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Most recommendations I've come across regarding solder mask expansion state that it is required so that misalignment between the copper pad and the solder mask opening do not interfere with the solder connection to a mounted component.

However, nearly every default recommendation I've seen for PCB design rules includes solder mask expansions for routing vias. Tented vias are possible, but used for specific cases and not by default.

Can anyone explain why soldermask expansion on default routing vias shouldn't be zero? If the soldermask overlaps the copper by a small amount, why does that matter?

EDIT: To clarify, in my CAD package, an expansion of "zero" means that the copper pad and the soldermask opening have the SAME dimension. It doesn't mean that the soldermask opening is non-existent. To tent a via in my CAD package, I would need to either check the appropriate box or use a negative solder mask opening value.

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  • \$\begingroup\$ I don't see why they would considering you can tent it if you want. \$\endgroup\$
    – DKNguyen
    Commented May 19, 2020 at 15:09

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You might leave them open if you want to use them as test-points.

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  • \$\begingroup\$ Add ---- We'd tent vias if they sit immediately adjacent to a component pad, such that there's a risk the via will "wick" or suck away the solder during reflow. Further, I tent them if they sit under some silkscreen as our PCB houses will automatically remove silkscreen that touches any component pads and the vias seem to get caught in that semi-automatic process. \$\endgroup\$
    – Kyle B
    Commented May 19, 2020 at 16:32
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I generally tent them all unless, as mentioned by Kyle B, they're test points.

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  • \$\begingroup\$ My understanding is it's bad practice to tent BOTH sides, as that can trap gasses which will expand during reflow and possibly damage the via. \$\endgroup\$
    – Kyle B
    Commented May 19, 2020 at 16:34
  • \$\begingroup\$ I hadn't heard that, or experienced it myself. Maybe it's a good idea for us all do check with our vendors...it may vary by process. \$\endgroup\$
    – Cristobol Polychronopolis
    Commented May 19, 2020 at 16:42
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    \$\begingroup\$ I've never had trouble with tenting both sides of a 0.2 mm via in a 1.6-mm-thick board. After plating there's not much volume left in that via to trap gasses. But for a larger via diameter I'd avoid it. \$\endgroup\$
    – The Photon
    Commented May 19, 2020 at 17:01
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    \$\begingroup\$ I think it is the kind of defect that you would only see when thousands of PCBs are being made. Unless you guys who tent do make thousands of PCBs. \$\endgroup\$
    – DKNguyen
    Commented May 19, 2020 at 17:28
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    \$\begingroup\$ @KyleB No prototypes! Every design is a production design! \$\endgroup\$
    – DKNguyen
    Commented May 19, 2020 at 19:20

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