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I'm trying to solder a 4x4mm QFN IC using the method outlined here The thermal pad is connected to bottom ground plane with thermal vias. The ground plane is not covered with soldermask because I'm gonna attach a heatsink below the IC.

The problem is as soon as I apply the heat, all the solder drains to bottom plane via the holes and spreads into the ground plane. So IC does not stick.

Then,

  1. Any methods to somehow fill the vias with thermal conductive material?
  2. How do you do filled vias in EAGLE CAD (or do you?) (for next version of the board)
  3. Or any other better advice?
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    \$\begingroup\$ I think you mean it is not covered with soldermask. Silkscreen is the writing. \$\endgroup\$ – DerStrom8 Aug 23 '17 at 15:18
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Print the solder paste over the thermal pad only and reflow (or tin it with a regular solder using larger iron), let cool, apply the paste over the board, populate and reflow.

On the next revision make thermal vias smaller. I tend to pick next to the smallest allowed drill size.

Some boardhouses do filled vias if you specify it along with a compatible finish, i.e.HASL.

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