General > General Technical Chat
Solder dots around mounting hole - what are they for?
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eTobey:
Hi,

can anyone explain to me, what the purpose of these dots is? It is a mounting hole, and a screw will sit on them.

Cheers,

Toby

Nominal Animal:
They're not dots, they're actually vias that happen to be filled with solder.

Their purpose is to strengthen the PCB, so that tightening the screw won't deform and/or crack the board.
AndyBeez:
They also provide contact points. As this is ground, that looks like a chassis connection. Did the PCB have a metal washer or metal post here?
wraper:

--- Quote from: Nominal Animal on October 20, 2023, 11:13:46 am ---They're not dots, they're actually vias that happen to be filled with solder.

Their purpose is to strengthen the PCB, so that tightening the screw won't deform and/or crack the board.

--- End quote ---
There are not necessarily vias and actually placing vias under those is problematic as solder will escape to opposite side during reflow. They get crushed/worn instead of copper while tightening the screws and prevent screw from loosening as well. Generally it's used with self-locking screws which can easily destroy thin copper foil in a few screw-unscrew cycles.
nctnico:

--- Quote from: Nominal Animal on October 20, 2023, 11:13:46 am ---They're not dots, they're actually vias that happen to be filled with solder.

Their purpose is to strengthen the PCB, so that tightening the screw won't deform and/or crack the board.

--- End quote ---
No. Tin does nothing where it comes to adding mechanical strength. Wraper is 'on the money'. Putting vias under screws is a bad idea because they will deform and are likely to crack.  If you want to stitch ground planes together at a screw hole, the vias must be placed around the screw head but not so many that the strength of the board is weakened (like mouse-bites to seperate boards).
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