> I am looking for help on determining recommended dimensions for thermal   
> vias and thermal via legs (spokes).  I understand there is a trade-off   
> between making the vias large enough with small enough legs to allow the   
> board to be soldered without the use of a blast furnace versus making the   
> vias smaller with larger legs to allow for the required current capacity   
> of the connection.  Any suggestions would be most appreciated.
> 
> John Urry

John,

Why are you even considering "spoking" the vias? Vias are not related to a
solderable component, requiring a solder process. We (I) have been doing
spokeless plane layers for years now and have had absolutely! no complaints
from countless fab houses on this design issue.

My one recommendation though is to use a good size (say .020-.028" via size)
for the thermal vias, and to have a thermal engineer give you a good formula
for determining the amount of vias required.

What I might also suggest is to open the soldermask around the thermal area
(copper etch fills) and possibly even add thermal etch on the opposite side
of the board (and open the soldermask from this area) for more thermal
dissipation.

Good Luck,

Mitch Morey
Cadence Design Supervisor
Praegitzer Design
San Diego, CA 92121
Phone: (619)450-0994 x-17
Pager: (619)982-7529
Fax:   (619)450-0529
EMAIL: [log in to unmask]
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