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.
Ceramic resistor heat sink.
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The aluminum cased types are designed to be attached to a heat sink to dissipate the heat.
Large wirewound resistors may be rated for 1 000 watts or more.
Simple metal heat sinks.
Nowadays the resistance wire itself may be replaced by a moulded spiral of metal oxide compound.
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.
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.
The rated power is dependent on being used with a suitable heat sink e g a 50 w power rated resistor overheats at a fraction of the power dissipation if not used with a heat sink.
A simple practical heat sink is an aluminum or steel plate as shown in figure 1.
They are most often wire wound resistors.
Ohmite now carries a line of heatsink solutions that are specifically designed for your bga and pbga applications.