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      <title>Writing an FE model for thermal analysis</title>
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      <div class="title_topic3" id="xps10_pagetitle">Writing an FE model for thermal analysis</div>
      <hr noshade="true">
      <p class="para_topic">Translating a PC Board assembly from ECAD format into an FE Model for thermal analysis is an automatic procedure. The work involved is in preparing for the write operation. However, once you have prepared a database of the components that are thermally significant, backed up by suitable defaults for undefined components, future translation of similar boards becomes an automatic procedure.</p>
      <p class="para_topic">The exported file is in Universal File Format, and can be imported into a simulation. The file contains the component and board thermal data in an FE model. When you export an FE Model, the universal file is created in the directory specified for Read/Write on the File Settings dialog box.</p>
      <div class="title_division"> Overview of the FE model</div>
      <p class="para_division">A basic understanding the thermal modeling techniques used by solvers NX Electronic System Cooling and NX Thermal and Flow will help you decide how to define the FE model. The entities and parameters needed for thermal analysis of the PC Board assembly are created automatically based on information you provide. The components and PC Board are meshed with 2-D finite elements which have material and physical properties defined in the thermal database file. Also in the thermal database file, components are assigned a heat load and the heat is conducted through a resistance to the PC Board by defining Thermal Couplings. The PC board is defined as a default flow surface, and its conduction properties are defined in the simulation.</p>
      <p class="para_division">The Options listed below apply to component heat loads defined in the Default Thermal Data file or the Thermal Database file.</p>
      <div class="title_division"> Options for modeling component heat loads</div>
      <p class="para_division">The <b class="uiTerm">Component Mesh Type</b> specifies how the NX mesher models the component, and how the component's heat load is coupled to the board. Three options are available
      </p>
      <ul>
         <li>
            <p class="para_item">None creates no mesh for the component, but rather spreads the heat load evenly over all the elements in the board. This option is appropriate for components with a small heat load, which may still be thermally significant if there are enough of these components in the assembly. </p>
            <p class="para_item"><img align="bottom" src="graphics/mesh_type_1.gif" border="0"></p>
         </li>
         <li>
            <p class="para_item"><em>Case</em> meshes the case of the component and couples the case mesh to the board mesh. 
            </p>
            <p class="para_item"><img align="bottom" src="graphics/mesh_type_2.gif" border="0"></p>
         </li>
         <li>
            <p class="para_item">Case + Junction meshes both the case and the junction of the component, then couples the junction mesh to the case mesh, and the case mesh to the board mesh. </p>
            <p class="para_item"><img align="bottom" src="graphics/mesh_type_3.gif" border="0"></p>
         </li>
      </ul>
      <p class="para_division">The option for Power Rating allows you to select  <em>Operational</em> or  <em>Maximum</em> power output for the component, provided both of these values are defined for a component in the Default Thermal Data file or the Thermal Database file. If you select an option that is undefined for that component, a warning is issued on FEM export.
      </p>
      <p class="para_division">The optional  <em>Multiplying Factor for Heat Loads</em> allows you to factor in environmental or other global variables that may affect overall heat generation.
      </p>
      <div class="title_division"> Specifying components and boards for thermal analysis</div>
      <p class="para_division">There are three ways to specify FE modeling options and thermal characteristics for components and boards in PCB.xchange:</p>
      <ol type="1" start="1">
         <li>
            <p class="para_item">In the Thermal Database, an XML file you create </p>
         </li>
         <li>
            <p class="para_item">In the  <em>Default Component Settings</em> dialog box 
            </p>
         </li>
         <li>
            <p class="para_item">In the Default Thermal Data file, an XML file you can modify </p>
         </li>
      </ol>
      <p class="para_division">Certain options and thermal characteristics can be specified using two or more of these methods. When this occurs, precedence is applied according to the sequence above. Thus, any value for a given option specified in the Default Thermal Data file is overridden by any value for the same option specified on the  <em>Default Component Settings</em> dialog box. Similarly, both these values are overridden by any value for the same option in the  <em>Thermal Database</em> file.
      </p>
      <p class="para_division">In addition to the three methods above, there is the Material Definition file, an XML file that defines materials in terms a material name and its associated material properties. These material names can be referenced in the Thermal Database file, the  <em>Default Thermal Data</em> file and on the Default Component Settings dialog box, but they are defined only in the  <em>Material Definition file</em>.
      </p>
      <p class="para_division">Summarizing these methods in tabular format we have:</p>
      <table border="3" cellpadding="3" width="100%" align="center">
         <tr>
            <th colspan="1" rowspan="1">
               <p class="para_th">Method</p>
            </th>
            <th colspan="1" rowspan="1">
               <p class="para_th">File Name</p>
            </th>
            <th colspan="1" rowspan="1">
               <p class="para_th">Function</p>
            </th>
            <th colspan="1" rowspan="1">
               <p class="para_th">Precedence</p>
            </th>
         </tr>
         <tr>
            <td colspan="1" rowspan="1">
               <p class="para_td">Thermal Database file</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">(as defined on Default Component Settings dialog box)</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">User-created file that specifies components indexed by reference designator, package name or part number</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">Precedence over Default Component/Board Settings dialog box and Default Thermal Data file</p>
            </td>
         </tr>
         <tr>
            <td colspan="1" rowspan="1">
               <p class="para_td">Default Component Settings dialog box</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">N.A.</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">Defines certain common component settings and applies them to the current session of PCB.xchange</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">Precedence over Default Thermal Data file</p>
            </td>
         </tr>
         <tr>
            <td colspan="1" rowspan="1">
               <p class="para_td">Default Board Settings dialog box</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">N.A.</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">Defines certain common board settings and applies them to the current session of PCB.xchange</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">Precedence over Default Thermal Data file</p>
            </td>
         </tr>
         <tr>
            <td colspan="1" rowspan="1">
               <p class="para_td">Default Thermal Data file</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">pcbx_ug_thermal.xml</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">Initialization file read at PCB.xchange start up to define default settings</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">N.A.</p>
            </td>
         </tr>
         <tr>
            <td colspan="1" rowspan="1">
               <p class="para_td">Material Definition file</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">pcbx_ug_mat.xml</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">Initialization file read at PCB.xchange start up to define materials referenced by the other methods.</p>
            </td>
            <td colspan="1" rowspan="1">
               <p class="para_td">N.A.</p>
            </td>
         </tr>
      </table><br><div class="title_division"> Creating local copies of the initialization files</div>
      <p class="para_division">When you start PCB.xchange, the application reads the  <em>Default Thermal Data</em> file <i class="fileName">pcbx_ug_thermal.xml</i> file and the Material Definition file <i class="fileName">pcbx_ug_mat.xml</i> stores the data in memory. These files are part of the installation and are normally left unchanged as an application default. If desired, the system administrator can modify them. However, as a user, you can copy these files and place them in the working directory or your home directory, where you can modify them to best serve the needs of a specific application. With these modifications you can adjust the default settings that appear in the dialog boxes, as well as certain defaults that are used by the software during FE model export.
      </p>
      <p class="para_division">The working directory is defined as the directory from which PCB.xchange is launched (typically the same directory from which NX is launched). The home directory is an environment variable defined by a system administrator for a specific user. PCB.xchange first looks for the initialization file in the working directory. If it does not find it, PCB.xchange looks in the user's home directory. If the file is not there, it uses the file in the PCB.xchange installation.</p>
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