CapeSym

Table of Contents

Extracting and Using Boundary Conditions

SYMMIC is intended to provide thermal analyses at all levels, from the gate details in a FET or MMIC, to modules, packaging and entire subsystems. As described in the previous section, SYMMIC can export any layout of devices into the device template format, thereby converting an assembly of devices into a sub-device that can be used in a new layout (as graphically depicted in the overview diagram). This can be done recursively as many times as necessary to create the subsystem desired (although the boundary conditions extraction will only trace back three levels). Typically, the desired subsystem is the smallest one in which the boundary conditions are known precisely. An example would be an LRU which is attached on one or more sides to a cold plate of known temperature.

Once the subsystem model has been created, SYMMIC can solve it (just like any other device or layout) and automatically extract precise boundary conditions for every layout (containing no exported devices) that was exported to make the subsystem. In other words, boundary conditions are automatically extracted for every layout composed of FETs and/or other devices that still contain all of the fine details (like the heated gate segments), since those are usually the layouts for which boundary conditions are desired. This is done by tracing back through the history information saved in each exported device template, and therefore it is necessary that all devices, layouts and exported devices be saved (and not moved from their original file locations before the boundary conditions are extracted).

To extract the boundary conditions for the exported layouts in a subsystem solution, select the menu item Solve > Boundary conditions > Extract at exports... SYMMIC will then find all the exported layouts (as described above), extract their boundary conditions, and generate an HTML-formatted report. A dialog box will appear asking for a file name for that report. (If no exported layouts are found, or if an error occurs, a message will be printed in the console window.) The HTML report will include a screenshot of the solution image, overlaid with numbers labeling each export boundary condition. Number labels are placed above the devices, and so will not be visible when a bottom view of the solution is shown or when a plot is displayed. Number labels are drawn in the wireframe color.

The boundary conditions report contains the following information.

1. Export history – showing the file sequence traced from the subsystem solution to the first export (or three levels, whichever comes first)

2. Location of the exported layout in the subsystem – X and Y ranges, and the bottom Z where the (slice) temperature boundary condition data is obtained

3. Boundary condition statistics – average heat flux (Q), and minimum, maximum and average temperatures at the above location

4. Average film boundary condition – the average heat transfer (h film) coefficient corresponding to the average flux and the (minimum) sink temperature (Tinf) in the subsystem solution

5. Boundary condition temperature file – containing the complete temperature distribution for SYMMIC to use in a simulation of the original, fine-detail layout before it was exported

The calculated average flux is dependent on the power parameters in the device template file and the percent power defined for the entire layout, so extracting boundary conditions on a loaded solution that was calculated with different values for these parameters may produce erroneous values for Q and h. The basic formulas are:

The boundary condition temperature files have the same name as the solved subsystem layout, but with “.bc#” extensions corresponding to the exported layout numbers. For example, extracting the boundary conditions from the solution to “layout3.xml” will generate “layout3.bc1” as the file with the temperatures on the bottom of the first MMIC. The first page of a sample boundary conditions report is given below.

Boundary Conditions Calculated for layout 3

Date: 2/17/2009 Time: 14:18:53

From the Steady State Solution:

Layout #1:

Export History:
E:\ProjectsRun\SYMMIC_Run\ExportBC\layout3.xml
- E:\ProjectsRun\SYMMIC_Run\ExportBC\layout2.xml
-- E:\ProjectsRun\SYMMIC_Run\ExportBC\layout1.xml
--- E:\ProjectsRun\SYMMIC_Run\ExportBC\XbandAmp.xml

X location = [2700,6500] µm
Y location = [700,3200] µm
Z location = 3500 µm

BC temperatures are saved in file E:\ProjectsRun\SYMMIC_Run\ExportBC\layout3.bc1

(Boundary conditions for layouts #2 – 7 are similar, and omitted for brevity.)

Two different boundary conditions are provided: (1) an average heat transfer coefficient (h film) and sink temperature (Tinf), and (2) a temperature distribution file (BC temperatures). Use of the temperatures file is much preferred over the heat transfer coefficient, as is shown in the export example in the Top-Down Analysis chapter, since it results in an exact match of the boundary temperatures. The average heat transfer coefficient is provided primarily as a measure of the heat transfer from the layout.

Once the boundary condition for a layout has been extracted, SYMMIC can use the temperature distribution file to precisely set the bottom temperatures on that layout. To use this feature, open the layout of interest and select Solve > Boundary conditions > Use temperature file... The following dialog will appear.

As shown above, the boundary condition file for the first layout above has been entered, and the Use boundary condition file check box selected. After pressing OK the simulation is ready to run. Note that this state and boundary condition file name will be stored in all of the device templates which are part of the current layout if File > Save device(s) is selected. Note also that the layout should remain the same size and with the same device positions as when it was exported in order to preserve the accuracy of the temperature boundary condition. Large changes to these parameters will not prevent the temperature boundary condition from being applied, but in some cases it may be wiser to use the average heat-transfer coefficient and sink temperature from the boundary condition extraction until all parameters are fixed and an updated subsystem solution and set of boundary conditions can be obtained.

In summary, to extract and use boundary conditions for the exported devices in a layout, do the following.

1. Build the desired subsystem by recursively exporting layouts to sub-devices, then laying out those sub-devices.

2. Solve the desired subsystem, which should have well-known boundary conditions.

3. Extract boundary conditions for all exported layouts by Solve > Boundary conditions > Extract at exports...

4. Return to the original, fine-detail layout of interest and use an extracted temperature file in place of the backside film boundary condition by Solve > Boundary conditions > Use temperature file...

Boundary Condition File Format

The file used to provide the temperature profile boundary condition consists of lines of ASCII text terminated by newline characters. Each line contains three floating point values separated by white space, representing an (x,y) location on the surface followed by the temperature in kelvin. The locations are listed in ascending order sorted by x, then by y. That is, the y values ascend most rapidly, restarting at each new x value. The lower-left corner of the surface as viewed from the top view is taken to be the origin (0,0).

The following partial listing demonstrates the format of the .bc file:

 0.000000000E+000  0.000000000E+000  3.419035842E+002
 0.000000000E+000  1.100000000E+002  3.431162468E+002
 0.000000000E+000  2.200000000E+002  3.439094690E+002
 :                 :                 :
 2.000000000E+003  2.611357500E+003  3.496611889E+002
 2.000000000E+003  2.739638611E+003  3.494740109E+002
 2.000000000E+003  2.899990000E+003  3.488572814E+002