No physical connection to what?  I usually have a fat trace to the pad
of the component, in an attempt to pull the heat into this square.  I
got the idea from a National note in I think the LM340 datasheet; don't
remember now if they spec'd soldermask over the metal or not now.

The designs I work on are generally pulsed, so in some ways I'm more
after thermal mass than anything.

Shawn Upton, KB1CKT
Test Engineer
Allegro MicroSystems, Inc
[log in to unmask]
603.626.2429/fax: 603.641.5336

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pete Houwen
Sent: Friday, September 10, 2010 9:03 AM
To: [log in to unmask]
Subject: Re: [TN] PCB Heatsink Pads

I'm not sure it's a violation, but certainly not common, and likely not
very 
effective at dissipating heat.

With no physical connection, and the solder mask in between, you aren't
going 
to dissipate enough heat to bother with the pad.  In the days of dry
film, it 
was a violation, but now the biggest concern is the time it takes to
answer 
the question each time.


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