Power semiconductors fail when the junction temperature is a guess. The RFQ should carry the watts to be removed, the maximum ambient, the allowed case or junction temperature, and whether air is forced. The extrusion length is a result, not the specification.
Thermal resistance has a condition
Natural-convection ratings assume a clear vertical fin and a free room. A sink laid flat in a closed box will not meet that number. Forced-air ratings assume a duct and a flow. If you bolt a fan to the end of an extrusion, ask which curve applies. Give the designer the power loss from the datasheet at your current, not the nameplate of the motor you are driving.
The interface is half the problem. Specify the thermal pad or grease, the screw torque, and whether the tab must stay isolated from the chassis. A mica kit and a silicone pad do not have the same thermal resistance or the same hipot result.
- Power loss in watts and the ambient inside the enclosure, not the room.
- Allowed case temperature, or junction temperature if you will calculate it.
- Air: natural, or forced, with the direction and any filter.
- Fan: voltage, size, airflow, noise, and bearing life at temperature.
- Interface material and isolation requirement.
| Item | Ask for | Reject |
|---|---|---|
| Heat sink | C/W at the airflow you have | A length with no thermal data |
| Fan | m3/h, voltage, and L10 life | A fan rated only as silent |
| Pad | Thickness and W/mK, isolation if needed | A random grey sheet |
Sample check
Weigh the loss you specified against the sink curve and write the expected temperature rise on the approval. On the sample, confirm fin orientation matches the drawing and the fan blows the way the duct expects. Check the fan label for voltage. A 24 V fan on 12 V moves less air than the curve you bought. Do not run a life test in incoming inspection. Record the bearing life the maker states and the ambient they stated it at.
What to lock on the order
Write the watts, the ambient inside the box, and the allowed temperature rise in one sentence at the top of the RFQ. The sink maker can then propose a length. If you already drew a length, say whether it is fixed by the sheet metal or only a guess. A fixed length needs a fan or a pad change when the numbers do not close. Specify the fan supply from the rail you actually have, including the tolerance, and the connector the harness already uses. Ask for the life figure and the temperature it was measured at, and file that page. On the sample build, measure the case temperature of the semiconductor at the load you specified, in the enclosure if you can. A bench test in open air will look better than the product and will approve the wrong sink.
Questions buyers actually ask
Is a longer heat sink always cooler?
Not if the airflow is blocked or the interface is dry. Thermal resistance is published at a stated airflow or in natural convection. Use the matching figure.
How long does a fan last?
Bearing life is an L10 figure at a temperature. A sleeve bearing and a ball bearing are different replacements. Write the hours you need.
What should the drawing say about the pad?
Thickness, conductivity, and whether the device is isolated. A pad that is too thick can be worse than no pad.