What is the best practice for tenting vias? Is it only one side or both. I used to know that tenting should be done on only one side to allow outgassing and avoid trapped air during reflow. But I heard the opposite recommendation, where it says that vias should be either tented on both sides or there should be no tenting at all left, as single sided tenting can introduce other reliability concerns.
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I think that the mask tenting recommendation has shifted over time as process control has improved. I remember something along the lines of tenting one one side as well, but now it is recommended that tenting should be both sides or hole open on both sides.
If you have filled and capped vias (like Type 7), then you can mask tent one side without concern.
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I used to do single side tenting a while back, but these days, most fabrication shops I work with prefer to tent both sides. I think that’s mainly due to process control and with one side tenting, the solder mask can sometimes bleed through the hole and partially exit on the opposite side. This can be a problem for fabs, especially when the opposite side needs to remain flat and clean for soldering or probing. These days, it’s safer to either tent both sides or leave the vias completely open, meaning that even the encroached pads can have open holes.
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That algins up with what I’ve read lately too. I also remember someone mentioning about barrel popping or something along those lines being a risk with single-sided tenting, not just the usual mask bleed-through issue.
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That’s a good point. I hadn’t considered how process control improvements might have changed the recommendation.
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Interesting. I’ve heard of barrel cracking from trapped moisture or expansion, but not specifically barrel popping.
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I think this was more of a problem in older processes, but modern fabrication techniques have largely addressed it. Today fabricators use advanced mask control, so single or double-sided tenting may rarely causes issues unless there are specific assembly or cleanliness requirements.
In practice the fabricator’s CAM process often resolves minor mask-related issues automatically. They may adjust the mask to eliminate slivers or ensure proper coverage, even if your design specifies single-sided tenting. These adjustments are typically made without notification provided your documentation permits minor modifications. As a result, the final boards may include subtle optimizations for reliability, regardless of the original design specifications.
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Heard from a colleague recently, they put an assembly through reflow and when it came out, QFNs were scattered around the board and not sitting on their pads. They traced it to the thermal vias that was being tented only at the bottom side but open on the top. The outgassing from the flux had no escape route and literally popped the components off their pads. Once they removed the bottom-side tenting from the thermal vias, the problem was solved.
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According to IPC-4761, single-sided tenting (Types I-a, II-a, III-a) is not recommended, so both-side tenting or open vias are preferred.
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I used to think it wouldn’t really matter whether vias were tented on one side or both. But after reading through this thread, I’m starting to think it might explain a soldering issue I ran into recently. On one of the build, several large SMD components shifted during reflow, all of them were on pads connected to thermal vias that were only tented on one side.
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