Heat Sink Surface Finish & Flatness

[EngAP]

I am looking for information regarding heat sink surface finish and flatness. My particular problem is as follows:

I have a small extruded aluminium heat sink (approx. 25mm long, 20mm wide and 13mm fin height) with 15W dissipated from a small (10 by 12mm) heater. The heater is screwed onto the rear face of the heat sink. Heat sink thermal resistance is approx. 3.4 degC/W.

For thermal interface I am using Graftech eGraf 1210 (0.25mm thick, 10W/m-K through conductivity). In tests with a machined sample heat sink with the mounting face sanded smooth this has given be performance as expected.

However I have no experience with extruted heat sinks so do not know what surface finish to expect. I have read that the surface finish of extrusions is often good enough as supplied, without post machining.

I would like recommendations on surface flatness and finish for the above application and whether a post machining operation to smooth the surface would be required.

If a post machining operation is recommended then what surface flatness would be required. The machined sample had a sanded (using P400 Wetordry) finish on the mounting face.

Thanks.

 

[Maxham1]

In regard to the surface finish, I think the fact that you have a thermal interface material will minimize the requirements for surface finish. I would think something like 16 microinches or 32 microinches would be sufficient. Ask the supplier of the thermal interface material what they recommend; that'll give you a sanity check.

The flatness specified should be such that any bow in the heatsink is smaller than the thickness of the thermal interface material.

 

[EngAP]

Thanks - that's pretty much what I thought.

 

[Tunalover]

Diecast heat sinks have higher infrared emmitance than the smoother extruded variety. If you have considerable radiation heat transfer this is something to consider. Also pin fins are usually a better choice over a ribbed heatsink (when they both occupy the same volume) when the pin fins have the right average diameter and spacing.


Tunalover

 

[Christine74]

This might help with your question about the surface finish:

http://www.mhtlab.uwaterloo.ca/pdf_papers/mhtl07-1.pdf

-Christine