Pete,
You need to be careful with these types of generalizations. In most
cases the opposite is true.

1. Vias are quite often used for thermal transfer, especially for TSOP,
BGA, QFN, and many other package styles.

2. Spoke-type thermal relief is quite often required for SMT components,
even when the board is convection reflowed. This is also sometimes done
to facilitate rework and hand-soldered parts.

3. For high-reliability designs, some vias are purposely made slightly
larger (.015-.022) and are filled with solder. They are used for thermal
transfer from one area of the PWB to another, and then typically to a
frame or heatsink plate. 

There are many other reasons wagon-wheel type thermal relief may be
required for both SMT and PTH technology in all of the above scenarios.



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pete Houwen
Sent: Tuesday, April 07, 2009 8:42 AM
To: [log in to unmask]
Subject: Re: [TN] when do vias require a thermal relief

When wave soldering, much of the metal heating is done by the solder
itself.  
So as solder travels up the pin and hole, a large heatsink (non-relieved
plane) 
can stop the solder flow.

When reflow soldering, the board itself is heated.  So even external
planes and 
SMT pads do not need thermal relief.

That's for holes/pads connected to pins/leads.   Those are NOT "vias".

Vias don't get a pin in them, don't need solder, there is no thermal 
consideration required.  Best to directly connect to the plane, for
electrical 
and fabrication concerns.  For the same reason your fabricator wants to 
remove non-functional pads, they would rather not have themal vias.  If
you 
have a via carrying enough current to need solder fill all the way
through, the 
thermal spokes are going to need to be large enough that they will not
provide 
much thermal relief anyway.

So the short answer is vias never need, and probably should not have,
thermal 
relief.  (but never say never)

Pete

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