> -----Original Message-----
> From: Dom Bragge [mailto:[EMAIL PROTECTED] 
> Sent: Thursday, January 29, 2004 5:23 AM
> To: protel
> Subject: [PEDA] TO-220 4th pin?
> 
> I just would like to ask a question about how you handle 
> TO-220 footprints (& the like)...
> 
> I have the three (electrical) pin device, that's fine.
> I'm placing the T)-220's flat on the board.
> 
> What if I want to (selectively) put copper on the top layer 
> under the TO-220 & have a suitable soldermask antipad? This 
> could aid in cooling without resorting to an actual heatsink.
> How should I best do that?
> 
> Do I place on the board a free pad, rectangle, with a hole 
> the same size as the hole for the TO-220 tab? Seems a bit 
> ugly, two holes etc etc but it should probably give me the 
> SMask opening.
> 
> Do I make a 4pin lib part, have a 4th pin on the footprint 
> being the large hole & add a polygon connected to that net? I 
> suppose I'll have to add an opening for the SMask on the 
> TO-220 footprint as well.

Dom

I usually keep 2 footprints available and one SCH symbol.

The SCH symbol is always the same, 3 pins plus Tab (pin4), the footprint
can then have several choices, normally I have a manufacturer prefix,
and then TO220H or TO220V.

If I intend to use this symbol with a vertical TO220 footprint I just
add the No ERC marker to the 4th pin. PCB update just ignores pin 4
then, as does the DRC. I never add the fill or copper to the horiz
version in the library, I do it manually afterwards as it is too much
hassle when doing updates to have to update free primitives from... All
the time.  

Easiest thing is to see it, so I uploaded a TO220 example from my user
library to http://www.proteluser.com/download/to220/Export.zip

Ignore the paste definitions layer, that is for something else ;)

Best Regards

John A. Ross

RSD Communications ltd
Email  [EMAIL PROTECTED]
WWW    http://www.rsd.tv
=========================


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