Mounting hole rules

[steaky1212]

Hi,

I'm designing a PCB that is to be screwed to a plastic assembly.

What general rules to people use regarding placing components close to screw holes?

Obviously there are the PCB manufacturers design rules detailing minimum distances between tracks and holes, but thats more for registration etc than for the assembly aspect.
It's pretty obvious that you probably shouldnt put a small QFN package right next to a mounting hole, but how close can you get without risking fractures in the solder?
And how does this change with different components (0603 resistors, SOT23 packages etc).

Also, do people use non-plated holes for screw holes, or does the plating give the hole extra strength? Can you get closer to a supported hole if you have a larger land to remove board flex?

What are peoples thoughts?

Thanks,

Steaky

[David_AVD]

I use footprints for mounting holes that include a circle to show where the edge of the screw/bolt/nut head will be.

For the nut versions, the circle reflects where the nut driver tool will go on as well so that parts don't get squished during assembly.

As for how close is too close for components, that may depend on how much flex the board sees in use as well.  (unit bouncing around and/or heavy components)

[Maxlor]

Same here, I define keepout areas for screws. Generally I also avoid running traces under screwheads, since repeated screwing and unscrewing might create shorts if the solder mask is rubbed off.

I haven't ever really thought about mechanical stress around screw holes. Except when I've made a mistake the boards have been mounted flat with several support points. And I'm not yet to the point where I use chips that require heavy heatsink that might actually put a noticeable stress on the PCB. Might I ask why you're worried about it? What are your parameters?

[steaky1212]

Hi,

Thanks for the reply.

My worry is that the FR4 will compress when tightening (over-tightening) a mounting screw, and fracture solder joints.
I found this information on Mentors website (http://blogs.mentor.com/tom-hausherr/blog/tag/pcb-mounting-holes/).

It mentions putting small vias inside the land of a mounting hole to give extra strength, and component placement courtyards but that seems more for the fact that washers wont stay central.

The PCB is to have small solenoids screwed on using 2 small screws. The solenoids have a MOSFET driver circuit that I want to place physically close, but I didnt know how close I could place the components to the mounting hole. Maybe I am worried over nothing...

Thanks again.
Steaky

[Bertho]

The same trick with a circle indicating screw/nut sizes and not traces under the screw (usually use M3 -> hole 3.3mm and circle of 6.6mm).

Generally mounting holes are non-plated. Only if I need a physical connection I have pads (or unconnected in case of cheap china print whose manufacturer cannot do NPTH).

Additionally, if the PCB is in an environment with vibrations, then the mounting-holes are not placed with perfect symmetry. Using an asymmetric pattern makes sure that a single vibrational mode or frequency cannot excite the board into high stress.

[kizzap]

The requirements for mounting holes will change depending on the complexity of the board. As others have said, keepouts for screw heads are important. Also important would be copper pullback from the mounting holes for all the layers. Dave talked about this in one of his videos where he was trying to repair a 'scope which had a short on one of the voltage rails.

As a common sense thing, try keep heavier parts closer to the mounting holes, the board will be stiffer in those places and thus the heavier parts will have less vibrations.

-kizzap

[Conrad Hoffman]

Don't forget that chassis flex under some conditions can warp the entire board. For very sensitive analog circuits I've used kinematic mounting (3 holes) and compliant washer (grommet) mounting. All sorts of bad things can happen prior to cracking SMT components and traces. I've seen problems with multilayer capacitors just from being mounted too near an edge, much less the mounting holes, because they crack when the board is flexed, or are damaged in handling.

[free_electron]

as far as placement goes : i use the same rules as for component to mousebite distances.

now, layer stack wise : do not forget to remove internal planes underneath the compression area !
if you have a simple hole ( no copper top and bottom , just a non plated hole the internal planes should be retracted from the compression area , unless theplanes carry the same potential.

if you have copper top and bottom and it is connected to ground on an intenral plane : retract thepower plane. no traces carrying a different potential than the screw potential should be inside the compression area.

i have seen people put in a screw , put a self locking washer and nut , tighten the nut , the washer 'bites' into the board and causes a short between the outer layer and one of the inside layer with a destroyed board as a result ....
If electircal connection to the screw is required : do not trust the metallisation in the screw hole. it may get damaged , or ripped out if self tapping screws are used. : insert 6 vias in the perimeter underneath the screw head.