No flexion to PCB when screwed in to enclosure

[Faringdon]

Hi,
We will screw our rectangular PCB down to flat metal rails which run along
two opposite sides of our metal enclosure.
The surface may not be absolutely perfectly flat, so given this, we wish to use
sponge washers so that we dont hard flex the PCB at all when its screwed in.
Do you know of any better than this?...

Soft washers
https://www.vital-parts.co.uk/rubber-washers---wash008-6030-p.asp?gad_source=1&gclid=EAIaIQobChMI172q5vqZhAMV1IBQBh2-UgAQEAQYAyABEgLbbvD_BwE

[jpanhalt]

How about fastening it by only 3 points?

(Not that your worry is legitimate.)

[ajb]

Do you know of any better than this?...

Not without a lot more information.  You would need to define how much flex is acceptable in your PCB (it will never be 'none' except in CAD!) and how flat you can expect the mounting location to be first.  It may turn out that you don't need anything special if the PCB can tolerate as much out-of-flatness as you expect to see.  Then you can start figuring out what -- if any -- special hardware you need.  It might be that you don't need anything, but it will depend on how far out-of-plane the mounting positions are, what the distance is between mounting locations, and how sensitive your PCB is.

Just throwing rubber washers between the PCB and mounting surface could be worse than hard mounting it.  If someone overtightens the screw, the washer could cause the PCB to 'dish' as the center of the washer deflects more than the edges, causing (potentially significant) localized bending in the PCB.  There are other mounting options, for example softer rubber grommets with shoulder bolts to ensure the grommets aren't compressed, that would be more reliable in that respect.  But remember that as you tighten a PCB against something like a soft rubber washer, as the washer deflects so will the PCB, according to how stiff the one is relative to the other, so that stiffness ratio needs to be considered in selecting how compliant to make the mount.

[jpanhalt]

Just throwing rubber washers between the PCB and mounting surface could be worse than hard mounting it.  If someone overtightens the screw, the washer could cause the PCB to 'dish' as the center of the washer deflects more than the edges, causing (potentially significant) localized bending in the PCB. 

You got me there.  I assumed it would go under the screw head with a metal washer.  I agree,  under the board could/would make it worse.   Of course, we need to know the Shore hardness.  Again, I assumed quite soft, just to keep the board from rattling.  After 100 comments, maybe those details will emerge.

[nali]

Squishy washers won't help, you won't be able to tighten the screw. As mentioned you need shoulder screws that you can tighten, along with some form of compression.

[jmelson]

Assuming your metal box with rails are not seriously out of parallelism, you should have no problem.  PC boards with SMT components are surprisingly resistant to damage from flexing.  If the ends of the rails are less than 1 mm out of plane, I would totally forget about any issues with it.  If more than a couple mm, then it could possibly be an issue.  But, vibration in shipping and use could be MUCH more of a problem.
Jon