
#51789
I updated my profile and added my first name to my signature.

Yes, as I mentioned earlier, I am just getting started with Eagle. I did one board using toner transfer but I want a professionally fabricated board for my current project. I know I could use ExpressPCB's software but learning Eagle appears to be a good skill to acquire. I'll research gerber file formats, which could be my solution but I'm also now considering splitting my project into two boards. Two boards does offer some mounting benefits. I could use the grooves in the enclosure I've selected and it might make future updates a bit easier with two boards. I just can't justify $750 for Eagle Standard right now. Thanks.
User avatar
By bigglez
#51794
Greetings RIck,
RC wrote:I just can't justify $750 for Eagle Standard right now. Thanks.
Understood. BTW, the EAGLE suite has
three modules, which you can mix-n-match. You may
only need the layout module to get larger PCB sizes.

It's worth a call to Ed Robelo in the USA (Florida) Office
on: 954-237-0932 or Edwin.Robledo@cadsoftusa.com
to comfirm. Should bring your budget down to ~$250.

Comments Welcome!
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