I need some help with a thermal question...

I am trying to design an aluminium die casting. It is for an enclosure but will have a PCB screwed to its inside face. The outside face must act as a heatsink.
I have two questions:

1) If I need to dissipate 20W of heat from that PCB, what should the heatsink “fin†dimensions be like (thickness ad height), and what should their spacing be? I need to ensure optimal performance, but don’t know how to best to design a heatsink. Is there a standard formula or standard rules for designing such a thing?

2) Secondly, I have seen heatsinks that have “pins†instead of “finsâ€. How much better is the performance of pins compared to fins? I presume pins are better because they have more surface area? How could I calculate the geometry of the pins to ensure optimal performance?

Any help appreciated.

 

Do you have an airflow or is it free convection.

 

Its for the outdoors so there will be some airflow but I want to design for zero airflow to be considering the worst situation possible. I already design a heatsink - it seems to work ok but I would still like an answer to my original question because maybe I can optimise it.,

 

Been away a bit, but try this....

http://www.frigprim.com/online/natconv_heatsink.html

Might get you close.


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