At what point does PCB hole interference become a real problem? .005"? .001"?

[Mark172]

I'm the sole mechanical engineer on an electronic package wherein a small polyimide PCB will be mounting to an aluminum 6061 housing via #4-40 screws. Due to my lack of experience, I'm not sure how much concern to place on a worst-case .0015" interference due to the tolerance stack of the holes - the housings have been made and the PCB drawing is headed to our vendor shortly, so making changes to controlled drawings at this point is to be avoided unless really necessary (I didn't make these drawings, just began work here). .0015" is small enough that it seems to me it wouldn't be a problem in the rare chance it did occur - you wouldn't even need a drill to open up the hole, just some caution to avoid cross threading, and the screw will deform the board that .0015".

Anyone who's had experience with misaligned PCB / housing holes - is .0015" that big of a deal?

 

[Tunalover]

The worst that will happen is that the circumference of the hole will be slightly threaded as the screw is turned in. How did you come up with the value for the interference? What's the positional tolerance, size tolerance, and size of the PCB holes? What's the positional tolerance of the threaded holes in the housing?


Tunalover

 

[MikeHalloran]

No, the worst that will happen is that the screw will shave a little copper off the hole bore, in just a few of the assemblies.

Tiny metal shavings have been known to cause trouble in electronics.



Mike Halloran
Pembroke Pines, FL, USA

 

[Tunalover]

Mike Halloran-
Quite true if the holes are plated. We don't know if they are.


Tunalover

 

[Mark172]

The holes are plated. So yes, thanks for pointing the shavings out - didn't think of that, and for space applications that's a HUGE no-no. Tunalover - the .0015" worst-case interference comes from a tolerance stackup that considers the hole PCB through-hole diameter, screw diameter, housing diameter, and all associated tolerances.

Thanks for your help = definitely has tipped the scales in my mind. I will bring this to the attention of my managers on Monday.

 

[mcgyvr]

Some standards require unplated through holes when they are used for mechanical fastening..

Also for a #4-40 screw I'd use a .125-.128" diameter hole.. or .116" dia if a "close fit" is required.
And make sure you leave plenty of clearance for the head of the screw so you don't have to use insulating washers.. I hate it when "designers" put traces so close to mounting holes. Solder mask should NEVER be relied upon for insulation.