# PCB Heat Transfer Problem

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1. Nov 6, 2015

### rsk2mc

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.

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2. Nov 6, 2015

### Ronie Bayron

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.

3. Nov 6, 2015

### Staff: Mentor

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?

4. Nov 6, 2015

### rsk2mc

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.

5. Nov 6, 2015

### Staff: Mentor

Well tell them the design makes no sense to experienced engineers. Their thermal design is wrong.

6. Nov 6, 2015

### rsk2mc

This design was actually made by Electrical Engineers, then the obvious problems occurred, MEs were brought in.

7. Nov 6, 2015

### Ronie Bayron

This is a capacitor don't you think? The dielectric is in between copper

8. Nov 6, 2015

### Staff: Mentor

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...

9. Nov 6, 2015

### rsk2mc

The material is ULTEM and the pcb is FR4.

10. Nov 6, 2015

### Ronie Bayron

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

11. Nov 6, 2015

### rsk2mc

There is air space between the pcb and the inner copper box.

12. Nov 6, 2015

### Ronie Bayron

Is that space in between ventilated or open to the outside?

13. Nov 6, 2015

### rsk2mc

No, it's all enclosed.

14. Nov 6, 2015

### Ronie Bayron

Is it depressurized or evacuated with air?

15. Nov 6, 2015

### Staff: Mentor

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

16. Nov 6, 2015

### Staff: Mentor

That is just a capacitor?

17. Nov 6, 2015

### Ronie Bayron

It's a form of shielding from interference I guess

18. Nov 6, 2015

### rsk2mc

No, the pcb is set inside the copper enclosure. The heat is mainly caused by rf and stray currents.

19. Nov 6, 2015

### rsk2mc

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

20. Nov 6, 2015

### Staff: Mentor

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...

21. Nov 6, 2015

### Ronie Bayron

If its not depressurized (the cavity inside or in between pcb and encasement and assuming that there are no vents as you said) then you would get ΔP/ΔT = constant relationship. RF does not heat up anything if there is no media in between pcb to heat up.

The space in between should be vented or not, evacuated/depressurized it so you can treat your analysis from the pcb pure radiative heat.[/SUP]

22. Nov 6, 2015

### Ronie Bayron

If its caused by that, try evacuate the cavity between pcb and copper enclosure from air, then ground first layer copper enclosure. It's worth to try.