Trace Layer Import for Printed Circuit Boards Under Icepak

Transcription

1 Tutorial 13. Trace Layer Import for Printed Circuit Boards Under Icepak Introduction: A printed circuit board (PCB) is generally a multi-layered board made of dielectric material and several layers of traces. From the thermal modeling point of view, a PCB may be treated as a homogeneous material with bi-directional thermal conductivity, i.e. thermal conductivity value is different in the normal-toplane direction than that of in-plane. This approach is reasonable as long as the trace distribution is more-or-less uniform in any given layer. However, with the continuing challenges to increase product functionality while decreasing product size, designers are compelled to place more and more functionality on individual PCB s. As PCB s become more densely populated, their trace layers are becoming more non-uniform and it is prudent to use locally varying thermal conductivity information on the board. Conducting a computational heat transfer simulation for each individual layer is costly and impractical for a system level model. In Icepak, it is possible to import trace layout of the board and compute locally varying orthotropic conductivity (k x, k y, and k z ) on the board using a background mesh size. The supported file formats are (1) MCM and BRD files and (2) Gerber files (.grb/.art/.pho) created using Cadence, Synopsys, Zuken, and Mentor. Please note Gerber files import option is only available on Windows platform and needs the artwork license feature. Further to be able to import MCM/BRD files the user needs to have Cadence Allegro installed. In this tutorial, we will show : How to import trace layout of a typical PCB in BRD format and solve two sample cases based on the trace layout information. How to use trace layers separately option for better accuracy. How to import Gerber format layer and via files. Prerequisites: This tutorial assumes that you are familiar with the menu structure in Icepak and that you have solved or read Tutorial 1. Some steps in the setup and solution procedure will not be shown explicitly. Problem Description: A PCB board, library files and traces are imported to create the model. The model is solved for conduction only, without the components and then solved using the actual components with forced convection. c Fluent Inc. October 31,

2 Trace Layer Import for Printed Circuit Boards Under Icepak Import Trace Layout of a Typical PCB in BRD Format Section 13.1: Importing Board and Library Files Section 13.2: Importing Trace Files 13.1 Importing Board and Library Files 1. Start Icepak, as described in Chapter 1 of the User s Guide. When Icepak starts, the New/existing panel will open automatically. 2. Click New in the New/existing panel to start a new Icepak project. The New project panel will appear. 3. Specify a name for your project and click Create. (a) In the Project text box, enter the name Test. (b) Click Create. To build the model, you will first import the board layout. The board and the associated library files have to be chosen at this step and the trace file can be imported later. File Import IDF file 1. In the IDF import panel, select the board (A1.bdf). You can keep the default project name A1 and click on Next. The associated library files will be imported automatically c Fluent Inc. October 31, 2007

3 13.1 Importing Board and Library Files 2. Select Next to see your board layout options. Keep Detail for the Import type, XY for the board plane and Rectangular for the board shape. Since we import the trace information later, we don t need to edit the board properties at this time. 3. Select Next to see the filtering options. Click Import all components. c Fluent Inc. October 31,

4 Trace Layer Import for Printed Circuit Boards Under Icepak You can filter certain components at this step by their size and power information, i.e. you can ignore the small components or the ones dissipating low power. We will import all of the components in this tutorial. 4. Select Next to see the Component models panel. If you have thin components on your board, they can be modeled as 2D sources. In this tutorial, we would like to model all the components as rectangular blocks. 5. Select Model all components as and select 3d blocks from the drop-down menu. If you click Next, you will go to the Miscellaneous options panel where you can specify the naming and monitor options. Just keep the default options and click Finish to start importing the files. This will take some time depending on the speed of your machine c Fluent Inc. October 31, 2007

5 13.2 Importing Trace Files You have learned how to import board and library files and in general you can import any IDF file by using the procedure above Importing Trace Files The next step in building the model is to import the trace files. A pre-built board model named A11, see Figure 13.1 will be used to demonstrate the trace file import. This pre-built model was extracted from the previous board file (A11.brd), a number of small components were removed and a non-conformal assembly was formed. 1. Unpack A11.tzr file to your desktop and name the project A11. As mentioned earlier, the trace file (.brd or.mcm) can either be imported during the IDF file import or the trace layout information can be assigned to the board after importing the IDF file. 2. Right click BOARD OUTLINE.1 in the Model manager window and click Edit Object to display the board object panel. To import the trace layout, follow the procedures below. (a) Select the Properties tab and click on Traces. (Figure 13.2) (b) Select Import.BRD file and click A1.brd from the Trace file panel. This process may take a few minutes depending on the speed of your computer. c Fluent Inc. October 31,

6 Trace Layer Import for Printed Circuit Boards Under Icepak Figure 13.1: A11 Board Layout Figure 13.2: Traces button 13-6 c Fluent Inc. October 31, 2007

7 13.2 Importing Trace Files Figure 13.3: Importing Trace Layout and Editing Layer Information Figure 13.4: Vias Information c Fluent Inc. October 31,

8 Trace Layer Import for Printed Circuit Boards Under Icepak (c) Once the import process is completed, you can edit the layer information. Select Edit layers from the Traces panel (Figure 13.3) to enter the board layer information. The number of layers in the board will automatically be imported to Icepak and you will have to enter the thickness of each layer and the material type. In this tutorial, the metal layers are pure Cu and the dielectric layers are FR-4. (d) Enter the layer thickness as shown in Table 13.1 and choose 100 rows and columns. (e) By default, layers are lumped for each sub-grid, therefore, the model layers separately option is off. They can also be modeled separately which will be discussed later. (f) Via information (e.g., material, plating thickness, filled/un-filled, via diameter etc.) is imported automatically (Figure 13.4), keep the default settings. (g) Click Accept to save your settings. The background mesh matrix (rows and columns) is used to compute the orthotropic conductivity on the board. The values of k x, k y, and k z on each cell are determined by the local trace density and the direction. Icepak does not include the trace geometry in the physical model; however, the locally varying orthotropic conductivity is mapped from the background mesh to the physical model mesh. Once the trace file is imported and assigned to the board geometry, the trace layers are associated with the board and are moved (in translation and/or rotation) with the board object. Table 13.1: Thickness Information on the Board (Layer 1: Top, Layer 7: Bottom layers) Thickness(mm) Thickness(mm) Thickness(mm) Layer Layer Layer Layer Layer Layer Layer (h) Right click on the object BOARD OUTLINE.1 and choose Traces from the menu. You can view the traces in three different ways, i.e. single color, color by layer, or color by trace (Figure 13.5). (i) Select color by layer and you will see the board traces as shown in Figure Conduction Only Model (PCB Without the Components) Section 13.3: Boundary Conditions and Meshing 13-8 c Fluent Inc. October 31, 2007

9 13.2 Importing Trace Files Figure 13.5: Displaying Traces on the Board Figure 13.6: Trace Layout on the PCB with the Color by trace Option c Fluent Inc. October 31,

10 Trace Layer Import for Printed Circuit Boards Under Icepak Section 13.4: Solving the Model Section 13.5: Post-Processing the Results 13.3 Boundary Conditions and Meshing You will generate a mesh for each sample problem. First we will consider a board without any components. 1. Make all objects inactive except the BOARD OUTLINE.1 object. 2. Select the cabinet and auto scale it so that the size of the board and the cabinet will be identical. 3. Create two wall objects at the Min z and Max z sides of the cabinet. 4. Apply a constant heat flux boundary condition for the wall on the Max z side at a value of W/m2 and a fixed temperature boundary condition for the Min z side at the ambient temperature (20 C). The rest of the sides are insulated. The board will be simulated using conduction only model. 5. Open the Mesh control panel and choose max X, Y, Z sizes as 5, 3, and 0.05 mm respectively,with minimum gap being 1 mm in all three directions. 6. Click Accept change value checks and then click Generate mesh Solving the model 1. Since this is a conduction only model, toggle off the Flow button from the General setup tab. Problem Setup Basic parameters General Setup. 2. Turn the radiation off and keep all the other values as default. 3. Select Solution settings Basic settings and keep the default number of iterations and choose the temperature convergence criteria as 1e-20 and click Accept to close the panel. 4. Under Solution settings Advanced settings, choose W cycle for temperature. 5. Enter 1e-6 for both the Termination criterion and Residual reduction tolerance. 6. Select Double for the solver precision. 7. Click Solve Run Solution to start the solution c Fluent Inc. October 31, 2007

11 13.5 Post-processing the Results 13.5 Post-processing the Results 1. Once the model has converged, select Post Plane cut. 2. Choose Point and normal as the set position. 3. Specify the point (PX, PY, PZ) as (0, 0, ) and the normal (NX, NY, NZ) as (0, 0, 1). Figure 13.1: Temperature Distribution on the PCB (mid-plane) The mid-plane temperature distribution shows that the high temperature regions occur at the no-trace areas where the opposite is true for the low temperature regions, i.e. maximum trace concentration leads to low temperatures. This is expected as the copper content is directly proportional to the trace concentration. It is worth noting that if a compact or detailed PCB were used in lieu of the traced PCB, one would obtain a fixed temperature for the entire mid-plane and this fixed temperature would be different from the average temperature of the traced PCB on the same plane. The spatially varying non-uniform conductivity of the board can also be viewed during post processing. The conductivities in the three direction K X, K Y, and K Z are available as post processing variables with plane cuts and object faces. Figure 13.2 plots k x at the board mid-plane. In the present case, since we chose not to model the layers separately there will be no variation of the conductivities in the board-normal direction. c Fluent Inc. October 31,

12 Trace Layer Import for Printed Circuit Boards Under Icepak Figure 13.2: K X Distribution on the PCB (mid-plane) c Fluent Inc. October 31, 2007

13 13.6 Boundary Conditions and Meshing PCB With the Actual Components Under Forced Convection Section 13.6: Boundary Conditions and Meshing Section 13.7: Solving the Model Section 13.8: Post-processing the Results 13.6 Boundary Conditions and Meshing 1. In order to put the actual components back into the model, highlight all the components under the Inactive folder and drag them back into the Model folder. 2. Click on the Cabinet and auto scale it. 3. Assign a velocity of -1.5 m/s through the opening on the Max x side of the cabinet (the minus sign shows that the flow is in the negative x direction). While not shown here, the trace import feature has a number of advantages on the meshing side. It should be remembered that detailed PCB s cannot intersect nonconformal assemblies; however, there is no such limitation for block objects. Since a PCB is represented as a block in the case of importing traces, non-conformal assemblies can intersect it. (Depending on the number of internal layers and thicknesses, detailed PCB objects may produce a large mesh count due to a high number of cells in the board-normal direction possibly with a number of slender (high aspect ratio) cells, which reduces the mesh quality). 4. Open the Mesh control panel and choose X, Y, Z sizes as 9.5, 7 and 0.7 mm respectively. 5. Click on Accept change value checks and click Generate Mesh Solving the model 1. Since we have forced convection, toggle on the Flow button and choose turbulent for the flow regime in the General Setup panel. Problem Setup Basic parameters General Setup 2. Select Solve Settings Basic 3. Set the number of iterations to 200 and click Accept to close the panel. Keep the advanced settings as of the previous case. 4. Click Solve Run Solution under the Advanced tab and click Enable sequential solution of flow and energy equations and start the solution. c Fluent Inc. October 31,

14 Trace Layer Import for Printed Circuit Boards Under Icepak 13.8 Post-processing the Results To display contours of temperature on the board, follow the procedures below. 1. Once the model has converged, select Post Object Face and choose Max z side of the BOARD OUTLINE.1 object. 2. Turn on the show contours and click on Parameters button. 3. Keep the default selection of Temperature. 4. For Color levels, select This object from the drop-down list. This will show the temperature distribution at the top of the surface of the board (Figure 13.1). Two hot spots are identified underneath the high heat flux components. Figure 13.1: Top Surface Temperature Distribution: PCB With Imported Traces (100 x 100) in Forced Convection Using the Model Layers Separately Option Next we revisit the conduction only model with the difference being that this time all the metal layers are modeled separately and not lumped together in the thickness direction c Fluent Inc. October 31, 2007

15 13.8 Post-processing the Results Figure 13.2: Contact Resistance Plates for Meshing the Individual Layers Separately 1. Go to the Post menu and click on Load solution ID. Post Load solution ID 2. Select the solution ID corresponding to the model which has just the PCB without any components. 3. Display the Edit layers panel as discussed earlier and check the Model layers separately box and click Create/Update Meshing plates. To model each of the layers separately we need to ensure that there is at least one cell across each of the metal and dielectric layers at the correct locations in the board-normal direction. The Create/Update Meshing plates option automatically creates contact resistance plates in the plane of the board at the start and end locations of each metal layer. Theses dummy plates have zero thermal resistance and their sole purpose is to ensure proper mesh resolution within the board. If the layer thicknesses are modified after creation of these plates the Create/Update Meshing plates button will update the positions of these plates. Additionally, the Remove plates button can be used to delete the existing plates. Figure 13.2 shows the plates created for the tracing layers on this board. Now the model can be meshed again (same mesh settings as earlier) and solved with the exact same boundary conditions. The temperature distribution and conductivc Fluent Inc. October 31,

16 Trace Layer Import for Printed Circuit Boards Under Icepak ity profiles on the board can be viewed again during post processing to examine the effect of modeling the layers separately as compared to the previous case. Importing Gerber Files As mentioned earlier, this option is only available for Windows and need an add-on artwork license feature. In this section, we will just discuss the procedure to import Gerber layers and via files. Editing layers and simulation procedure will be the same for both the Gerber and BRD format files and will not be repeated here. 1. Open a new Icepak session and unpack the file A11.tzr. 2. Right click on the object BOARD OUTLINE.1 and select Edit Object from the menu. In order to import Gerber trace and via files. Under the Properties tab, click on the Traces button to display the Traces panel. Click on Import Gerber files in the Traces panel to display the Import Gerber files panel. Figure 13.3: The Import Gerber Files Panel (a) Click the Browse button in the Metal Layer Gerber Files panel to display the Metal layer file dialog. (b) Select a file or hold down the <CTRL> or <SHIFT> key to select BOTTOM.art, INT1.art, INT2.art, TOP.art and click Open to import files c Fluent Inc. October 31, 2007

17 13.8 Post-processing the Results (c) To change the order of Gerber files, use the up (Up) and down (Dn) buttons or to delete a file, select Delete. Put the files in the order as shown in Figure (d) Click the Browse button in the Via Files panel and select file V14.art. Define start layer as 1 and end layer as 4. (This is to define the starting and ending layers that those vias connect.) (e) Select Accept to import the files. Once the import process is completed, you can edit the layer information and do the necessary simulation following the procedure outlined in Section You may continue from step 2c in Section Summary: In this tutorial, you imported the board layout and trace files. Then you simulated the board using a conduction only model. Post-processing of this model, resulted in high temperature regions occurring at the no-trace areas and the opposite for the low temperature regions. Next, the components were put back into the model and simulated under forced convection. Additional Exercise: You can change the number of rows and columns to 7 x 7, 30 x 30 and 500 x 500 and rerun the model. The results will almost be identical in the case of the last two arrangements. As another case study, the BOARD OUTLINE.1 object can also be replaced with a detailed PCB object with the same dimensions and the results can be compared. c Fluent Inc. October 31,

18 Trace Layer Import for Printed Circuit Boards Under Icepak c Fluent Inc. October 31, 2007