> I've been meaning to ask this for quite some time. I have a lot of > experience designing circuits, but not printed circuit boards; Neal's > question just reminded me to ask.
Thermals are so named because they create a thermal isolation between the pin and the large chunk of copper that makes up a ground plane. Without it, the heat from soldering would be sucked into the ground plane so fast it would be difficult to get a good solder joint. The thermal acts to slow heat loss to the plane, allowing it to concentrate in the pin and form a good melt. A non-thermal "pin", such as a connected via, has solid copper all the way around it, providing the maximum current load. An isolated pin (i.e. not connected to the ground plane) has a ring of non-copper (i.e. removed during etching) around it. To make a thermal, you add a couple of thin bridge traces across the ring of non-copper. This provides electrical connectivity without adding a lot of thermal connectivity.
