Use of Export to Model a T/R Module and LRU

SYMMIC provides thermal analyses at all levels, from gate details in a FET or MMIC, to modules, packaging and entire subsystems (e.g. an LRU). To do this, SYMMIC has an device template creation feature (also known as “export”) which can create an efficient device template from any layout of devices, thereby converting an assembly of devices into a sub-device that can be used in a new layout. In this example a number of MMICs and several devices are exported and then arranged into a representative T/R Module with a 70 Watt heat load, and solved. The boundary conditions are then extracted and used in the solution of the Power Amp, yielding precise peak temperatures. A similar procedure is followed to model and solve an LRU. (The FETs, MMICs, T/R Module and LRU layouts shown are representative; they do not model an existing radar system.)

3. Export every layout as a new device (template file) for the T/R Module layout.

6. Extract the boundary conditions for every original, fine-detail layout that was exported.

11. Extract the boundary conditions for every original, fine-detail layout that was exported.

12. Solve the center Power Amp again, but with the LRU temperature boundary condition.

This example assumes that one is familiar with the use of SYMMIC to configure device parameters in the Generic FET Template, and to lay out devices in MMICs. If not, please review the previous sections of Top-Down Analysis before proceeding.

Export a Layout for Every MMIC

The export capability of SYMMIC only works for layouts (i.e. not for stand-alone devices which are not in a layout)¹. Four different layouts need to be created for this example, three for MMICs and one for a simple device to represent a generic heat source.

1. Power Amp

The power amplifier for this example will be a resized version of the 2-stage X-band amp created in MMIC Thermal Analysis. Begin by copying the XbandAmp.xml layout file and its device templates, Stage1FET.xml and Stage2FET.xml, from the SYMMIC examples folder into your working directory. Rename the layout file as PowerAmp.xml. Also copy over the module type file: “TypeModule.xml”.

Start SYMMIC and open PowerAmp.xml. Select the Edit layout... command in the File menu. Change the Title to Power Amp. Change the FET positions to: (400,1000), (1200,470), and (1200,1560), in that order. These are the bottom-left corner positions (don't select the Center at box). Then change the Length to 2000, the Width to 2900, and the Meshsize to 100. Click the OK button to see the changed power amp model in the main window (Figure 1). Select Save layout from the File menu to save your changes.

Now export the power amp layout by selecting Create device template... from the File menu, and fill in the dialogs as shown in Figure 2. Note that the device type file being used is “TypeModule.xml”, which contains two layers – an attachment layer (to the LRU beneath it) made of Indium and a heat-spreading shim of Copper-Molybdenum-Copper laminate – both of which we want to add to the bottom of the MMIC. At the bottom of the dialog is a button titled Choose Components to Include. Press this button to bring up a second dialog that lists all of the unique components in the layout. For modeling the T/R module or LRU subsystems, the component detail can be removed and the heated areas (BCs) simplified, so select the button Clear All and then OK to save these settings and return to the first dialog. Enter a value of “100” in the box labeled Smallest BC feature to retain; BCs closer together than this will be merged together, so a value greater than the gate pitch for a FET will cause them all to be merged together.

Figure 2. Create Device Template from Layout dialog (left) for the Power Amp export, and its Choose Components to Include dialog (right) after selecting the Clear All button.

On selecting OK, the file is exported and the new template “PowerAmpExport.xml” is saved.

On closing this template SYMMIC will ask if you want to save the device(s) because they have changed. What it is really asking is whether you want to save the component export selections to their “toExport” flags, which in this case would be “-1” for all since all were cleared. Similarly, SYMMIC will ask to save the layout file in order to set the export defaults to the settings for the export just completed.

2. Driver Amp

The driver amp for this example will be built from the Generic FET Template. Begin by copying FET.xml from the SYMMIC examples folder and renaming it as DA-FET.xml. Open DA-FET.xml and make another copy by selecting Save as... and using File Name: DA-FET2.xml, and Template Title: FET 2. Now change the Number of Gates to 6. In Heat Generation... change the On Power to 2 W/mm, then save your changes to FET 2 by selecting Save device(s) in the File menu.

Next create a new layout for the driver amp MMIC by selecting Create layout... in the File menu, and skip the Create Array of Device? wizard. Fill in the fields as shown in Figure 3 (DA-FET.xml at (275,225), mirrored, 90° CCW; DA-FET2.xml at (825,120), mirrored, 90° CCW; Title: Driver Amp; Length: 1600; Width: 1000; Meshsize: 100). Save the layout as DriverAmp.xml. The driver amp should now look like Figure 4. If it doesn't, check that the two devices have boundary distances of 100 on all sides (e.g. Device > Pad/Bus/Boundary... > Gate Bus to Boundary).

Now create the driver amp module template (File > Create device template...) using Save as: “DriverAmpExport.xml” with device title: “Driver Amp on module”, clearing all of the components, with a Smallest BC feature to retain of 100, and adding 2 layers from the file “TypeModule.xml” on the bottom as before.

3. Post Amp

The post amp model is similar to the driver amp. Start by copying DA-FET.xml to PA-FET.xml, DA-FET2.xml to PA-FET2.xml, and DriverAmp.xml to PostAmp.xml. Open PA-FET.xml in SYMMIC and change the Number of Gates to 4, then save it. Open PA-FET2.xml and change the Number of Gates to 8, then save it. Finally, open PostAmp.xml, select Edit layout... and modify it so that it matches Figure 5 (PA-FET.xml at (475,290); PA-FET2.xml at (1075,150); Title: Post Amp; Length: 2000; Width: 1200), then save the layout.

Now create the post amp module template using Save as: “PostAmpExport.xml” with device title: “Post Amp on module”, clearing all of the components, with a Smallest BC feature to retain of 100, and adding 2 layers from the file “TypeModule.xml” as before.

4. PSAL Device

The final component needed for our T/R Module example is a simple template used to represent a uniform heat source of some kind. (The name “PSAL” implies that it could be a phase shifter, attenuator, and/or limiter in our T/R Module.) Begin by copying the MesaResistorS.xml template from the SYMMIC examples folder into the working directory, and renaming it SimpleDevice1.xml. Then open it in SYMMIC and set the Geometry... and Heat Generation... parameters as follows.

Save the changes by using Save device as..., leaving the file name unchanged, but changing the Template Title to “Simple Device 1”.

Next select Create layout... from the File menu, and skip the array wizard. Set the Template Title to “Simple Device 1” and change the Meshsize to 100. Leave the rest of the fields as they are and click OK. (If the Length and Width fields are zero, the layout is sized just to the minimum required to include the device.) Now select Save layout as... and give it the File Name: “PSAL.xml”.

Now create a module template for this device using Save as: “PSALexport.xml” with device title: “Simple Device on module”, clearing the one component, and adding 2 layers from the file “TypeModule.xml” as before. The Smallest BC feature to retain is irrelevant in this case since there is only one rectangular BC.

Create a layout for the T/R Module

Now that all the components for the T/R Module have been created, we are almost ready to create the module layout. The only remaining task is to modify the module size and check the mesh sizes of the new templates. First open DriverAmpExport.xml. In Module Geometry... change the parameter Left Module Offset to 50, then save the device. Note that the Export feature sets the module offsets to 200 µm by default; if one wants to place two exported devices closer together than 400 µm, then one or more offsets need to be changed. For this template, in Meshing... the Smallest Element Size is 14 microns, which is fine.

Next open PowerAmpExport.xml. In Module Geometry... change the parameters Back Module Offset and Front Module Offset to 50. In Meshing... change the Smallest Element Size to 15, then save the device.

Finally, open PSALexport.xml. In Module Geometry... change the parameter Right Module Offset to 50. In Meshing... change the Smallest Element Size to 15, then save the device.

Now we are ready to create the T/R Module layout. To begin, open PSALexport.xml (if necessary), and compare your display with Figure 6. Then select Create layout... skip the array wizard and enter the following information. The (X,Y) locations are for the bottom left template corner (i.e. don't select Center at).

Title is T/R Module, Length = 20000 µm, Width = 9700 µm, Meshsize = 200 µm

Device Template File 6 is PSALexport.xml, (X,Y) = (14500,820), Rotation = 90° CW

Figure 6. Exported PSAL device after changing the Right Module Offset to 50.

Click OK to exit the dialog and load the module components. Since the module device type file was used for the appended layers, their properties don't need changing. Select Components... in the Device menu. If device 1 is currently selected, then the components list should be as shown in Figure 7.

If no changes are needed, then just save the layout: select Save layout as... and use the File Name: “TR_Module.xml”. (If any changes were made to the devices, then save them first.) The T/R Module geometry should now appear as in Figure 8. Note that the two power amps have SiC substrates, whereas the rest of the module items use GaAs.

Solve the T/R Module

The T/R Module is now ready to solve. Before doing so, you may want to change the power to one or more of the devices. To do that, first select the device from the menu Device > Select device > (choose one) and then select the Heat Generation... parameters. Each (exported) device has a parameter called Percent Power. If the exported MMIC contained more than one device, then each device will have its own parameter (e.g. d1 Percent Power). Every heat flux area on that device is controlled by this parameter, so if one wanted to turn a FET on an exported MMIC off, the d# Percent Power for that device would be set to zero. For this example, all of the devices will be left at the default setting of 100%.

Before running the simulation, check the solver settings by selecting Configure run > Solver settings... from the Solve menu. Enable the solver nonlinear iterations, as well as the “Repeat superposition” item (unless you are willing to exchange high accuracy for a reduction of run time by about one-half). Check that RC calculations are not enabled in Solve > Configure run > RC calcuations... Finally, save all the devices again (to update their solver settings) before running.

Now run the simulation, which should take under minute, depending on your computer, and produce the image in Figure 9. In this case, since the bottom of the module was set to 300 K, the components do not heat each other up significantly. This can be seen in a left view of the max temperature plot in Figure 10 (Results > Plot max temperature, View > Left view, Results > Set plot axes > Show axis > Frame). If the Power Amps were solved in isolation, then the max temperature of each Stage2FET would be the same. As it is, they differ by 1 ºC. Please note that these peak temperatures ARE NOT the actual peak power amp FET temperatures, since all of the FET structure has been removed. What is correct is the temperatures in the module layers, since the heat fluxes out of each MMIC are preserved during export. In order to find the actual peak temperatures in a FET, one needs to return to the MMIC containing that FET and solve it with the bottom boundary condition taken from the solution of the entire module, as will be demonstrated.

This concludes the export example, per se, but often one might want to use the module solution to provide an accurate boundary condition for one of the MMICs on the module, as mentioned earlier. In the next section we will do just that for a power amp.

Extract Boundary Conditions for MMICs

The PowerAmp.xml MMIC layout requires a boundary condition on its bottom face, which can consist of either a film coefficient and sink temperature or a temperature distribution from a file. The boundary condition extraction feature of SYMMIC will provide both of those options, as explained in Chapter 4: Extracting and Using Boundary Conditions.

To obtain the boundary conditions for every exported MMIC (layout) in the T/R Module, first orient the module so that all the MMICs are visible (since a screen shot will be taken), then simply select Solve > Boundary conditions > Extract at exports... After several seconds, a dialog box should come up asking for a file name; name it TR_ModuleBCs.htm. The HTML report produced should be as follows.

Boundary Conditions Calculated

Boundary #1:

Boundary #2:

Boundary #3:

Boundary #4:

Boundary #5:

Boundary #6:

Boundary conditions are provided for all of the exported layouts. For this example we will demonstrate the procedure of using an extracted boundary condition for one of the power amps. The lower (or more central) of the two power amps is Layout #3. It has a temperature range of 305.5 – 318.2 K, and its boundary condition temperature distribution file is named TR_Module.bc3.

To run with the new boundary condition, open PowerAmp.xml and select Solve > Boundary conditions > Use temperature file... to bring up the Apply Temperature File dialog. Enable the temperature boundary condition file and enter its name, as shown below.

Run the simulation. This is a much larger simulation than the T/R Module, because of the fine mesh surrounding the gates. It should take less than 20 minutes to solve, depending on the computer speed.

Once the solution has been obtained, one can see that the temperature boundary condition has been applied by comparing the bottom of the MMIC with that same surface in the solution from which the boundary condition was obtained: Z equals 750 µm under Layout #3 in the T/R Module, in this case. In Figure 12 we compare those two cases with a solution obtained using an average heat transfer coefficient.

Figure 12. Bottom view of the center Power Amp in the T/R Module (left), from a solution of the Power Amp alone using the boundary condition temperature file (center), and from a solution of the Power Amp using an average heat transfer coefficient boundary condition (right) (h = 3.14E-7 W/μm²-C, T = 300 K).

Note how the presence of the adjacent Power Amp pulls the hot spot away from the center of the MMIC. If the average heat transfer coefficient is used instead (right), the hot spot is centered on the FETs and the bottom temperature range does not match that of the module solution. The temperature range with the heat transfer coefficient is 312.8 – 316.7 K, with a simple average of 314.8 K (which is the same as the area-weighted average from the T/R Module solution). So with an average heat transfer coefficient as the boundary condition, the total heat flux and average temperature will match, but the temperature range can differ considerably. If the boundary condition temperature file is used instead, the results match exactly. Therefore the boundary condition temperature file should be used whenever possible.

Export the T/R Module for the LRU

The Export feature can be used recursively. In other words, there is no limit to how many times a device can be exported. For the current example, we will export the entire T/R Module for use in an LRU layout.

In creating the exported devices for the T/R Module, a device type file called “TypeModule.xml” was used. As described in the Exporting Layouts section of Chapter 4, the device type file is a simple way to specify the layers and boundary conditions to be appended on export. Before making use of the TypeLRU.xml template, open it in a text editor and take a look at the boundary conditions specified.

Three film (heat transfer) boundary conditions are specified, one for the left side of each layer. The values for “h” and “temperature” will be replaced by parameters during the export process, so they are not important. What one may need to change, though, is where the boundary conditions are applied. For example, if the LRU were connected to a cold plate on three sides (left, front and back), then the boundary conditions in TypeLRU.xml should read as follows.

Go ahead and make this change, and then save the new type file as TypeLRU1.xml in the working directory. One advantage of cooling the LRU from three sides will be lower average temperatures, but there is a significant disadvantage as well, as will be seen.

Now open TR_Module.xml in SYMMIC and export it with all three layers of the new LRU type file TypeLRU1.xml. Save the exported device as TR_ModuleExport1.xml with device title: “T/R Module on LRU1”. This time, keep all of the components and leave the BC resolution set at -1, since we don't want any additional simplification. In the exported device, set the Back Module Offset to 100 µm.

Two other devices are needed for our representative LRU model, both of them derived from the simple resistor model already used for the T/R Module. For the first one, open MesaResistorS.xml in SYMMIC and change the following parameters.

Save device as: “MesaResistorS4.xml” with the title: “7x7 15W Mesa Resistor”. Now place it in a layout by itself so that it can be exported, using File > Create layout... Leave the settings at their defaults, except for the Title: “7x7 15W Mesa Resistor”, and save the layout as “MesaResistorS4L.xml”. Now export this layout with all three layers of the new LRU type file TypeLRU1.xml as before. Clear the 1 component, then save the exported device as MesaResistorS4LExport1.xml with device title: “7x7 15W Mesa Resistor on LRU1”. The Smallest BC feature to retain is irrelevant since there is only 1 BC.

For the second simple device, open MesaResistorS.xml in SYMMIC and change the following parameters.

Save (this) device as MesaResistorS5.xml with the title “19x8.7 40W Mesa Resistor”. Now place it in a layout by itself so that it can be exported, using File > Create layout... Leave the settings at their defaults, except for the Title: “19x8.7 40W Mesa Resistor”, and save the layout as “MesaResistorS5L.xml”. Now export this layout with all three layers of the new LRU type file TypeLRU1.xml as before. Clear the 1 component, then save the exported device as MesaResistorS5LExport1.xml with the device title: “19x8.7 40W Mesa Resistor on LRU1”.

Now all the needed devices have been created and the LRU layout can be made. The LRU for this example has the following layout parameters. Title is “LRU with 3 Sides Cooled”, Length = 70000 µm, Width = 46000 µm, Meshsize = 1000 µm.

Figure 13. The example LRU geometry, containing four T/R Modules and five other (simple) heat sources.

Before running the simulation, set the Device > Mesh... parameters for every device. For all, set the one or two Appended Layers Mesh Size(s) to 500. In addition, for the T/R Modules, set the Smallest Element Size to 100, and for all the Mesa Resistors set the Smallest Element Size to 300. The other mesh parameters should not need to be changed from their default values: Mesh Bias in X and Y = 1.5; Mesh Refinement = 1.0. The default boundary condition temperature on the three sides of the LRU is 300 K, which this example uses. Check also that the Solve > Configure run > Solver settings... are as desired, then save all the devices (File > Save device(s)). Run the simulation, which will take about 5 to 15 minutes. The meshed result is shown in Figure 14. (To show the mesh lines, enable Settings > Mesh lines.)

Note that the Internal Mesh Size of 100 is probably the maximum value that one should use for the T/R Modules, in order to maintain sufficient resolution. For the other devices, however, even coarser meshes (than the 300 used) would be sufficient.

Next, turn off the mesh lines and plot the maximum temperatures (Results > Plot max. temperature). As seen in Figure 15, although the majority of the LRU surface is near the boundary condition temperature (300 K), a problem with front and back edge cooling is that the power amps are at different temperatures. To quantify this, return to the device view and extract the boundary conditions (Solve > Boundary conditions > Extract at exports...). Name the BC's report file as LRU1-BCs.htm. The boundary conditions are numbered as shown in Figure 16. Compare, for example, the temperatures seen for Power Amp #10 (center) with Power Amp #22 (back edge). They differ by 12 – 19 ºC.

Figure 15. Left view of maximum temperatures plot for the LRU with 3 sides cooled.

Figure 16. Numbered boundary conditions extracted from the LRU with 3 sides cooled.

If the original boundary conditions in TypeLRU.xml had been used – left side cooling only – then the LRU average temperatures would be higher, but the temperatures under each Power Amp would be very similar. The solution is shown in Figure 17. Average Power Amp temperatures range from 414 – 418 K.

To run with a temperature boundary condition file from the LRU, such as #10, open PowerAmp.xml and select Solve > Boundary conditions > Use temperature file... to bring up the Apply Temperature File dialog. Enable using the boundary condition file and enter its name, e.g. “LRU1.bc10”. Now save the devices with their current names, and save the layout file as PowerAmp-bc10.xml so that any previous solutions won't be overwritten. Running the simulation calculates a maximum temperature of 589 K for the Power Amp.

This completes the example of using the Export feature to model subsystems such as T/R Modules and LRUs, but there are other uses for Export as well. For example, Export can be used to create subsystems for analysis for which the details of the FET structures in the MMICs are not of interest, such as in the design of thermal management schemes for the LRU. A summary of the requirements and limitations of the export feature is given in the Device Template File Format chapter.

1 Layout files are easy to determine, either by (a) opening the file with a text editor and looking at the “DOCTYPE” parameter on the second line – it will either be “Template” (stand-alone) or “MMIC” (layout), or by (b) opening the file with SYMMIC and looking at the File menu – it will either have the item Create layout... (stand-alone) or Edit layout... (layout).

Boundary Size = 300 µm	Mesa Length = 1600 µm
Mesa Width = 2000 µm	ON Power = 4.5 W

Tavg	302.543	K
Tmin	301.022	K
Tmax	303.278	K

Tavg	303.857	K
Tmin	302.047	K
Tmax	304.580	K

Tavg	314.770	K
Tmin	305.517	K
Tmax	318.220	K

Tavg	314.738	K
Tmin	305.613	K
Tmax	318.173	K

Boundary Size = 500 µm	Mesa Length = 6000 µm
Mesa Width = 6000 µm	ON Power = 15 W

Boundary Size = 500 µm	Mesa Length = 19000 µm
Mesa Width = 8700 µm	ON Power = 40 W

BC #10 Tmin = 343.8 K	BC #22 Tmin = 324.4 K
BC #10 Tmax = 357.5 K	BC #22 Tmax = 345.0 K

Tavg	304.188	K
Tmin	302.124	K
Tmax	305.005	K

Tavg	302.341	K
Tmin	300.969	K
Tmax	302.917	K

Use of Export to Model T/R Module and LRU