Steady-State and Transient Thermal Analysis of a Circuit Board

Transcription

1 Steady-State and Transient Thermal Analysis of a Circuit Board Problem Description The circuit board shown below includes three chips that produce heat during normal operation. One chip stays energized as long as power is applied to the board, and two others energize and de-energize periodically at different times and for different durations. A steady-state thermal analysis and transient thermal analysis are used to study the resulting temperatures caused by the heat developed in these chips. Features Illustrated Linked analyses Attaching geometry Model manipulation Mesh method and sizing controls Constant and time-varying loads Solving Time-history results Result probes Charts 1. Create analysis system. You need to establish a transient thermal analysis that is linked to a steady-state thermal analysis. a. Start ANSYS Workbench. b. From the Toolbox, drag a Steady-State Thermal system onto the Project Schematic. c. From the Toolbox, drag a Transient Thermal system onto the Steady-State Thermal system such that cells 2, 3, 4, and 6 are highlighted in red. d. d. Release the mouse button to define the linked analysis system.

2 2. Attach geometry. a. In the Steady-State Thermal schematic, right-click the Geometry cell, and then choose Import Geometry. b. Browse to open the file BoardWithChips.x_t. This file is included in the tutorial input file download. 3. Continue preparing the analysis in the Mechanical Application. a. In the Steady-State Thermal schematic, right-click the Model cell, and then choose Edit... The Mechanical Application opens and displays the model. b. For convenience, use the Rotate toolbar button to manipulate the model so it displays as shown below. Note You can perform the same model manipulations by holding down the mouse wheel or middle button while dragging the mouse. c. From the main menu, choose Units> Metric (m, kg, N, s, V, A). 4. Set mesh controls and generate mesh. Setting a specific mesh method control and mesh sizing controls will ensure a good quality mesh. Mesh Method: a. Right-click Mesh in the tree and choose Insert> Method. b. Select all bodies by choosing Edit> Select All from the toolbar, then clicking the Apply button in the Details view. c. In the Details view, set Method to Hex Dominant, and Free Face Mesh Type to All Quad. Mesh Body Sizing Board Components: a. Right-click Mesh in the tree and choose Insert> Sizing.

3 b. Select all bodies except the board by first enabling the Body selection toolbar button, then holding the Ctrl keyboard button and clicking on the 15 individual bodies. Click the Apply button in the Details view when you are done selecting the bodies. c. Change Element Size from Default to m. Mesh Body Sizing Board: a. Right-click Mesh in the tree and choose Insert> Sizing. b. Select the board only and change Element Size from Default to m. Generate Mesh: Right-click Mesh in the tree and choose Generate Mesh. 5. Apply internal heat generation load to chip. The chip on the board that is constantly energized represents an internal heat generation load of 5e7 W/m3. a. Select the chip shown below by first enabling the Body selection toolbar button, then clicking on the chip. b. Right-click Steady-State Thermal in the tree and choose Insert> Internal Heat Generation. c. Type 5e7 in the Magnitude field and press Enter. General items to note: The applied loads are shown using color coded labels in the graphics. Time is used even in a steady-state thermal analysis. The default end time of the analysis is 1 second.

4 In a steady-state thermal analysis, the loads are ramped from zero. You can edit the table of load vs. time to modify the load behavior. You can also type in expressions that are functions of time for loads. 6. Apply a convection load to the entire circuit board. The entire circuit board is subjected to a convection load representing Stagnant Air - Simplified Case. a. Select all bodies by choosing Edit> Select All. b. Choose Convection from the Environment toolbar. c. Import temperature dependent convection coefficient and choose Stagnant Air - Simplified Case. Note that the Ambient Temperature defaults to 22oC. i. Click the flyout menu in the Film Coefficient field and choose Import... (adjacent to the thermometer icon). ii. Click the radio button for Stagnant Air - Simplified Case, then click OK. 7. Prepare for a temperature result. The resulting temperature of the entire model will be reviewed. Right-click Solution in the tree under Steady-State Thermal and choose Insert> Thermal> Temperature. 8. Solve the steady-state thermal analysis. Choose Solve from the toolbar. 9. Review the temperature result. Highlight Temperature in the tree. You have completed the steady-state thermal analysis, which is the first part of the overall objective for this tutorial. You will perform the transient thermal analysis in the remaining steps. Items to note in preparation for the transient thermal analysis: If you highlight Initial Temperature under Transient Thermal in the tree, you will notice in the Details view, the read only displays of Initial Temperature and Initial Temperature Environment. In general, the initial temperature can be: Uniform Temperature - where you specify a temperature for all bodies in the structure at time =0, or Non-Uniform Temperature - (as in this example) where you import the temperature specification at time = 0 from a steady-state analysis. The initial temperature environment is from the steady-state thermal analysis that you just performed. By default the last set of results from the steady-state analysis will be used as the initial condition. You can specify a different set (different time point) if multiple result sets are available. 10. Specify a time duration for the transient analysis. A time duration of the transient study will be 200 seconds.

5 Under Transient Thermal, highlight the Analysis Settings object and enter 200 in either the Step End Time field in the Details view or in the End Time column in the Tabular Data window. Also note and accept the default initial, maximum, and minimum time step controls for this analysis. 11. Apply internal heat generation to simulate on/off switching on first chip. A chip on the board is energized between 20 and 40 seconds and represents an internal heat generation load of 5e7 W/m3 during this period. a. Select the chip shown below by first enabling the Body selection toolbar button, then clicking on the chip. b. Right-click Transient Thermal in the tree and choose Insert> Internal Heat Generation. c. Enter the following data in the Tabular Data window: Time = 0; Internal Heat Generation = 0 Note Enter each of the following sets of data in the row beneath the end time of 200 s. Time = 20; Internal Heat Generation = 0 Time = 20.1; Internal Heat Generation = 5e7 Time = 40; Internal Heat Generation = 5e7 Time = 40.1; Internal Heat Generation = 0 The Graph window reflects the data that you entered. General items to note: Loads can be specified as one of three types: - Constant remains constant throughout the time history of the transient. - Tabular (Time) (as in this example) define a table of load vs. time.

6 - Function enter a function such as =10*sin(time) to define a variation of load with respect to time. The function definition requires you to start with a = as the first character. 12. Apply internal heat generation to simulate on/off switching on second chip. Another chip on the board is energized between 60 and 70 seconds and represents an internal heat generation load of 1e8 W/m3 during this period. a. Select the chip shown below by first enabling the Body selection toolbar button, then clicking on the chip. b. Right-click Transient Thermal in the tree and choose Insert> Internal Heat Generation. c. Enter the following data in the Tabular Data window: Time = 0; Internal Heat Generation = 0 Note Enter each of the following sets of data in the row beneath the end time of 200 s. Time = 60; Internal Heat Generation = 0 Time = 60.1; Internal Heat Generation = 1e8 Time = 70; Internal Heat Generation = 1e8 Time = 70.1; Internal Heat Generation = 0 The Graph window reflects the data that you entered. 13. Prepare for a temperature result. The resulting temperature of the entire model will be reviewed. Right-click Solution in the tree under Transient Thermal and choose Insert> Thermal> Temperature. 14. Solve the transient thermal analysis. Click the right mouse button again on Solution and choose Solve. The solution is complete when green checks are displayed next to all of the objects. You can ignore the Warning message and click the Graph tab.

7 15. Review the time history of the temperature result for the entire model. Highlight the Temperature object. The time history of the temperature result for the entire model is evaluated and displayed. The Tabular Data window shows the min/max values of temperature at a time point. By moving the mouse, you can move the bar along the Graph as shown, to any time, click the right mouse button and Retrieve this Result to review the results at a particular time. You can also animate the solution. 16. Review the time history of the temperature result for each of the chips. Temperature probes are used to obtain temperatures at specific locations on the model. a. Right-click Solution and choose Insert> Probe> Temperature. b. Select the chip to which internal heat generation was applied in the steady state analysis and click the Apply button in the Details view. c. Follow the same procedure to insert two more probes for the two chips with internal heat generations in the transient thermal analysis. d. Right-click Solution or Temperature Probe and choose Evaluate All Results. 17. Plot probe results on a chart. a. Select the three temperature probes in the tree and select the New Chart and Table button from the toolbar. A Chart object is added to the tree. b. Right-click in the white space outside the chart in the Graph window and choose Show Legend.

8 c. In the Details view, you can change the X Axis variable as well as selectively omit data from being displayed. You have completed the transient thermal analysis and accomplished the second part of the overall objective for this tutorial.