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I recently made a board for the TPA3004D2 Class-D Amplifier Chip. It has a fairly large thermal pad on the bottom for heat dissipation. I used an existing TQFP-48 library in Eagle that had a thermal pad which I resized (my first mistake). I then put a 4x4 grid of vias connecting the thermal pad to the bottom ground plane, as recommended in the datasheet. Anyways, I got the boards yesterday and this is what they look like:

Top Layer Bottom Layer

As you can see from the picture, the 4 vias in the center of the top-layer grid got grouped together which is good. But for some reason, the thermal pad that was supposed to be there never got fully "etched" out, and the copper is actually under the purple, thus reducing the surface area that the thermal pad will contact the vias with. So what are my options for soldering this? I was thinking about going in from the bottom and soldering through each via, thus giving me maximum area and heat transfer. Thoughts?

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  • \$\begingroup\$ can't you just scrape away the solder resist? \$\endgroup\$ – Icy Dec 3 '15 at 15:06
  • \$\begingroup\$ @Icy I tried to do that, but that stuff is resilient and I think if I do too much scraping the surface is going to be too rough \$\endgroup\$ – crocboy Dec 3 '15 at 15:24
  • \$\begingroup\$ Could try masking the area off with high-quality blue painter's tape (or probably better yet, kapton tape) and gently rub with a cotton swab dipped in acetone, xylene, tolulene, or other such paint thinner. \$\endgroup\$ – rdtsc Dec 3 '15 at 15:30
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    \$\begingroup\$ @rdtsc Neither Acetone, nor Toluene, nor Xylene, will do anything at all to the solder mask. Pretty much only mechanical action (scratching, scraping, sanding) or intense heat (laser!) will remove it. \$\endgroup\$ – Tom Carpenter Dec 3 '15 at 15:37
  • \$\begingroup\$ I have a hard time believing tolulene won't remove it, but haven't had a reason to try. \$\endgroup\$ – rdtsc Dec 3 '15 at 15:44
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Are you using solder paste or just soldering by hand?

First fill the vias with just enough solder that the holes are filled. You don't want solder above the surface of the holes.

If using solder paste, add paste on top of and in the space between the vias as well as the lead pads.

If soldering by hand, add lots of flux on top of and between the vias. After soldering the leads to the pads, turn the board over and apply heat to the vias. The idea is to reflow the solder in the via holes onto the thermal pad.

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  • \$\begingroup\$ I'll most likely be doing this by hand \$\endgroup\$ – crocboy Dec 3 '15 at 15:38
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If there is room on the opposite side, consider soldering in pins/wires (that will stick out from the board) to act as more of a heat sink, rather than just solder-fill - also, copper conducts heat better than solder.

An abrasive approach might remove the solder resist without too much roughness (use a fine grit) but will probably mess up the surface plating (gold?) - probably not a big deal if you are soldering right after you do it - that's mostly for solderability after long storage IIRC.

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