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Solder dots around mounting hole - what are they for?
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eTobey:

--- Quote from: Nominal Animal on October 20, 2023, 11:13:46 am ---They're not dots, they're actually vias that happen to be filled with solder.

--- End quote ---
How can you say that they are vias, when you actually dont know? Because they are not vias!


--- Quote from: AndyBeez on October 20, 2023, 11:25:26 am ---They also provide contact points. As this is ground, that looks like a chassis connection. Did the PCB have a metal washer or metal post here?

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It had neither. Just a screw. On the other side to the chassis, there is copper.

Those two big vias connect to the other side with copper that is around the mounting hole.

I hate, that so many people do it because "everybody does it". This mentality is purely dangerous for society (as history proves) and does prevent improvement. Also its kinda stupid, doing something you dont know why.

If you say, that it would strengthen the PCB, then compare the properties of copper/solder vs. those of the PCB. Holes also rarely strengthen something. Protecting the copper layer or securing the screw, sounds more appropriate to me. I can also think of, that it could equalize any height differences.



Nominal Animal:

--- Quote from: eTobey on October 21, 2023, 07:54:07 am ---How can you say that they are vias, when you actually dont know?
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Because every time I've seen such "dots" –– on several dozens of different boards from PC and server motherboards, SBCs, and routers –– they have always been vias and not "solder dots".  A solder dot would make very little sense, as a crush washer would be better, simpler and cheaper.  (The simplest crush washer is simply a thin annealed copper washer when electrical connectivity is desired, and a teflon or nylon washer when not.)


--- Quote from: eTobey on October 21, 2023, 07:54:07 am ---Holes also rarely strengthen something.
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Wrong.  Holes are an extremely important feature that can make a structure stronger than if it were solid.

The key is mechanical properties.  PCBs are laminates: layers of thin conductive metal and bulk material; for FR4, glass fiber reinforced epoxy.
When the screw is tightened, the glass fiber reinforced epoxy (or comparable substrate material) will contract along the axis of the screw, and enlarge in the plane of the PCB.  Without the holes, the expansion can cause fractures –– because the conductive copper layers break before they stretch ––, even delaminate the structure, and break the board.  The via holes provide room for expansion of the epoxy material, allowing local deformation around the mounting hole, without crack formation; plus the plating material acts a bit like glue helping keep the layers from delaminating.
DavidAlfa:
They provide mechanical relief (Act as a cushion when tightening), protecting the copper layer while also enhancing the electrical connection to the chassis, as these solder dots will squeeze and make a much better contact than plain copper.
Anyways, modern computers's nonsense aesthetics like to paint the screws and the case, so they actually make no contact at all.
Nominal Animal:
Just as a data point:

If you have a standard 1.6mm thick PCB, and you use a M3 or #4-40 mounting screw, starting from just snug or making a contact but exerting no force, a single quarter turn reduces the PCB thickness by 7.8% and 10%, respectively (pitches being 0.5mm and 1"/40).  Decide for yourself if you think that matters or not.
nctnico:
And how much torque is needed to turn a screw that much? Won't it snap off before that? There is a reason why there are torque specifications and not rotation specifications for fasteners (unless you are using stretching bolts but that is a different story).

There is an interesting discussion here:
https://www.reddit.com/r/engineering/comments/12csy9j/setting_torque_values_based_on_clamped_material/
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