General > General Technical Chat
Solder dots around mounting hole - what are they for?
wraper:
--- Quote from: Smokey on October 21, 2023, 12:40:23 am ---
--- Quote from: wraper on October 20, 2023, 11:37:12 pm ---
--- Quote from: thm_w on October 20, 2023, 08:49:23 pm ---I've had no issues with properly torqued screws directly on ENIG, for normal use cases.
--- End quote ---
ENIG is not that common for mass produced devices except maybe smartphones and tablets. Usually it's either OSP or HASL depending on use case.
--- End quote ---
Tell that to all the people using fine pitched parts that aren't tablets and smartphones. You need to do a tradeoff analysis. Is saving an extra $0.10 on the PCB worth having a higher % failure rate because of HASL assembly issues and tiny parts?
--- End quote ---
Those people use OSP. And did you miss "depending on use case"?
EPAIII:
So, no one really knows for sure.
Smokey:
--- Quote from: wraper on October 21, 2023, 01:48:01 am ---
--- Quote from: Smokey on October 21, 2023, 12:40:23 am ---
--- Quote from: wraper on October 20, 2023, 11:37:12 pm ---
--- Quote from: thm_w on October 20, 2023, 08:49:23 pm ---I've had no issues with properly torqued screws directly on ENIG, for normal use cases.
--- End quote ---
ENIG is not that common for mass produced devices except maybe smartphones and tablets. Usually it's either OSP or HASL depending on use case.
--- End quote ---
Tell that to all the people using fine pitched parts that aren't tablets and smartphones. You need to do a tradeoff analysis. Is saving an extra $0.10 on the PCB worth having a higher % failure rate because of HASL assembly issues and tiny parts?
--- End quote ---
Those people use OSP. And did you miss "depending on use case"?
--- End quote ---
I actually did miss that. :)
Smokey:
To get back on topic, It's pretty obvious the OP found PCB crop circles:
Nominal Animal:
:-//
I see vias placed around mounting holes in most larger boards I have. Structurally, they do seem to help avoid cracking due to the static forces exerted by the fastening screw: instead of behaving like a laminate, the region around the hole acts more like an uniform solid.
On PC motherboards, self-locking screws are typically used, but on all I've seen –– many –– the "dots" have been vias. As the mounting hole distance is quite large (100mm or more), and forces relatively large (inserting connectors, expansion cards, memory modules take quite a bit of force, relatively speaking), the mounting holes get stressed laterally, in the plane of the board. Without the vias, the board could crack from the mounting hole. While the screws are not supposed to be very tight, just snug, overtightening a self-locking screw without the hole strengthened with vias could delaminate the board due to the excess pressure while twisting, too.
There is always a good chance I've understood the situation wrong, or that the vias around mounting holes are there just because "everyone does it this way", but I don't think either is the case here.
Edited to add: While many PCB design guides talk about guard vias around plated holes used for grounding (the board to chassis or similar, when screwed in), on many boards I have the mounting holes' guard rings are isolated and not grounded.
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