The time spent on the pad (by a soldering iron during soldering) is called “dwell time.”
When components are soldered on the PCB pads say a two terminal component to a footprint containing two pads,May be one of the pad resides in a ground plane while other is attached to a signal trace , when automated soldering process the ground plane pad will act as a good thermal conductor and sinks more heat and cools faster.This brings about uneven cooling time and may cause bad joints.to avoid that and to increase the dwell time,the copper around the pad residing in the ground plane is removed.
IPC2221 Section 9.1.3 says,
“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 sol-
dering process.”
