When a resistor is attached directly to the heat sink the heat generated by the resistor conducts to the metal plate and then into the air as a result of convection.
Ceramic resistor heat sink.
They are most often wire wound resistors.
Ohmite s heatsinks provide a low cost thermal solution for these specific packages in multiple sizes.
Simple metal heat sinks.
Ohmite now carries a line of heatsink solutions that are specifically designed for your bga and pbga applications.
Nowadays the resistance wire itself may be replaced by a moulded spiral of metal oxide compound.
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Additionally you may want to ensure that the resistor is mounted with an air gap below it as well not touching the pcb to promote air circulation on all sides.
Large wirewound resistors may be rated for 1 000 watts or more.
A surface mount power resistor with a pad for heatsink may require a copper pour heatsink or a physical heat sink of some kind attached see the datasheet for the component in question.
However even this description is out of date.
A simple practical heat sink is an aluminum or steel plate as shown in figure 1.