Dear Dennis Beerman,

OK, this sounds great for through-hole components.

But my cow-orkers speculate that my surface-mount transistors (SOT223, TO252,
DPAK, etc.) will swim out of position unless I add some green solder mask. Not
to mention that large area will bring too much or too little (depending on who
is spreading the FUD) solder to the drain pad.

Do surface-mount transistors need solder mask to hold them in position during
reflow (when the PWB drain pad is much larger than the component's drain pad) ?
(Is the surface tension on the other 2 feet of the package enough to hold it in
position ?). Do surface mount transistors need solder mask  to bring the "right"
amount of solder to the drain pad of the component ?

Since I'm soldering the drain pad directly to the copper on the PWB, that will
give better thermal transfer than any kind of thermal grease or other compound.

Thermal vias sound good, although Jeffrey G. Holmes found that arrays of
through-plated holes make no difference in still-air testing (my product has no
cooling fan).
Jeffrey G. Holmes, "Surface Mount Technology" September 1993
http://www.geocities.com/Heartland/Hills/5131/thermal.htm

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"Beerman, Dennis" <[log in to unmask]> on 2000-10-13 12:38:38 PM


My experience has been to provide a land area with no soldermask covering.
This provides a better method for the heat to dissipate into the surrounding
air.

John Morin <[log in to unmask]> on 2000-10-13 01:09:16 PM

...
Dave
     My experience here is that you need to open the soldermask in the area
where the component will contact the board. The mask, air or any other
material will hinder the transfer of heat. Furthermore you may also
consider using a thermal compound between the component and the board.
You may also want to add several small vias under the component if the
metal under the part is a part of some internal net that you can use to
help dissipate more heat.

     Hope this helps.
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