[SOLVED] How to find Thermal Resistivity of PCB?

[knikhil271083]

Hi,

Can anybody let me know how to calculate the thermal resistivity of a PCB? I want to calculate the effect of heating of a high wattage SMD resistor on the heat dissipation of PCB .
I have referred to the manufacturer's datasheet for the FR4 material used for manufacturing PCB, but it only mentions the Thermal Conductivity. I have attached the datasheet of the PCB material for reference.

Is the regular formula for electrical conductivity (R=ρl/A) applicable to thermal conductivity too?

Thanks in advance for help!

Regards
Nikhil

View attachment isola_370HR.pdf

 

[dick_freebird]

Thermal conductivity is the inverse of thermal resistivity.
But that is just a material property. The larger problem
has to comprehend the detail geometry as heat removal
is dominated by the lateral conduction close-in to the
"heater", including copper layers as well as the 'glass.

 

[kspalla]

You can minimize thermal resistance if hot vias used underneath the heat dissipating component.

 

[knikhil271083]

Does this mean that I have to increase the thickness of Cu layer on my PCB? It is a 4 layer board. Top and bottom are signal layers and Inner1 and Inner 2 are power and ground layers respectively. All layers are 35 microns.

 

[kspalla]

Increased thickness will help in lateral flow of heat. if you plan hanging boards wit clamps then better increase the Cu thickness.

 

[FvM]

I want to calculate the effect of heating of a high wattage SMD resistor on the heat dissipation of PCB.

What's your exact question? Finally, the input power of the resistor will be dissipated from the PCB surface, whatever it internal thermal resistance is. The thermal resistance will however affect the temperature distribution.

The second important point is the total PCB to ambient thermal resistance. It depends on the PCB size and natural or forced convection.

It's a realistic assumption that the horizontal (lateral) thermal conduction will be mainly provided by copper planes and copper pour on outer layers. It can be estimated based on known Cu thermal conductivity and geometry.