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| Solder dots around mounting hole - what are they for? |
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| Siwastaja:
--- Quote from: Nominal Animal on October 22, 2023, 04:10:03 pm ---So no, it is not my idea. --- End quote --- Certainly not. To me, this was always common sense; the idea of material expanding under pressure, and holes adding places where to expand to, minimizing the risk of nearby copper tracks stretching to breakage, has been totally intuitive to me, to the point I never thought about it much and just copied the usual pattern of just adding those vias; I can confirm having seen it in gazillon of PCBs. It's worth noting though that filling the vias with solder would compromise this property and probably cause more problems instead, so that would explain why wraper sees the "solder dots" and vias at different places. I think DavidAlfa said it well, too. I really don't understand why you are getting so much backlash. To me all you write is obvious. |
| nctnico:
That is more like copying something everybody else is doing rather than looking at material engineering. This page shows a stress / strain curve for some FR4 material: https://www.researchgate.net/figure/Stress-strain-curve-for-FR4-PCB-material_fig4_4058600 This page explains what such a curve means: https://www.xometry.com/resources/3d-printing/stress-strain-curve/ So the strain is the amount deformation according to the stress level. The curve shows a maximum deformation of 1.8% at the point of failure which does not look like the properties of a material that 'flows'. The curve for FR4 resembles the curve shown for a brittle material. Which is logical if you look at what FR4 is made of: glass fibers glued together with epoxy resin. When you put pressure on FR4, you are pulling on the inner glass fibers which will snap at some point. As wraper already noted: If you are putting vias under a screw head, these vias are breaking the weave pattern and thus weakening the board at that point. The only reason I see for putting vias under a screw head is to ensure contact between the screw and all ground layers in a board without needing too much space. For a mild steel M3 screw this is probably just fine (assuming there are only a few vias) as you can't really damage a board with a mild steel M3 screw (see my previous post for the calculation). For bigger screws it might be wise to place the vias outside the area of the screw head in order to prevent cracking the vias and increase strength of the board under the screw head. |
| Nominal Animal:
--- Quote from: nctnico on October 22, 2023, 07:54:35 pm ---This page shows a stress / strain curve for some FR4 material: https://www.researchgate.net/figure/Stress-strain-curve-for-FR4-PCB-material_fig4_4058600 --- End quote --- Read that paper! That curve is derived from impact tests, not from bending or stretching. For a viscous or viscoelastic material, the stress is directly proportional to the rate of strain, and not to strain itself like for elastic materials, for small deformations. Thus, bending/stretching differs wildly from dropping and impact behaviour. You'll get better understanding of the topic from e.g. Characterization of the material properties of two FR4 printed circuit board laminates by Haugan and Dalsjø; the full PDF article is open access. |
| SiliconWizard:
--- Quote from: nctnico on October 22, 2023, 07:54:35 pm ---The only reason I see for putting vias under a screw head is to ensure contact between the screw and all ground layers in a board without needing too much space. --- End quote --- Yeah I think so too. Not convinced about the benefits against stress. Now using several "dots" instead of a solid pad probably helps preventing the screw from getting loose over time to some degree. |
| Siwastaja:
Especially when it comes to PCB design, we all "copy" things we don't exactly and perfectly understand. If we didn't, no one could do anything (the classical trap of perfectionism). There's still a long way to cargo cult engineering, which means copying basically everything with very little understanding about anything to the point of creating an unworkable design. So far, I'm unconvinced about the scientific evidence of anything regarding those holes. Half of the people here are saying they make the PCB stronger (specifically against copper track breakage) and others say they make it weaker (by cutting the glass fibers of the FR4 itself). If the latter is the case, I wonder why having them is so common in professionally designed products*. On the other hand, not having the vias seems quite common, too; then the question is, are they "left out" because the designer thinks they are just unnecessary, or because they think they are detrimental? A big difference. *) and ground plane stitching is not the explanation; for that, the holes could be further away, not under the screw head Probably this is one of the cases where it usually does not matter either way. |
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