How to Set Up Flip-Chip Junction to Board and Junction to Case Thermal Analysis Using Ansys Fluent

How to Set Up Flip-Chip Junction to Board and Junction to Case Thermal Analysis Using Ansys Fluent Hi everyone, this is Luis Maldonado from Ozen Engineering. Today I'm going to show you how you can set up the junction to board and junction to case thermal resistance for a flip chip.

So we are starting to set up all the different parameters for our simulations. For models, we require the energy equation. For the biscuit model, I recommend setting it to laminar because, in this case, we don't have any fluid region, so we actually don't require solving fluid equations.

For materials, I already added all the different materials, so you don't have to worry about it. The only thing that I want to mention here is that Ansys Fluent allowed us to set up orthotropic material for the thermal conductivity.

So you can go to thermal conductivity, click here, you can move from constant to orthotropic. To set up orthotropic, you have to select two different main directions, in this case, X and Y, and Ansys Fluent calculates the third direction for itself.

In each of the directions, you can have a different conductivity. So, for our case, we have the substrate and the underfill die as orthotropic materials for the thermal conductivity, the other half thermal conductivity as constant values.

Okay, so the first thing that we have to do is select the source term for our die. So, to do that, I would like to create a named expression to facilitate our setup. So I will click select new. I will name it power and I will set that as 5 watts. Okay.

If you are going to use this case for a parametric study, you can just select use as an input parameter. So you can use the power name expression for a parametric study, okay? Okay, I'll resave it, so for the solids, first, it's essential to select the materials.

So, for the balls, we're going to select solder, click here, and paste in the other balls. Next is the die; for the die, we are going to select a silicon typical, and then we are going to add a source term, click here, and add source term.

Please be sure to correct any misspelled Ansys product names as you transcribe, e.g., 'OptiSling' should be 'optiSLang'. Click here to make sure that the actual value is placed as a source term. Click OK.

For the underflip chip, just under for the first layer of the copper, since we are using a PCB 2S2P, we are going to have a copper of 5. The other layers are completely copper, so let's copy this.

For the layers of ERP4, we select ERP 4. Just click here, so we just finished setting up everything in the cell zones, so let's go to the boundary conditions. So, for balls, most of these walls are internal walls, so we are not going to worry about it.

For the die sides, we are going to have two different kinds of boundary conditions. For junction to board, the idea is that all the heat flows from the chip to the PCB. So, the idea is that we are going to isolate all the walls of the flip chip. So, we are going to set a heat flux equal to zero.

Then, to make it easier, I'm going to copy this for all the other boundaries. I know PCB bottom ring is going to be different, PCB ring is going to be different. The same as bottom, the sides, and the other walls that we select will have a heat flux equal to zero. So, let's copy them.

Okay, so for the bottoms and the boundaries that are outside the ring, we are going to set a convective boundary condition. We are going to set a heat transfer coefficient of 10, that is common in natural convection, and the ambient temperature is going to be 298.15 degrees Kelvin or 25 degrees C.

So, let's apply it. And this same boundary condition is going to be replicated in the sides and in the top. Finally, we have our fixed temperature that is going to be placed in the rings. We are going to select a thermal boundary condition as a temperature, set the value 25 degrees C.

Let's apply and then copy this also to the top. Ok, so we finished setting up our case, cell zones, and boundary conditions, then let's go to methods, so here just let all default and click high order term relaxation factor, relaxation term.

Click on the Residuals tab, and select the default Residuals.

For the residuals, we recommend that for this kind of application, use a lower residual than the default, something in the order of 1 to the power of minus 12. And let's create some report definitions, so we are interested in the maximum temperature inside our die, so let's add maximum, so maximum die, just select report file temperature die, okay?

The other value that is important to calculate the junction to board is the average temperature in the connections. So, let's call it AverageTemperatureBGAPCB and select this temperature, and I already created a name selection, so let's do all the BGAPCBs, ok?

Okay, we also are interested to see if our simulation is stabilized or not with the total heat transfer rate. So, let's screen total heat transfer.

For this junction to work, the heat is going to escape through the rings, the bottom, the top, and the sides of the domain because we have the natural convection boundary condition in this region. So, let's go to compute.

And to check if everything is going okay, I could practice just to calculate the average temperature in all the domains. So, we can, the idea of this monitor is to check if all the cells achieve a constant value after the iterations. Okay, so we already have our reports. Let's go to initialization.

For initialization, I will start with ambient temperature. And for run calculation, I will recommend using the time scale factor for solid use 10. Ok, so let's start the simulation.

Ok, so for the sake of the video, to make it short, I'm going to pause here and I will continue and resume when this calculation finishes. Our simulation already converged after 145 iterations, so it converged in the threshold that we set up for the energy equation.

So, let's check that we have everything okay. So, first let's check the Convergency of the energy equation. So, we know that we only have losses of energy, and the next one is the equation in this in PCB bottom, PCB top, in the sides, and in the rings.

So, as you can see here, the net value is 2 to the power of minus 5, that is really low compared to the power that we are using in our flagship. So, in terms of numerical iteration, we achieve a good convergence.

Also, here you can see the monitors where you can see that they flatten out in some values for the max temperature, for the average temperature, the heat flux that we already checked, and the average temperature of the domains.

So, for the calculation for junction to board, we require the maximum temperature, so let's go here, the maximum temperature, compute it, we require this value, and for the average, no, this one, no, for the average temperature in the PCB, let's compute this, so with these two values, we can calculate the junction and the power, we can calculate the junction to water thermal resistance.

Ok, so let's modify this case to calculate the junction to case thermal resistance. So, for doing that, we require to change the boundary conditions. In this case, what we want is to make the heat go only to the top of our flagship.

In order to do that, we are going to set up an isolate boundary condition in all the boundaries except the top, where we are going to set a fixed temperature value.

Copy this to all the boundaries, and then let's select top, ah, no, PCB top, no, I was wrong, SHC top, sorry, it's, uh, I'm not finding, ah, sorry, it's the top. I'm going to stop for some minutes, so I let this case run.

Okay, here, our case just finished with the same conversions value that we select for the receivers, so again, you can see here how the receivers go to the value, you can also check the constant value for the max temperature, average temperature, the heat flux, well, actually, this heat flux is going to be 5 because we have 0, so we are going to check this correctly now, and this is the average temperature in all the domains.

So, let's go to check the energy balance in the energy equation, energy equation, so flux is total energy for this junction to work. What we have is that all the heat is going through the thermal paste top.

So, we are going to select this only, and as you can see here, the net value is 5 to the power of minus 5. If you have any questions, please contact us at https://ozeninc.com/contact for more information.

So, with these two values and the power, you can calculate the junction to case thermal resistance. One thing that we didn't check in the previous is that you can actually plot different contour plots depending on what you want to see.

So, let's use this, and let's just plot all the external walls, so you can see how the temperature contours. Save and display.

So, in this case, you can see that almost all the flow is going to the top of the thermal base, and then we have a constant value in the other regions of the PCB because of the boundary conditions. So, well, basically, this is what I wanted to show you today. Thank you for watching this video.

Please contact us at https://ozeninc.com/contact for more information.