I have often heard this "equal and even" heating of the entire board explanation for not using thermal relief on SMD's. This is fine for getting the solder to a molten state but it fails when one considers the fact that the solder joint is formed as the solder cools. Now, how does your solder joint look when you consider the thermal properties for forming the joint. Would you not want the solder joint areas to be isolated from large copper masses? Thermal relief still serves a purpose for SMD's. As for rules - those written for through-hole parts serve well for SMD's also. The 'rule of thumb' I normally employ is that a connecting copper strap should be no wider than the plane gap.

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Pete
Sent: Monday, September 26, 2011 7:20 AM
To: [log in to unmask]
Subject: Re: [DC] SMT land/pad thermal relief

Lorand,

(there's still some debate on this, but:)

There's no real need for thermal relief on SMT lands.  The wave process on PTH uses the molten solder to heat the component lead and pad.  Thermal relief is used to keep that heat on the pad and lead instead of being conducted into large copper areas.  In SMT, the solder is melted by heating the entire board evenly.  So the large copper areas are heated just as much as the pad, regardless of any thermal relief.  A solid connection between the pad and plane will solder just as well as a thermally relieved pad.

Of course, that creates a soldermask defined pad, which adds a stress point to the solder joint.  If that is a concern for a specific solder joint, you can use the thermal relief style pad to create a copper defined pad.  But the rules for those  spokes are then electrical, not thermal.

Pete

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