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December 22, 2011

Comments

jws

I would expect there to be some effect of the distributed thermal resistance inside the block as well. Especially in less conductive materials, this could dwarf the interface "R". I'd be interested to see how this is treated, it becomes even more of an interest when there's "something" on the other side of the main mass that is the real site of interest (like, say, a die on top of a Cu-W heat slug on top of that thermal plane).

Rick Collins

There are some assumptions missing in the above example. To get a temperature from a thermal resistance I believe there has to be a constant heat flow from the base plate through the mass. That means there has to be a thermal resistance from the mass into the environment, normally considered a type of "heat sink" at a constant temperature.

I suppose this is detailed in the previous article referred to here but not cited in a way I can find it.

John Dunn

It wasn't detailed in the previous item either. This is admittedly simplified in that sense, but temperature excursions do take time to happen. Thermal time constants are involved but often overlooked which is the point at hand.

Nick Name

In fact the problem is much more complex than described. The temperature rise time constant at the interface between source and heat sink is function of : thermal resistance at contact (as mentioned), diffusibility of heat in the sink material (not mentioned since conductivity is high) and thermal resistance at the sink-fluid surface. The assumption is made that the whole sink mass is at same temperature which is valid ONLY if the heat source has a quite low dynamics with respect to the heat diffusion, a fast changing source will lead to local higher temperatures since the heat cannot be diffused in the whole mass. From the other side the highest temperature depends on the possibility for the sink to transfer heat to environment. This last property is not only function of area and fluid velocity but also of the boundary layer thickness. This is the reason why profiles are designed so that a high turbulence occurs.

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