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I am currently designing a heat sink for OPA 549. Among the many considerations, for example, the choice of material, the geometry of heat sink, the design of fins and all the parameters that come into play during the modelling, I am most curious about the simple choice of material. Copper has more conductivity than Aluminium while the heat capacity of Aluminium is more than twice of copper. So naturally there has to be a trade off between the two. And I have no constraint on weight. I cannot use the combination of the two as there is bonding problem. My question is simple- Which one to use and why? Relevant details: Max Power 90W; Forced Convection with 21 cfm; Overall thermal resistance is around 0.5 SI units. thanks in advance

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The heat capacity isn't relevant for steady state heat removal, that's all about conductivity and good coupling to whatever your ambient is. Heat capacity only matters for short spikes of heat that you have to absorb because there is no time to transfer them away immediately. –  Olin Lathrop Jul 3 '13 at 13:37
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closed as off-topic by Manishearth Jul 12 '13 at 14:15

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Assuming a relatively small heatsink (you aren't trying to conduct heat to a fin a meter away) then the thermal conductivity isn't a major factor. And as the heatsink has a constant heat inflow and outflow the heat capacity doesn't really matter.

What is important is the ratio of surface area of the fins, the thermal contact to the hot side supporting the weight on the component.

That's why aluminium is so popular, it's cheap, easy to extrude/mould/machine, doesn't need any surface coating/painting and is light.

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