Re: gEDA-user: What really ARE thermals, and why are they call such?

[DJ Delorie <dj@xxxxxxxxxxx>]

> 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.