Thermal via questions

[LethalCorpse]

I've been thinking a lot about thermal strategies and when and why to use or not use thermal relief. A very good discussion on the subject happened here, but the forumbot suggested thread resurrection was a bad idea, so here I am.
Do you use thermal relief on the thermal pad of high power devices (vregs, transistors, etc)? On one hand, no thermal relief gives a much better heatsink for the device. On the other hand, if your plane and tab are big enough, it makes it much more likely to fail soldering.

My theory is it's better to keep them in, because I've likened heat flow to current. If you have a very high potential difference (temperature differential, eg while soldering), a small increase in resistance will result in a big reduction in current (heat flow). When the potential difference (temperature difference) is much smaller, the same increase in resistance results in a much smaller reduction in current flow. QED, the advantages while soldering outweigh the disadvantages in operation.

I'd be interested in the opinions of folks experienced in thermal modelling.

 

[ads-ee]

I'm not experienced at thermal modeling, but at the last place I worked we had a 16 layer board that had multiple parts with large 1cm^2 or larger thermal slugs on the back of the parts. The entire board was surface mount with active components on both sides of the board. The board had both a large power hungry FPGA (11W) and multiple PAs (something like 8W each). The FPGA was mounted on the backside of the board where the heatsink was located (mounted over the entire length of the board). The PAs were on the top side and they had about 16 thermal vias without any thermal relief whatsoever, I looked at the layout. There was also a large copper pad on the backside of the board to draw heat not only into the ground plane, but also out the heat sink.

Of course the manufacturer had to heat the PCB quite a bit to ensure the spikes in the heat profile would melt the solder paste. There were also some interesting changes to the solder paste stencil to help out with the solder reflow process.