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Solder dots around mounting hole - what are they for?

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Nominal Animal:

--- Quote from: nctnico on October 21, 2023, 10:07:17 am ---And how much torque is needed to turn a screw that much? Won't it snap off before that?
--- End quote ---
For FR4, no.  It just deforms.  I don't have a torque wrench to measure, but it's what I can do by hand when extending the screwdriver in the same direction as my arm to M3 or #4-40 with a Philips head, i.e. using my wrist only.

It's easy to verify for yourself, no need to take my word for it.

nctnico:
And how are you sure you aren't stretching the screw + thread? The link I provided shows some good starting points for going from compressive strength versus tensile strength of a screw.

Going by the 350N/mm2 number from that link, an M3 screw with a head diameter of 6mm and a hole size of 3.2mm should stay below a total compressive force of 7000N (20mm2 * 350N) before the FR4 gets damaged. A mild steel (4.6 class) M3 screw is rated for a tensile strength of 2100N. IOW: You'll snap a mild steel M3 screw long before damaging FR4.

eTobey:

--- Quote from: Nominal Animal on October 21, 2023, 09:05:08 am --- A solder dot would make very little sense, as a crush washer would be better, simpler and cheaper.

--- End quote ---
Thats just wrong, as the dot comes with no additional cost. You cant go cheaper than for free ;-)


--- Quote from: Nominal Animal on October 21, 2023, 09:05:08 am ---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.

--- End quote ---
I may make some cookies, to see if holes really add structural strength ;-) Or maybe i just handle them appropriately and eat them, to not make a mess.

I just found this picture in google:
My guess is, it should make contact with the screw, but the soldermask has to stay (dunno why).

eTobey:

--- Quote from: Nominal Animal on October 21, 2023, 09:52:30 am ---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).

--- End quote ---

Arent those vias just underneath the screwhead? Wouldnt they then become rather useless?

Nominal Animal:

--- Quote from: eTobey on October 21, 2023, 11:20:36 am ---
--- Quote from: Nominal Animal on October 21, 2023, 09:52:30 am ---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).

--- End quote ---

Arent those vias just underneath the screwhead? Wouldnt they then become rather useless?

--- End quote ---
Yes.  No, because the epoxy material doesn't compress, it deforms.  Think of clay, play-doh, or dough, and not foam.  Instead of expanding in every direction in the plane of the PCB and potentially causing the damage I described (especially to inner conductive layers up to an inch away, say), most of the expansion happens into the via holes, with the plating material acting a bit like glue, helping avoid delamination there.

The reason it makes the PCB stronger is that it allows larger and more varied dynamic forces to elastically deform the PCB near the mounting hole without propagating the deformation further; keeping the deformation localized there.

Like I said, there is no need to take my word for it: this is easy to test with an unneeded board.  If this is something that really interests someone, they could design a multilayer board with comb-like capacitive stress measuring structures and various types of mounting holes placed symmetrically near such structures, and test it.  One could manufacture a set of five for just a few dollars at JLCPCB even with six layer boards right now.  My claim is simple: with vias within the screw head area, at any given screw torque the PCB stress/deformation is minimized, compared to similar mounting holes without vias, or with solder dots.

The image you showed of small "pads" around the mounting hole is new to me; I've never seen that in real life.  But plenty of the via kind.  I'm not an EE myself, but my background is in physics (mostly HPC and computational materials physics; not this kind of mechanical stress stuff, but atomic level stuff) and before that, in custom full-stack development.

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