I'm adding extra copper area for more thermal heat on through hole components pads. I now wonder if it's okey to have the solder resist overlapping the edge of this extra large copper area and what the pro and con is?
Best regards
\$\begingroup\$
\$\endgroup\$
1
-
\$\begingroup\$ The overlap is ok. What interests me about this is if anyone will address how the solder-mask affects heat dissipation. I've read that it does not, but this seems counter-intuitive to me. I can't recall the source(s) of the information. I may need to put together a question about this after I do some more research. \$\endgroup\$– TutCommented Mar 7, 2017 at 11:06
Add a comment
|
2 Answers
\$\begingroup\$
\$\endgroup\$
3
Yes you can do that. Often for smaller pads the expand the soldermask to allow for PCB fabrication tolerances and prevent overlapping the pad with soldermask, which of course will prevent solder from reaching the pad. But for large pads, there's nothing really wrong with keeping the soldermask solder if you don't need to (or want to) allow solder on the entire area.
The good thing about doing it is to keep the board looking cleaner. You can also use the area to apply silkscreen if you want.
But, in your case as you are doing this for thermal management, leaving the copper exposed may be preferable. To Tut's comment, yes certainly soldermask will impede heat dissipation somewhat, but not too much, e.g. as compared to dissipation from an inner layer which has thicker core material covering it. What the soldermask prevents though is applying more solder to the copper to increase the copper volume which will greatly improve heat dissipation.
-
1\$\begingroup\$ Three thumbs up for paragraph 3. I was going to add that as a comment if you had not mentioned it. In fact I have seen really long pads.... half inch or more used down each side of an I.C. and fairly heavily soldered to produce a kind of in-situ "finned" heat-sink. \$\endgroup\$– Trevor_GCommented Mar 7, 2017 at 13:57
-
\$\begingroup\$ Yes @Trevor, thanks, that's exactly what I was thinking about. Unfortunately I couldn't find any images of it online yet. \$\endgroup\$– AngeloQCommented Mar 7, 2017 at 14:05
-
\$\begingroup\$ +1 ... TI has a couple related app notes about their PowerPAD packages that discuss issues related to heat dissipation and soldermask: PowerPAD™ Thermally Enhanced Package and PowerPAD™ Made Easy. \$\endgroup\$– TutCommented Mar 7, 2017 at 14:24
\$\begingroup\$
\$\endgroup\$
One issue with NOT uncovering it, is if the copper area is large enough the solder will actually wick UNDER the mask. The mask will wrinkle up and look awful and may actually start to flake off.
This can be alleviated a lot by relieving the pad using a cross or star connection.
But for heat transfer, that's net minus, better to pull back the solder resist instead. Plus, as mentioned by AngeloQ in a previous answer, for heat removal, the more solder the better. Not only does the extra solder give you more metal to conduct the heat away, it also increases the pads surface area to improve convection.
