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What is the suggestion for thermal vias under the exposed pad of ICs ? I understand that copper plugging would be ideal. But if I only have the choice of tenting or not tenting, should I leave the bottom end exposed or tented?

  • Exposing the vias is said to cause problems with flux wicking away from the IC to the bottom via pads (see below). This discussion here sharply opposes leaving vias open and says solder wicking is bad under all circumstances. However, doesn't solder-filling the via lead to way better thermal transport ?

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  • Bottom-side tenting would prevent flux wicking during reflow to some extent but could lead to trapped air. Also wouldn't the solder anyway wick into the via and the air bubble float up to the IC pad ?

  • Top-side tenting (as also suggested in the link before) the via-in-pad would prevent wicking and air bubbles altogether, but would greatly affect the thermal performance I guess, so I am highly sceptical of this.

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For large heat pads filling the via with solder isn’t as good as copper but much better than air. Solder wicking down the via isn’t a problem since the part will drop down onto the pad. This is preferable to it sitting on a blob of solder and possibly lifting the pins off their pads. Solder is much more dense than air so it will do what it wants to and the buoyancy of the air will have no significant effect. For small pads though you’re right that wicking solder down the via could be a problem and so tenting would be wise. I suppose that the air in the via would expand during the reflow cycle but that’s the lower-risk option.

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  • \$\begingroup\$ Ok, but the vias would be filled either way... Do I interpret you correctly that you are saying one should expose the bottom via side (because the flux wicking causes actually no problems) ? \$\endgroup\$
    – tobalt
    Commented Oct 28, 2021 at 8:07
  • \$\begingroup\$ I edited my answer - for small pads you might be better with tenting the underside. The only via-in-pad I’ve ever done has been plated over. \$\endgroup\$
    – Frog
    Commented Oct 28, 2021 at 8:13
  • \$\begingroup\$ I wonder whether making the paste mask apertures oversize might help. I’m speculating though. If you have enough control over the assembly process you could paste and reflow the underside first, which would fill the vias, and then paste the top side. \$\endgroup\$
    – Frog
    Commented Oct 28, 2021 at 8:18
  • \$\begingroup\$ The bottom side has no parts, so it is a single side design. But based on your reply, I believe that adding paste on the entire pad (instead of several paste squares) would provide additional solder to wick into the holes. In addition, I can leave the hole itself untented, but cover the bottom-side annular via rings with solder-mask. That would allow solder to wick only into the via drill, but no further. \$\endgroup\$
    – tobalt
    Commented Oct 28, 2021 at 8:21
  • \$\begingroup\$ If you’re suggesting that you paste the entire area of the package as a big rectangle I’d be cautious - that might be ok but I’d want to be sure that enough solder wicks into the vias to be sure that the pads won’t be left shorted on the topside. Masking all of the copper on the bottom side sounds very sensible. \$\endgroup\$
    – Frog
    Commented Oct 28, 2021 at 8:28

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