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I understand that it's bad practice to put a via on a pad as it can wick solder during reflow.

How about putting the via directly adjacent so the ring is touching the pad but the hole is outside? I'm working on a project with very tight size constraints and being able to do this would be really helpful.

enter image description here

Thank you,
Stateful

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    \$\begingroup\$ When your annualar ring copper meets the pad, there is a sharp angle that can act as an acid trap during the etching process. you want to reduce that angle or you could potentially cut your trace during manufacturing. I have seen it happen a couple of times. \$\endgroup\$ – slightlynybbled Mar 22 '16 at 1:03
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As long as there is a reasonable amount of soldermask between the pad and the via hole, you won't have issues with solder wicking into the via, which is typically the main concern with via-in-pad. Make sure that you have enough soldermask so that the soldermask width minus your PCB shop's soldermask tolerance is sufficient to guarantee a strip of soldermask.

However, as others have pointed out, you may have issues with acid traps or uneven heating. For a one-off, these are likely a non-issue, but for a product, these should be taken care of. Because it's an LED (and therefore shouldn't be drawing too much current), use the smallest via and annular ring your boardhouse allows, and see if that works. If that fails, two vias on either side, overlapping the pads but with soldermask in between pad and via should be reasonable. However, if you are under such tight size constraints that this is necessary, consider opting for filled via-in-pad, and then you can put vias wherever you like. Sort of.

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I think the problem is then a differential heating characteristic of the pads, possibly leading to a tombstoned part. I'm no expert on soldering process though. I would ask your assembly house.

That annular ring appears to be much larger than the radius of the hole... have you thought about tightening up the drill tolerances and making a smaller ring?

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  • \$\begingroup\$ That's a great idea, Daniel. Making the ring smaller hadn't occurred to me. Min drill from my place is 0.15mm. \$\endgroup\$ – Stateful Mar 22 '16 at 0:10
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    \$\begingroup\$ Your pcb manufacturer will also have a minimum annular ring dimension as well. Make sure you don't violate that. \$\endgroup\$ – Dan Laks Mar 22 '16 at 0:12
  • \$\begingroup\$ IIRC, that is based on the drill hit tolerance. If you spec a higher tolerance, drilling can take longer (if it's within the capability of the manufacturer) But your rings just look huge. \$\endgroup\$ – Daniel Mar 22 '16 at 2:36

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