Hi everyone,

I’m trying to understand how solder masks work on a PCB. If I have two parallel copper traces on a PCB that are covered by a solder mask, and I accidentally create a solder bridge between them, will the solder mask prevent a short circuit? In other words, does the solder mask act as an insulator that stops current from flowing between the traces, or would the bridge still cause a short?

Thank you

 

The solder will not stick to a track covered with solder mask. Any accidental drip should slide right off. Are you trying to solder something to one of the tracks?

 

How is it that you are creating this solder bridge between traces that are covered by the solder mask? The only way would be to somehow get through the solder mask in order to create the bridge. Whether that means that the solder mask was mechanically removed (scraped away) or thermally removed (burned away with the soldering iron) doesn't matter. Once you penetrate the solder mask and create a solder connection to a trace, you have an electrical connection to that trace.

The solder mask makes it harder to create solder bridges, it does not prevent someone from doing it if they try hard enough.

 

Rule 101 of PWBs. Never rely on solder masks as insulation.

 

How is it that you are creating this solder bridge between traces that are covered by the solder mask? The only way would be to somehow get through the solder mask in order to create the bridge. Whether that means that the solder mask was mechanically removed (scraped away) or thermally removed (burned away with the soldering iron) doesn't matter. Once you penetrate the solder mask and create a solder connection to a trace, you have an electrical connection to that trace.

The solder mask makes it harder to create solder bridges, it does not prevent someone from doing it if they try hard enough.

WBahn, You're right. Reflecting on it now, I realize how off-track my question was. For the solder to form a bridge, it obviously has to make contact with the copper itself, not just the solder mask . If the solder stuck, it must have penetrated the solder mask and made direct contact with the copper traces. My initial thought was misguided , fatigue must have clouded my thinking when I asked the question hahah.
Thank you for clarifying this.

 

Rule 101 of PWBs. Never rely on solder masks as insulation.

Just to make sure I’m understanding it right: if I test continuity on a single copper trace that’s covered with a solder mask and it still beeps, would this be an example of what you’re talking about, where the solder mask isn't providing complete insulation?

 

I don’t understand this last question.

Ate you probing it on the solder mask covered portion of the track or on the uncovered ends?

If the former, your probes have penetrated the mask. If the latter, of course it will beep. if it doesn’t, the track is broken.

But, once again, the purpose of solder mask is not to insulate, but to prevent solder from sticking there. There may be exceptions, but you could remove all the solder mask from a working board and it would have no affect on its operation.

Are you actually diagnosing a problem, or are these just theoretical questions?

 

I don’t understand this last question.

Ate you probing it on the solder mask covered portion of the track or on the uncovered ends?

If the former, your probes have penetrated the mask. If the latter, of course it will beep. if it doesn’t, the track is broken.

But, once again, the purpose of solder mask is not to insulate, but to prevent solder from sticking there. There may be exceptions, but you could remove all the solder mask from a working board and it would have no affect on its operation.

Are you actually diagnosing a problem, or are these just theoretical questions?

For the first part, my thinking was that testing continuity on a non-conductive or insulated area, like a solder mask, shouldn’t cause the multimeter to beep. That seemed logical to me. What I was trying to understand was whether the probes were penetrating the mask. After testing it myself, I realized that the solder mask isn’t 100% insulating, which explains why the multimeter beeped. This explained to me that the probes might easily penetrate the mask and that completely aligns with what schmitt trigger mentioned: "Rule 101 of PWBs: Never rely on solder masks as insulation." That’s why I wanted to confirm with him by asking, "Just to make sure I’m understanding this right: if I test continuity on a single copper trace that’s covered with a solder mask and it still beeps, would this be an example of what you’re talking about, where the solder mask isn't providing complete insulation?"
In this case, placing the probes on top of the solder mask does seem to penetrate it, allowing continuity to be checked, which indicates that the solder mask isn’t fully insulating. This is what I wanted to confirm with him.

As for the second part of your question, my issues are both theoretical and practical. They started from observing things in tutorials I watched, and then I experimented with a small PCB I have, which is covered in solder mask. I didn't initially know what the solder mask was or how it behaved, so I started testing continuity to understand it better and asked some questions here in the forum to get more information.

 

The primary purpose of the solder mask is to prevent solder from flowing where it is not wanted. An example is the pins of an IC where the traces/pads are in very close proximity to each other, so the mask helps prevent bridging the pins. But it also provides some protection against oxidation. The mask material itself does not conduct. If a continuity test of mask covered trace causes a "beep" from the tester, the test probes have likely penetrated the mask and are making contact with the metal (solder plated) trace below the mask.

 

Just to make sure I’m understanding it right: if I test continuity on a single copper trace that’s covered with a solder mask and it still beeps, would this be an example of what you’re talking about, where the solder mask isn't providing complete insulation?

Yes, exactly.
You might be ok for low level logic signals, but for higher voltages you would be playing a Russian Roulette.

 

I tried testing continuity in some traces covered with solder mask. With blunt probes, I could not get continuity pressing rather hard and scratching them across the trace. I the tried some pointy ones and was able to quickly get continuity. It left the solder mask visibly scratched, so I am reasonably sure that is what you did, scratch through the solder mask, which is not very durable.

 

The solder mask purpose is described correctly. But the solder mask goes on AFTER the board is etched.
BUT it is quite possible for an incomplete etching process to have an unintended connection between traces, and for that to be covered by a solder mask.
The sole purpose of the solder mask is to keep solder from sticking where it is not wanted!! It happens that solder mask also provides other benefits, but that is totally secondary!

 

I skimmed, but if you have metal connecting 2 traces, it's going to be short. The beeping meter will verify this. If you have trouble controlling the solder, put a metric ton of flux on it, flood the area. That will make the solder flow like water. If there's too much solder, use some solder wick (copper braid) to remove the extra, or remove all of it and start again, using plenty of flux. Flux is your friend, just clean it off when you're done. There are several good fluxes, my personal favorite is the MG Chemicals that comes in the syringe.