Dear circuit designers,

I have a few transistors that get hot on my board, and I'm thinking of using the
PWB as the heat sink.
Obviously, the more copper area, the better.

It's not so obvious to me how to handle the (typical green) solder mask. My
cow-orkers speculate that it acts as a thermally insulating blanket. That's
exactly what I don't want. Should I
  (a) Punch a very large rectangle in the solder mask to "let more heat out".
  (b) Use the standard holes in the solder mask just big enough for each pin and
the drain pad.
  (c) have little strips of green solder mask to "hold the part in place",
leaving most of the heat sink uncovered.
?

Using copper areas on the PWB as a heat sink:
  http://www.geocities.com/Heartland/Hills/5131/thermal.htm
  http://www.irf.com/technical-info/an994/an-994.htm
  http://onsemi.com/pub/Collateral/DataSheet/lm317mrev6.pdf
all have graphs of thermal resistance vs. copper area.

--
David Cary