Solder dots around mounting hole - what are they for?

[VK3DRB]

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.

It's hard to generalize about "FR4" other than to say that it is made of some sort of fire-retardant material with an E_r 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.

That's because they are mounted poorly. Heat/cold cycles can loosen screws that don't have appropriate spring washers on them over time. Loctite can also prevent movement.

[wraper]

That's because they are mounted poorly. Heat/cold cycles can loosen screws that don't have appropriate spring washers on them over time. Loctite can also prevent movement.

Spring washers are among the most useless things that can be used for prevent screw from loosening though. https://ntrs.nasa.gov/citations/19900009424

The typical lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking.

[thm_w]

Having solder on these vias is pure stupidity.

Its not vias, its just a pad, and it could be necessary in this case as the board appears to be OSP, as we discussed above.

That's because they are mounted poorly. Heat/cold cycles can loosen screws that don't have appropriate spring washers on them over time. Loctite can also prevent movement.

Spring washers are among the most useless things that can be used for prevent screw from loosening though. https://ntrs.nasa.gov/citations/19900009424

The typical lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking.

They work better than a plain washer, that NASA quote is a bit deceiving. They still don't work that well though (skip to 1:40):

[SiliconWizard]

That's because they are mounted poorly. Heat/cold cycles can loosen screws that don't have appropriate spring washers on them over time. Loctite can also prevent movement.

Spring washers are among the most useless things that can be used for prevent screw from loosening though. https://ntrs.nasa.gov/citations/19900009424

Nice reading. Well recommended.

[VK3DRB]

That's because they are mounted poorly. Heat/cold cycles can loosen screws that don't have appropriate spring washers on them over time. Loctite can also prevent movement.

Spring washers are among the most useless things that can be used for prevent screw from loosening though. https://ntrs.nasa.gov/citations/19900009424

Nice reading. Well recommended.

Good doco. They mentioned nyloc nuts which have the greatest efficacy. It is a safety requirement in Australia that earthed chassis terminals have a nyloc nut securing the earth wire to the chassis and NO star washer between the lug and the chassis. I have also used nylocs on circuit boards where something must frame earthed.

Another point is that over time, metallic bonds are formed on threads and other mating metals over time, making them more diffucult to remove or shake loose.

A bloke came around here to buy my 30-year old 7' x 4' trailer earlier this year. The trailer had an oversized ball, which prevented theft. So he had to remove the 2 inch ball from his 4WD and put my 2.5 inch ball on. Trouble is the old ball had been left untouched in his 4WD for 25 years. To remove it, no amount of WD40 could shift it. I did not want to use a blow torch near a fuel tank or the greasy underside of his vehicle. A large pipe wrench with an extension handle could not budge it. What worked was putting the pipe wrench on the nut and backing the 4WD into a large power pole. The heavy duty pipe wrench handle was actually flexing. Eventually it released with an almighty BANG. Looking at the threads, I don't think rust played a big part. It was more likely the metallic bonds in the thread which had formed over 25 years. Anyone who has left metallic gauge blocks up against each other for a few weeks will know how strong these bonds can become.

[wraper]

They work better than a plain washer, that NASA quote is a bit deceiving. They still don't work that well though (skip to 1:40)

Initially it lost tension almost as fast as with plain washer, it's only total loss of tension that took a bit more time. So it does not prevent unscrewing any better than plain washer, it just delays total failure.

[Someone]

It is a safety requirement in Australia that earthed chassis terminals have a nyloc nut securing the earth wire to the chassis and NO star washer between the lug and the chassis.

Citation of that requirement? Doesnt seem to appear in the usual suspects.

[thm_w]

They work better than a plain washer, that NASA quote is a bit deceiving. They still don't work that well though (skip to 1:40)

Initially it lost tension almost as fast as with plain washer, it's only total loss of tension that took a bit more time. So it does not prevent unscrewing any better than plain washer, it just delays total failure.

Delaying failure means its better than a plain washer... Everything in the test other than the nordlock delayed failure, including the nyloc.

Of course this test was optimized to some level of vibration, and in other scenarios you won't have as much as this test, and the split washer may survive while a plain washer would fail.

[nctnico]

IMHO the Nord-lock video is doctored. Just look at a car and how many bolts are put into the engine block alone without a washer and they won't run lose when torqued (and some bolts do not require a lot of torque; like 40Nm to 60Nm ) to spec. It is just a matter of good engineering. What kind of worried me on my previous car where the 6x M6 bolts holding the crank shaft pulley in place while needing like 40Nm of torque on each bolt. They never ran lose though.

[thm_w]

Its an official test: DIN 65151, the bolt is hit directly with shear load vibration. They show initial preload is 75Kn, I guess that is M12 bolt? then it would be 180Nm of tightening torque.

If you have a setup with enough clamping force to prevent sliding movement, the bolt shouldn't easily loosen: https://www.boltscience.com/pages/vibloose.htm
In the case of your crank shaft pulley, I assume its balanced well, and the forces are opposing each other, that the shear load on those bolts is minimal. But yeah, not all automotive fasteners will have loctite or washers at all, surprising to me too.

Here is another example:

[eTobey]

I found a PCB, where there where these solderblobs around the mounting hole. And it was covered in a silicone film. That looks like they are used to pierce through the silicone  (hopefully) to make contact. Thats not really a great way to do it IMHO.