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Shouldn't the junction to case (top or bottom) thermal resistance depend only on package or IC internals? How do the PCB details play a role as mentioned in footnote 1?

enter image description here enter image description here

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  • \$\begingroup\$ What is the part number? Maybe we can check if this is common with other parts from the same manufacturer or family. \$\endgroup\$
    – Raul M.
    Commented Sep 14, 2021 at 19:51
  • \$\begingroup\$ That note (1) is basically telling you the test configuration under which theta-jc was measured. Think of this in the same way the switching speed of a digital part is specified - with XXX load resistance, YYY load capacitance, etc. \$\endgroup\$
    – SteveSh
    Commented Apr 11, 2022 at 11:41

3 Answers 3

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Yes, normally the Junction-To-Case thermal resistance doesn't depend of the PCB.
The problem is that a thermal resistance can only be defined between to isothermal surfaces and the PCB is not isotherm.
What should be understood here is that the temperature difference between the point Tb and the point Ta is proportional to the power dissipated by the die, and the value is 2.1°C/W. This value depends of the PCB geometry.

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Thermal dissipation of electronic devices is a "ugly beast".

You are right, and in fact, generally speaking the junction-to-case thermal resistance is normally independent from the PCB details...

The point here is that the attached data refer to a very high temperature sensitive device, at the point that the measurement of thermal resistance (junction-to-ambient, and even junction-to-case) with this device type is extremely difficult without mounting it on a very good heat spreader (for example, over a CU pad on a PCB, having certain area dimension and thickness).

This is the main reason why the supplier is giving the thermal resistance data in that way, that is, referring to CU pad dimensions/thickness.

Besides, you can see that also tha data referring to the top are given, and this justifies the need for this high temperature sensitive device to possibly dissipate the heat from the top of the package, too (even if the major contribute will come from the bottom side).

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Junction to case thermal resistance will not depend on your board characteristics, it is explained in the FDD86102 datasheet.

RθJC is guaranteed by design while RθJA is determined by the user's board design.

FDD86102 (Farchild) --> Not dependant enter image description here
Checking other datasheets, none of them show any dependency with the board characteristics

BSC005N03LS5I (Infineon) --> Not dependant enter image description here IPTG007N06NM5ATMA1 (Infineon) --> Not dependant enter image description here SIZ340BDT-T1-GE3 (Vishay) --> Not dependant enter image description here SCTL90N65G2V (STMicroelectronics) --> Not dependant enter image description here

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