# Hand calculation for Junction temperature. PCB thermal analysis using excel.

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 February 27, 2021, 23:40 Hand calculation for Junction temperature. PCB thermal analysis using excel. #1 New Member   Join Date: Oct 2018 Posts: 24 Rep Power: 7 Dear Thermal Designers, I am trying to validate CFD results of Junction temperature using theoretical calculations. I have simple PCB of thickness 1.5 mm with K(in plane) and K(through) as 35 and 0.9 W/mK resp. In middle of PCB I have single IC component with power load of 0.88W. Rjb= 2 C/W, Rjc = 1C/W. System is supposed to have natural cooling without any heat sink so whatever cooling is achieved is through PCB board. h = 0.955*(delT/L) ^0.25 I am modeling PCB heat transfer assuming it as circular disc fin with r1 and r2 as inner and outer radius. Temperature profile can be represented by 2nd order differential equation having complex number. Please refer https://www.google.com/url?sa=t&sour...kCmH6JJq0dENJh https://www.google.com/url?sa=t&sour...=1614486348298 Solving it requires bessel calculator (METHOD 1)and which is available online. Another way (METHOD 2) I am doing heat balance by assuming (heat conduction through disc fin) = (heat connected + heat radiated through disc fin) = heat load of IC Unknown in method 1 is outer radius of disc fin r2 or effective area of cooling. In second method unknowns are r2 and temperature T(r2). Here I am assuming that conduction and (convection+radiation) are in series. To evaluate r2 I am assuming T(r2) = 1.6 *T(ambient) based on judgment. Using goal seek I'm funding to and T(r1). Both methods give fair enough results which matches with CFD Tj value. But for small IC size only (like < 10x10mm) Personally I feel bottle neck is to find the effective cooling area for conduction convection. Is anybody aware of how to decide effective cooling area nearby IC? Is there any thumb rule? Thank you in advance. Rt_design likes this.

 October 2, 2021, 09:20 #2 New Member   Sami Safarini Join Date: Sep 2021 Posts: 8 Rep Power: 4 If you don't mind me asking, what is the purpose of validation exercise? I am guessing that the PCB and IC are your own design and you want to provide a spec?

 Tags electronic cooling, junction temperature, pcb