Greetings - This has a bit more to do with board assembly practice but maybe it fits here. The question is whether or not to use washers under screw heads when screws are used to mount boards or secure hardware to boards. I wonder about screw material (steel vs stainless steel vs plated steel vs brass vs …?) A washer could serve as an intermediary between the plated copper on the board and the screw head. I wonder if corrosion could be an issue, especially over longer time frames. And then there is the rubbing friction between the board and the screw as the screw is tightened. There are a few places where I NEED to use washers to account for minimum screw length being too long for holes in enclosure mounting posts. I guess that I am asking about the conventional wisdom on using washers under screw heads on ECBs? Thanks for your thoughts.
At the least, a mounting hole intended for use with a washer requires more clearance around it compared to a hole intended for use with a fastener alone.
If your organization’s Standard Design Practice calls for washers under mounting hardware then your footprint library will show mounting holes with expanded clearances around them. Conversely, if the Standard Design Practice omits washers then the library will show holes with reduced clearances.
Although the question is not KiCad-specific, it DOES relate to general PCB Best Practices so I vote to keep it on the Forum.
If your PCB is in an environment that yours screws are going to rust (which also needs moisture), then your PCB is going to have a tough life. I have also encountered a few occasions of zinc plated metal parts getting covered with white “hairy” dust like material. This powder is similar to the “tin whisker” problem, but made of the zinc plating. Whisker growth on zinc plated parts have also been a problem from floor rosters and metal cabinets used in server rooms.
And if you’re worried about dissimilar metals, you can also use plastic washers.
If washers are to be used, then the footprint for the screw needs to reflect that size. There are no formal standards. Us is me! I was uncertain what standards others use because that effects how I lay out the board.
I know (many of) the specs that drive the choice of 0805 vs 0603 resistors. But have no clue about lots of mechanical things.
