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Instead of having a heatsink directly connected to the diode, I want to have a 2 mm layer of cold rolled steel in between case and heat sink(aluminium){for waterproofing as needed in the application}.

I have taken in account of the Rth(J-C) and Rth(C-H) for the diode and the thermal resistances of the steel plate and the conductive paste.I want to check the feasibility of the design by ensuring the temperature of the junction does not cross 150 degC. By considering the thermal resistance of the steel plate, there is not much difference in the temperature of the junction with or without the steel plate.

Kindly let me know if I should consider the contact resistance between steel plate and heat sink. If yes, how to calculate the value for the same?

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Yes, you need to consider the thermal resistance of the metal-to-metal interface. It is common to use thermal grease between two metal surfaces to reduce the thermal resistance. You mention "conductive paste" in your question but it was not clear where this paste was used in the assembly, or whether you specifically meant a thermally conductive paste. You can also buy thin sheets of a rubbery material that can be used for the same purpose.

Since you are creating a non-standard assembly you will need to measure the thermal resistance of the interface yourself.

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  • \$\begingroup\$ I know the conductivity of the thermally conductive paste. it is available in the datasheet.I have taken the thickness of the layer of paste to be 1 micron(from some sources) should this be sufficient or do i need to find anything else? \$\endgroup\$ – Aseem Jul 30 at 5:38
  • \$\begingroup\$ The thermal grease improves the thermal conductivity but if you are creating a new kind of structure then you may have to do some testing and measure the actual thermal conductivity. Or, try asking at the physics SE site. \$\endgroup\$ – Elliot Alderson Jul 30 at 10:51

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