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I have some PCBs that were wave soldered but the vias still remain open, they are not filled nor plugged form what I can see. I do not see any solder on the annular rings of the vias either.

How do they go about doing so?

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    \$\begingroup\$ Are the vias covered in solder resist? A good photo of the board may be useful \$\endgroup\$
    – user16324
    Jul 7, 2023 at 16:36

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Without at least a photo of your PCB, we have way too little information to even begin to answer your question, but here's a guess anyway.

They did?

Maybe: Masking?

They could have covered the vias and then removed the masking after wave soldering.

Maybe: It's not wave-soldered?

Wave soldering is way less common than reflow soldering these days. It's quite possible that they simply stenciled and applied the solder paste selectively (e.g. not on your via's or their annular rings).

They didn't?

Maybe: It's actually soldered

I highly doubt that they "prevent[ed]" solder from entering your vias. If your vias are large enough that you can easily see through them, the solder likely entered the via and then plated the barrel of the hole after surface tension proved insufficient to tent over the opening.

Maybe: It's oxidized

Copper will oxidize if left exposed to the air. If the via's are copper plated without further protection, the solder won't "stick" to the oxide top layer of the via.

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They either masked the vias and then pulled off the mask, but it's more likely they used selective wave soldering where the board is only soldered at certain points with a wave. They also could have hand soldered it (if you see a bit of flux residue around the through holes this is likely the case, that may not be there if they washed it well)

Selective Soldering

Selective soldering, also known as mini-wave soldering, offers cost-effective, repeatable results for THT and mixed technology soldering applications. Soldering points are individually programmed and monitored to control flux volumes and soldering time. And it is the only reproducible method for soldering THT components on a two-sided PCB assembly.

Selective soldering typically consists of three stages:

Fluxing or the application of liquid flux.
Preheating of the PCB assembly.
Soldering with a site-specific solder nozzle.

Source: https://www.nordson.com/en/about-us/nordson-blog/electronics-solutions-blogs/selective-vs-wave-soldering

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You can get peelable solder mask if you need to prevent solder from getting over certain areas. Call it out with another layer on your gerbers and the factory will silk screen it on.

Or get someone to tape over the holes on each board with polyimide tape before wave soldering.

Or sometimes what appear to be wave-soldered is actually paste-in-hole technology.

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