# How can a complex shape's heat sink thermal resistance be determined?

I need a heat sink with a thermal resistance of 0.7 °C/W/3" or less. C/W/3" represents how many degrees Celsius per watt a 3" long section of heat sink can dissipate without exceeding a max temperature, usually specified as 75°C.

I did find thermal resistance calculators online but only for simple rectangular heat sinks.

I'm interested in the large, differently configured heat sinks on eBay but none list the thermal resistance. How can the TR of odd shaped heat sinks be determined?

The photo is typical of the complex shapes I'm interested in.

• I cleaned up your post a bit but I had no idea what "C/W/3" 0.7 or less" means. Hit the edit link to fix it. Tip: use &deg; for the degrees symbol if you can't generate it otherwise. Preview the post before saving edits. Jun 28, 2021 at 18:24
• Have you looked in heatsink catalogs to find similar shapes?
– qrk
Jun 28, 2021 at 18:29

Thermal resistance is an highly non-linear thing and, as far as I know is determined… experimentally. As is, take a piece of aluminum/copper/whatever, put some Watts of heat to it and see how it heats up.

In fact due to convection the resistance varies depending on the orientation and airflow has a huge impact on the final result.

I'd suggest to look up on a manufacturer catalog the most similar shape to the one you have and start from there.

Alternative: if you are rich and competent there is thermal simulation software, too.

• With "see how it heats up" you probably refere to the temperature at equilibrium. Just to make clear that it's not about how fast it heats up", which is a matter of *thermal mass, not thermal resistance. Jun 28, 2021 at 18:41
• Thermal transients are sometimes important too (MOSFETs often have a derating graph for these) altough yes, I think that thermal resistance is determined in equilibrium keeping Ta constant Jun 28, 2021 at 18:49

That heatsink pictured on the blue surface is designed for hockey puck diode and thyristor packages. The device is clamped between that heatsink and an identical or similar one. The small hole in the center is for a centering pin. The clamping hardware includes two bolts, on on each side of the device. The clamping hardware includes some kind of spring that determines the clamping pressure. There should be detailed thermal design information available from the heatsink manufacturer or supplier. The original supplier may have been a heatsink company like Wakefield. It is also possible that the semiconductor supplier or the power electronic equipment manufacturer had it manufactured by an aluminum extrusion company like Alcoa.

This is not a full answer but rather to clarify some points in your question.

C/W/3" represents how many degrees Celsius per watt a 3" long section of heat sink can dissipate without exceeding a max temperature, usually specified as 75°C.

You haven't got that quite right. °C/W tells you the temperature rise at the centre of the heatsink relative to ambient air per watt of heat energy into the heatsink. The upper temperature is usually not specified as it depends on the power in and the ambient temperature. Usually the designer wants to control the junction temperature of the semiconductor and this can be calculated when all the thermal resistances are known.

Note that if the heat source is concentrated in the centre of the heatsinks pictured some derating may be required. The optimum will be having the heat sources spread out evenly along the heatsink as the device mounting holes in your photo suggest.