thermal reliefs
Thermal reliefs on a PCB are to make it easier to solder and desolder thru-hole components when you have a circuit board with internal planes, large poured planes, or wide conducting traces. Without the relief patterns on copper planes (or wide traces), the planes (or traces) act as a heat sink drawing much of the heat away from the lead you are trying to solder or desolder. The result could be a cold solder joint, or the need to apply excess heat while soldering. Often it is impossible to desolder a component without damaging the board if that component was placed without thermal reliefs on the large connected copper areas.
Section 9.1.3 of IPC-2221 "Generic Standard on Printed Board Design" says:
QUOTE
9.1.3 Thermal Relief in Conductor Planes - Thermal relief is only required for holes that are subject to soldering in large conductor areas (ground planes, voltage planes, thermal planes, etc). Relief is required to reduce soldering dwell time by providing thermal resistance during the soldering process.
These type connections SHALL be relieved in a manner similar to that shown in Figure 9-4. The relationship between the hole size, land and web area is critical. See the sectional standards for more detailed information.
UNQUOTE