General PCB mounting question

Thread Starter

stoopkid

Joined Mar 3, 2011
146
So I got my PCB all drawn up and transferred to a copper board before I realized that there's no room for mounting screws or anything. The board will have some large components and will need to be very stationary to my wood case. I realized I don't really know about any PCB screw standard or anything. Google results are vague. Is there a standard size I should clear for screws or do I just need to grab what I can find and make it work for my project?

Basically I just want to know how it ought to be done. I'm looking for dimensions and such.

Thanks
 

debjit625

Joined Apr 17, 2010
790
Their are options in PCB CADs for creating drill holes like that,but if you forgot,its ok just use any screw that will go with your PCB to mount it properly just don't mess up with your tracks.

Good Luck
 

mcasale

Joined Jul 18, 2011
210
You did not say anything about your application. Is this a "one-off" project, or will this be a product? Note that much of the heat generated on a PCB (especially SMT) goes through the board to the mounting holes and to a metal chassis (unless you have a fan in the package). You also need good grounds and shielding to help minimize RFI issues.
 

mcgyvr

Joined Oct 15, 2009
5,394
Pick any screw.. Typically #4-40 or #6-32 (or comparable metric for those across the pond) are pretty standard for regular pcb assemblies.. But its all up to how much support you need and how much area you have. But typically you would provided enough clearance for the diameter of the head (make sure to think about slop from the actual hole in the board and the screw not being perfectly centered) then add the required clearance based on your voltage levels (higher voltage = more clearance to prevent arcing..
 

tracecom

Joined Apr 16, 2010
3,944
I use 4-40 machine screws, .125" holes in the PCB, and try to keep a .25" diameter area clear around the center of the hole. The 4-40 machine screw heads are .200" in diameter, so that leaves .025" clear all around the screw head. Of course, if you are placing a nut on the PCB surface, that takes more clearance and you should leave .300" diameter clear around the center of the hole.
 
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kubeek

Joined Sep 20, 2005
5,794
Or if you have no place for the screws, no high voltage present on the pcb and you are confident it will work, you can use something like sanitary silicone to stick it to the bottom. Don´t power it up until the silicone is completely hardened.
 

ErnieM

Joined Apr 24, 2011
8,377
For a one-off hobby project don't forget about hot melt glue. You can find the glue and the gun in many dollar stores. I keep a gun and some sticks in my tool box.
 

kubeek

Joined Sep 20, 2005
5,794
For a one-off hobby project don't forget about hot melt glue. You can find the glue and the gun in many dollar stores. I keep a gun and some sticks in my tool box.
Well yes, but it´s the melting when hot which makes it a little tricky to use, because when the device should for any reason overheat the pcb might fall off and cause even more damage when it comes in contact with something else.
 

KMoffett

Joined Dec 19, 2007
2,918
Or if you have no place for the screws, no high voltage present on the pcb and you are confident it will work, you can use something like sanitary silicone to stick it to the bottom. Don´t power it up until the silicone is completely hardened.
Be very selective in the silicon adhesive/caulk you use. Ones containing acetic acid will corrode metals on PCBs. You can recognize them by the strong, pungent odor. There are electronics-compatible silicon adhesives.

Ken
 

ErnieM

Joined Apr 24, 2011
8,377
Well yes, but it´s the melting when hot which makes it a little tricky to use, because when the device should for any reason overheat the pcb might fall off and cause even more damage when it comes in contact with something else.
If it gets hot enough to re-flow hot melt glue then it shouldn't be inside a wooden case to begin with.
 
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