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.
Ohmite now carries a line of heatsink solutions that are specifically designed for your bga and pbga applications.
Simple metal heat sinks.
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The aluminum cased types are designed to be attached to a heat sink to dissipate the heat.
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.
Large wirewound resistors may be rated for 1 000 watts or more.
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.
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.
They are most often wire wound resistors.
A simple practical heat sink is an aluminum or steel plate as shown in figure 1.
However even this description is out of date.