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| Solder dots around mounting hole - what are they for? |
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| nctnico:
It is an interesting subject. I would like to see some numbers about how much FR4 'flows' though. I have not been able to find those -yet-. I doubt it is a large amount though otherwise screws would get lose over time by themselves. And the 'flow' has to be elastic otherwise you'd get a permanent dimple in the board (I don't recall seeing that ever). Keep in mind that the screw itself acts as a spring because it extends when tightened. I have seen quite a few high current applications where wire lugs or busbars are bolted to an FR4 board directly. So it looks like this is accepted as a reliable method to get a lot of current in & out of a board. |
| nctnico:
--- Quote from: Nominal Animal on October 22, 2023, 08:20:53 pm ---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. --- End quote --- Sorry, but like many other reports, this is about the 'glass transition phase' at high temperatures to research the effect of the soldering and the effect of high temperatures on board stiffness. It doesn't provide any information about effects of putting local compression stress on FR4. Also the fact that the report was written as part of a summer internship makes it just another collection of datapoints. |
| Nominal Animal:
--- Quote from: nctnico on October 23, 2023, 10:56:39 am --- --- Quote from: Nominal Animal on October 22, 2023, 08:20:53 pm ---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. --- End quote --- Sorry, but like many other reports, this is about the 'glass transition phase' at high temperatures to research the effect of the soldering and the effect of high temperatures on board stiffness. It doesn't provide any information about effects of putting local compression stress on FR4. Also the fact that the report was written as part of a summer internship makes it just another collection of datapoints. --- End quote --- Like I wrote earlier, I cannot find any peer reviewed articles about mounting stresses on FR4 or similar materials for different styles of mounting holes. For someone who does not even recognize the difference between elastic and viscoelastic materials, that PDF is a good starter on how these things are measured, giving you a better understanding of the topic at hand, instead of picking individual sentences or images at random that you think bolsters your argument. If you are hoping for a good, possibly peer-reviewed article on FR4 mounting stresses, there do not seem to be any; only "anecdotal" evidence from PCB designers having done stuff for decades, and "understanding" based on materials physics (viscoelasticity) and mechanical engineering (controlling and localizing strain under various stresses using physical structures, i.e. shape). The next best thing, in my opinion, would be a Finite Element Analysis with a few different forces, both on the mounting hole itself (corresponding to forces in the PCB plane) and thus with multiple different hole models, and on the PCB (perpendicular) with different holes symmetrically placed around the stress point in typical rectangular configuration. This yields images where the resulting strain is easy to see; compare to this one[/ULR]. (That [URL=https://www.researchgate.net/publication/337968594_Analysis_of_Printed_Circuit_Boards_strains_using_finite_element_analysis_and_digital_image_correlation]publication is open access, comparing digital image analysis of real boards to finite element simulations. So even that one is just an example of the type of such investigation, and does not answer the question at hand.) A set of boards with strain rosettes and different mounting hole styles for measuring the actual deformation would be easy to manufacture, but even with minimum trace widths, you'd need some high-resolution test equipment to measure. You'd also want to have a good microscope to examine the results of a crush test, say with thin angular (not perpendicular nor tangent to the hole) traces near the screw head just to make observing the effects easier. Using minimally thin test traces in inner layers close enough (too close!) to the mounting holes should show breakage at high enough deformations, but I suspect one one would have to test batches of boards, dozens of mounting holes of each type, to get any kind of meaningful results. Even if I did such a test and posted the results here, most of you would not believe me anyway; and it would be utterly wasted effort for everyone. Thus, the only real possibility is for someone else to investigate and report. I could be wrong here, even if I don't think so. My own opinion is based on relatively simple physics I've explained already, but seems to be countered with "I feel you're wrong" -type truthiness without any basis, which annoys me to no end. I'm done arguing from simple physics principles against vague gut feelings. |
| Siwastaja:
--- Quote from: nctnico on October 23, 2023, 10:11:02 am ---I have seen quite a few high current applications where wire lugs or busbars are bolted to an FR4 board directly. So it looks like this is accepted as a reliable method to get a lot of current in & out of a board. --- End quote --- This is also something you equally often hear described as unreliable; even something you should "never" do. What are the exact conditions where it would fail, beats me. I'm sure it's OK when not overtightened but what's the maximum acceptable torque on typical materials? I guess no one really knows. For the record, I have done that, too, and never had problems, but I still feel a bit uncomfortable knowing that maybe someone who repairs the thing seriously overtightens the bolts to the point of causing irreversible damage to FR4, beyond the elastic range, and with say 8mm bolt with 13mm hex nut this could really happen; compare it to a, say, M2.5 screw with PH0 head used to mount a PCB to a plastic case, it would be impossible to overtighten that. |
| thm_w:
--- Quote from: Siwastaja on October 23, 2023, 01:42:16 pm --- --- Quote from: nctnico on October 23, 2023, 10:11:02 am ---I have seen quite a few high current applications where wire lugs or busbars are bolted to an FR4 board directly. So it looks like this is accepted as a reliable method to get a lot of current in & out of a board. --- End quote --- This is also something you equally often hear described as unreliable; even something you should "never" do. What are the exact conditions where it would fail, beats me. I'm sure it's OK when not overtightened but what's the maximum acceptable torque on typical materials? I guess no one really knows. For the record, I have done that, too, and never had problems, but I still feel a bit uncomfortable knowing that maybe someone who repairs the thing seriously overtightens the bolts to the point of causing irreversible damage to FR4, beyond the elastic range, and with say 8mm bolt with 13mm hex nut this could really happen; compare it to a, say, M2.5 screw with PH0 head used to mount a PCB to a plastic case, it would be impossible to overtighten that. --- End quote --- Probably should never do in a very high reliability product (military, avionics). Otherwise, most applications I would expect to be OK, I agree. Creep could come into play at high temps and pressures, I don't know what a normal SS 304 M3 or M4 screw would apply pressure wise.. you'd have to do some calculations. The max allowable torque for A2-70: M3 1Nm, and M4 2.6Nm. They will shear off around twice that. 3Nm already feels like too much for the M4 on FR4. PCB PEM inserts are commonly used on high end gear, which rely on the strength of the PCB to stay in place. They mention that the hole for swaged (press fit) does not need to be plated, so plating probably does not affect strength. They don't mention vias near the hole, just board edge distance. I'm sure if you asked them about putting a ton of vias around the hole they would recommend against it. https://www.hindawi.com/journals/ijps/2022/9733138/ https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/pc.10130 https://www.researchgate.net/publication/268209268_Creep_Behavior_in_Fiber-Reinforced_Epoxy_DGEBA_Composites |
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