Solder dots around mounting hole - what are they for?

[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]

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.

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]

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.

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).

[thm_w]

I would guess these are not vias, just solderpaste as mentioned. Though some people do use vias as well.

I don't know if I buy the "vias increase strength" claim, without seeing some evidence presented.
The solder contact point claim, I see it working for those self-locking screws wraper posted. But for a regular smooth screw I can't see it being superior. I've had no issues with properly torqued screws directly on ENIG, for normal use cases.

I'm sure NASA has some recommendations for high vibration environments, if someone can find it.


https://electronics.stackexchange.com/questions/137394/mounting-hole-on-a-pcb (this one has an interesting point about using non-plated through holes to avoid metal shards)
https://electronics.stackexchange.com/questions/240092/why-do-some-pcbs-put-vias-around-mechanical-through-holes
https://electronics.stackexchange.com/questions/524791/why-do-some-pcbs-have-these-circular-rings-are-they-plated-through-hole-vias

[nctnico]

If you really want to get into the details to ensure good, long term contact, you need to take material compatibility into account as well. Probably most manufacturers / system assemblers won't care though for grounding purposes.

[wraper]

I've had no issues with properly torqued screws directly on ENIG, for normal use cases.

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.

[thm_w]

I've had no issues with properly torqued screws directly on ENIG, for normal use cases.

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.

So then for OSP you'd have to put solder dots as shown in OP to get proper connection.
HASL already has the tin there, wouldn't expect anything to be needed.

[Smokey]

I've had no issues with properly torqued screws directly on ENIG, for normal use cases.

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.

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?

[wraper]

I've had no issues with properly torqued screws directly on ENIG, for normal use cases.

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.

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?

Those people use OSP. And did you miss "depending on use case"?

[EPAIII]

So, no one really knows for sure.

[Smokey]

I've had no issues with properly torqued screws directly on ENIG, for normal use cases.

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.

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?

Those people use OSP. And did you miss "depending on use case"?

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.

[eTobey]

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

How can you say that they are vias, when you actually dont know? Because they are not vias!

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?

It had neither. Just a screw. On the other side to the chassis, there is copper.

Those two big vias connect to the other side with copper that is around the mounting hole.

I hate, that so many people do it because "everybody does it". This mentality is purely dangerous for society (as history proves) and does prevent improvement. Also its kinda stupid, doing something you dont know why.

If you say, that it would strengthen the PCB, then compare the properties of copper/solder vs. those of the PCB. Holes also rarely strengthen something. Protecting the copper layer or securing the screw, sounds more appropriate to me. I can also think of, that it could equalize any height differences.

[Nominal Animal]

How can you say that they are vias, when you actually dont know?

Because every time I've seen such "dots" –– on several dozens of different boards from PC and server motherboards, SBCs, and routers –– they have always been vias and not "solder dots".  A solder dot would make very little sense, as a crush washer would be better, simpler and cheaper.  (The simplest crush washer is simply a thin annealed copper washer when electrical connectivity is desired, and a teflon or nylon washer when not.)

Holes also rarely strengthen something.

Wrong.  Holes are an extremely important feature that can make a structure stronger than if it were solid.

The key is mechanical properties.  PCBs are laminates: layers of thin conductive metal and bulk material; for FR4, glass fiber reinforced epoxy.
When the screw is tightened, the glass fiber reinforced epoxy (or comparable substrate material) will contract along the axis of the screw, and enlarge in the plane of the PCB.  Without the holes, the expansion can cause fractures –– because the conductive copper layers break before they stretch ––, even delaminate the structure, and break the board.  The via holes provide room for expansion of the epoxy material, allowing local deformation around the mounting hole, without crack formation; plus the plating material acts a bit like glue helping keep the layers from delaminating.

[DavidAlfa]

They provide mechanical relief (Act as a cushion when tightening), protecting the copper layer while also enhancing the electrical connection to the chassis, as these solder dots will squeeze and make a much better contact than plain copper.
Anyways, modern computers's nonsense aesthetics like to paint the screws and the case, so they actually make no contact at all.

[Nominal Animal]

Just as a data point:

If you have a standard 1.6mm thick PCB, and you use a M3 or #4-40 mounting screw, starting from just snug or making a contact but exerting no force, a single quarter turn reduces the PCB thickness by 7.8% and 10%, respectively (pitches being 0.5mm and 1"/40).  Decide for yourself if you think that matters or not.

[nctnico]

And how much torque is needed to turn a screw that much? Won't it snap off before that? There is a reason why there are torque specifications and not rotation specifications for fasteners (unless you are using stretching bolts but that is a different story).

There is an interesting discussion here:
https://www.reddit.com/r/engineering/comments/12csy9j/setting_torque_values_based_on_clamped_material/

[Nominal Animal]

And how much torque is needed to turn a screw that much? Won't it snap off before that?

For FR4, no.  It just deforms.  I don't have a torque wrench to measure, but it's what I can do by hand when extending the screwdriver in the same direction as my arm to M3 or #4-40 with a Philips head, i.e. using my wrist only.

It's easy to verify for yourself, no need to take my word for it.

[nctnico]

And how are you sure you aren't stretching the screw + thread? The link I provided shows some good starting points for going from compressive strength versus tensile strength of a screw.

Going by the 350N/mm2 number from that link, an M3 screw with a head diameter of 6mm and a hole size of 3.2mm should stay below a total compressive force of 7000N (20mm2 * 350N) before the FR4 gets damaged. A mild steel (4.6 class) M3 screw is rated for a tensile strength of 2100N. IOW: You'll snap a mild steel M3 screw long before damaging FR4.

[eTobey]

A solder dot would make very little sense, as a crush washer would be better, simpler and cheaper.

Thats just wrong, as the dot comes with no additional cost. You cant go cheaper than for free ;-)

When the screw is tightened, the glass fiber reinforced epoxy (or comparable substrate material) will contract along the axis of the screw, and enlarge in the plane of the PCB.  Without the holes, the expansion can cause fractures –– because the conductive copper layers break before they stretch ––, even delaminate the structure, and break the board.  The via holes provide room for expansion of the epoxy material, allowing local deformation around the mounting hole, without crack formation; plus the plating material acts a bit like glue helping keep the layers from delaminating.

I may make some cookies, to see if holes really add structural strength ;-) Or maybe i just handle them appropriately and eat them, to not make a mess.

I just found this picture in google:
My guess is, it should make contact with the screw, but the soldermask has to stay (dunno why).

[eTobey]

If you have a standard 1.6mm thick PCB, and you use a M3 or #4-40 mounting screw, starting from just snug or making a contact but exerting no force, a single quarter turn reduces the PCB thickness by 7.8% and 10%, respectively (pitches being 0.5mm and 1"/40).

Arent those vias just underneath the screwhead? Wouldnt they then become rather useless?

[Nominal Animal]

If you have a standard 1.6mm thick PCB, and you use a M3 or #4-40 mounting screw, starting from just snug or making a contact but exerting no force, a single quarter turn reduces the PCB thickness by 7.8% and 10%, respectively (pitches being 0.5mm and 1"/40).

Arent those vias just underneath the screwhead? Wouldnt they then become rather useless?

Yes.  No, because the epoxy material doesn't compress, it deforms.  Think of clay, play-doh, or dough, and not foam.  Instead of expanding in every direction in the plane of the PCB and potentially causing the damage I described (especially to inner conductive layers up to an inch away, say), most of the expansion happens into the via holes, with the plating material acting a bit like glue, helping avoid delamination there.

The reason it makes the PCB stronger is that it allows larger and more varied dynamic forces to elastically deform the PCB near the mounting hole without propagating the deformation further; keeping the deformation localized there.

Like I said, there is no need to take my word for it: this is easy to test with an unneeded board.  If this is something that really interests someone, they could design a multilayer board with comb-like capacitive stress measuring structures and various types of mounting holes placed symmetrically near such structures, and test it.  One could manufacture a set of five for just a few dollars at JLCPCB even with six layer boards right now.  My claim is simple: with vias within the screw head area, at any given screw torque the PCB stress/deformation is minimized, compared to similar mounting holes without vias, or with solder dots.

The image you showed of small "pads" around the mounting hole is new to me; I've never seen that in real life.  But plenty of the via kind.  I'm not an EE myself, but my background is in physics (mostly HPC and computational materials physics; not this kind of mechanical stress stuff, but atomic level stuff) and before that, in custom full-stack development.