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My PCB fab said they can't fill the via-in-pad for flex PCB. The min hole size is 8mil with 20mil diameter. From google search, the concern is the solder will be drawn into an open via which could potentially starve the solder joint of solder. I am not a PCB manufacturing or reliability engineer. What's the impact of this? Any other concerns? Thanks.

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    \$\begingroup\$ As long as your via isn't massive, it's more of a concern when you're making hundreds or thousands of PCBs and statistics kick in and it's impractical to rework every board that has a problem. If you're soldering with an iron it isn't an issue since you just add more solder. Even if you reflow a few boards it's usually not a problem since you can just rework it if it is. \$\endgroup\$
    – DKNguyen
    Mar 15, 2022 at 17:23

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For flex PCB it should not matter, because the substrate is so thin that capillary effects don't really come into effect, and the only way solder could get lost is if it drains to the other side -- but surface tension is stronger than gravity.

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  • \$\begingroup\$ Thanks, Simon. It's a 8mil via hole. Is this still ok? Thanks, \$\endgroup\$
    – jsmith
    Mar 15, 2022 at 17:49
  • \$\begingroup\$ I think it should be, the via itself doesn't have a lot of volume, and an acceptable length-to-width ratio. You can make this more reliable by getting tented vias on the bottom, i.e. have the soldermask printed over the via. \$\endgroup\$ Mar 15, 2022 at 20:44

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