This slide shows some things to consider when working with heatsinks. In the previous example, the thermal resistance of the PF2202 was 5.9°C/W. This meant there was a 42.7°C temperature rise internal to the part. Using a PF2205 resistor will lower this value to 18.4°C because this device has a thermal resistance of only a 2.3°C/W. The PF2205 is more expensive than the PF2202, but chances are the heatsink savings will more than offset this additional cost. For example, the PF2205 would allow a higher package-to-ambient thermal resistance of 13.95°C/W vs. the previously calculated 10.35°C/W for the PF2202. As a result, the heatsink performance requirements can be reduced by 25% thereby reducing the cost and the physical size of the heatsink.