General > General Technical Chat
Solder dots around mounting hole - what are they for?
KE5FX:
--- Quote from: nctnico on October 23, 2023, 10:11:02 am ---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.
--- End quote ---
It's hard to generalize about "FR4" other than to say that it is made of some sort of fire-retardant material with an Er somewhere near 4. But I've seen a lot of TO-3 mounting screws that have loosened over time. Far too many to think that most PCB laminates -- or more likely, prepregs -- don't cold-flow to some extent.
nctnico:
--- Quote from: KE5FX on October 23, 2023, 11:53:35 pm ---
--- Quote from: nctnico on October 23, 2023, 10:11:02 am ---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.
--- End quote ---
It's hard to generalize about "FR4" other than to say that it is made of some sort of fire-retardant material with an Er somewhere near 4. But I've seen a lot of TO-3 mounting screws that have loosened over time. Far too many to think that most PCB laminates -- or more likely, prepregs -- don't cold-flow to some extent.
--- End quote ---
Interesting observation / good data point for sure. But it makes me question whether temperature and temperature cycling also have an influence on the screws getting lose. Or put differently: how can we be sure the flow of the laminate is the only factor at play in such a situation?
eTobey:
--- Quote from: KE5FX on October 23, 2023, 11:53:35 pm ---But I've seen a lot of TO-3 mounting screws that have loosened over time. Far too many to think that most PCB laminates -- or more likely, prepregs -- don't cold-flow to some extent.
--- End quote ---
If you have mostly seen TO-3 screws coming lose, then it might be, that those joints were a bit different. Also take into consideration, that every contact surface pair will set (? - i dont know the exact translation) and reduce the tension of the joint.
Anyone has a good connection to Dave? He could make some tests with his vibration device!
2N3055:
--- Quote from: nctnico on October 24, 2023, 12:21:16 am ---
--- Quote from: KE5FX on October 23, 2023, 11:53:35 pm ---
--- Quote from: nctnico on October 23, 2023, 10:11:02 am ---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.
--- End quote ---
It's hard to generalize about "FR4" other than to say that it is made of some sort of fire-retardant material with an Er somewhere near 4. But I've seen a lot of TO-3 mounting screws that have loosened over time. Far too many to think that most PCB laminates -- or more likely, prepregs -- don't cold-flow to some extent.
--- End quote ---
Interesting observation / good data point for sure. But it makes me question whether temperature and temperature cycling also have an influence on the screws getting lose. Or put differently: how can we be sure the flow of the laminate is the only factor at play in such a situation?
--- End quote ---
They do (temp and vibration) There was a Motorola AN on the topic.
I troubleshooted a batch of PSU boards with this problem long time ago..
VK3DRB:
Having solder on these vias is pure stupidity.
Solder will deform over time with pressure and the screw head may well have have a poor electrical contact or come loose. This is the exact reason you should never, NEVER , solder wires prior to inserting them in the screw holes where there is fire or other safety risks, such as mains wires on the back of a general power outlet.
The area on the PCB should be ENIG that is bigger than than the screw head diameter, and applicable creepage and clearances must be observed. For PE contacts, relevant regulatory standards must apply.
Don't ever assume PCB's laid out by big "professional" corporations do a good job.
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