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PCB Heat Transfer Problem

  1. Nov 6, 2015 #1
    Hello, I will try and provide as much detail to the problem as I can with the hope someone can help me solve this. We are currently working on a wireless project that contains high electrical currents, because of this the box the electronics are enclosed in experience quite a temperature rise. The basic concept is I have a PCB that is housed inside a copper box, the copper box has 3 layers, outer layer is copper, middle layer is plastic, inner layer is copper. I want to calculate the temperature inside the copper box and outside the copper box.

    Here are some details that we know:
    The PCB dimensions are: 3mm H x 37mm W x 159mm L
    The outer copper box outer dimensions are: 17.9mm H x 41.9mm W x 163.9mm L; wall thickness is 0.2 mm
    The plastic box outer dimensions are: 17.5mm H x 41.5mm W x 163.5mm L; wall thickness is 3 mm
    The inner copper box outer dimensions are: 15mm H x 39mm W x 161mm L; wall thickness is 0.2 mm
    PCB Thermal conductivity: .25 W/m*K
    Copper thermal conductivity: 400 W/m*K
    Plastic thermal conductivity: .146 W/m*K
    bPlastic emissivity: 0.9
    Copper wall emissivity: 0.2
    Ambient temperature: 20 C
    max Temp: 41 C
    Power: 7.6 W
    Current: .48 amps
    Resistance: 33 ohms

    i have attached some pictures to help. Please let me know if you have any questions, I have been stuck on this problem.
     

    Attached Files:

  2. jcsd
  3. Nov 6, 2015 #2
    Hire a mechanical engineer, if you are not one, or collaborate with and undergrad in ME.
    Start your analysis with the source from then you can calculate all the rest. Study heat transfer dynamics.
    Your target ΔT = 41-20 =21 centigrade
     
  4. Nov 6, 2015 #3

    berkeman

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    Why are you thermally insulationg the PCB if you are worried about heat transfer and temperature rise? And why are you specifying Tambient ast 20C? Is this i a controlled lab environment?
     
  5. Nov 6, 2015 #4
    The above info is what was given to me, I did not derive these numbers. I have been away from heat transfer for quite a while, I'm more focused in stress analysis. This is in a controlled environment, this is a healthcare application.
     
  6. Nov 6, 2015 #5

    berkeman

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    Well tell them the design makes no sense to experienced engineers. Their thermal design is wrong.
     
  7. Nov 6, 2015 #6
    This design was actually made by Electrical Engineers, then the obvious problems occurred, MEs were brought in.
     
  8. Nov 6, 2015 #7
    This is a capacitor don't you think? The dielectric is in between copper
     
  9. Nov 6, 2015 #8

    berkeman

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    I'm an EE. and I do thermal analyses all the time. Ask the EEs for the junction temperature analysis of the semiconductors in this design. They are responsible for that, not the MEs by themselves.

    In more cooperative work environments, we work together with the MEs to optimize the thermal transfer from the semiconductor junctions on our PCBs to the ambient air. We use "thermal resistance" numbers in degrees C per Watt for these calculations.

    And can you say what the insulation plastic layer is for in the design. It goes against drawing heat energy out of the core, obviously...
     
  10. Nov 6, 2015 #9
    The material is ULTEM and the pcb is FR4.
     
  11. Nov 6, 2015 #10
    Are all the materials arranged in compact not spaced void in between, when you cut it in a cross section? If so this is a conduction heat transfer. This may help 3-heat-conduction-equation-compatibility-mode-4-638.jpg
     
  12. Nov 6, 2015 #11
    There is air space between the pcb and the inner copper box.
     
  13. Nov 6, 2015 #12
    Is that space in between ventilated or open to the outside?
     
  14. Nov 6, 2015 #13
    No, it's all enclosed.
     
  15. Nov 6, 2015 #14
    Is it depressurized or evacuated with air?
     
  16. Nov 6, 2015 #15

    berkeman

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    Wait, this is a capacitor, and you are worried about thermal effects? I need to re-read the thread. Wait one...
     
  17. Nov 6, 2015 #16

    berkeman

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    That is just a capacitor?
     
  18. Nov 6, 2015 #17
    It's a form of shielding from interference I guess
     
  19. Nov 6, 2015 #18
    No, the pcb is set inside the copper enclosure. The heat is mainly caused by rf and stray currents.
     
  20. Nov 6, 2015 #19
    It is not depresserized. It is a form of shielding. The reason I posted this is because my initial calcs are off by 80 degrees and I'm not sure what I'm missing. I am on the process of running an Ansys analysis but I was hoping to solve this by hand. We have tested this which is how I know the temperature reached 180 F
     
  21. Nov 6, 2015 #20

    berkeman

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    The EE working on this shielding should post here so we can help him/her. There are ways to design good electrical shielding with adequate thermal transfer...
     
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