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8 Electrothermal Mechanical Stress Reference Design Flow for Printed Circuit Boards and Electronic Packages // / Stage 6: Generate Temperature Profiles for the Board in Icepak The power dissipated within the PCB planes and traces defines Joule heating. This can impact the board adversely by introducing mechanical reliability issues. Because temperature variation across the board gives rise to structural problems, it makes sense to generate a temperature profile for the board and its components in Icepak f rom the CFD solution. Soon in this design flow, you will see that this temperature profile can easily be incorporated into the ANSYS Mechanical calculations for thermal stress, deformation and elastic strain on the board. Generating the temperature profile f rom Icepak is very easy. The CFD Post/ Mechanical Data menu option in Icepak writes out a file with the temperatures for the entire board and the ICs that were included in the thermal simulation. This file comes with a .loads extension. This temperature file acts as a thermal load within Mechanical during the structural stress analyses. A sample data set f rom the .loads file is shown in Figure 16. The columns represent the x, y and z coordinates with temperatures defined in Kelvin. The CFD Post/Mechanical data writes out this LOADS file in the SIwave and Icepak results folder. / Stage 7: PCB Preparation for Use in ANSYS Mechanical Localized temperature increases cause thermal expansion and stress that can significantly reduce product reliability. Before we perform the structural analysis, we need to prepare the PCB in ANSYS SpaceClaim Direct Modeler (SCDM). The PCB is imported into SCDM where it is prepped with the heat sinks and component outlines needed for our structural analysis. These heat sinks can be exported as .step files f rom Icepak. This technique for preparing the geometry is used because it is convenient to first develop a simplified layout topology version of the board separately since a true 3-D ECAD model in acis is undesirable due to performance limitations. The ECAD data will be imported later during the trace mapping step. Best practices show that mapping the ECAD data with a hexahedral mesh provides the best performance and accuracy, while meshing the heat sinks using a tetrahedral mesh provides the greatest accuracy within ANSYS Mechanical. Preparing this MCAD geometry is accomplished very easily within SCDM. The ODB++ directory of the board was archived in the form of a .tgz file by Altium Designer. A simple drag & drop operation of the .tgz file translates it into SCDM 3-D interface. The exported file f rom Icepak that contains all the heat sinks is then imported into SCDM for placement on the PCB. The heat sinks are combined in SCDM and will be treated as solid objects in Mechanical. The 3-D SCDM file (of the PCB and the corresponding heat sinks) eventually gets imported into Mechanical through the ANSYS Workbench platform. Figure 13. Temperature map Figure 14. Contour plot Figure 15. Vector plot Figure 16. Sample data set from. loads file

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