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9 Electrothermal Mechanical Stress Reference Design Flow for Printed Circuit Boards and Electronic Packages // / Stage 8: Assemble the Project in ANSYS Workbench ANSYS tools offer a lot of f reedom in the order in which you can generate the temperature profile f rom Icepak, prepare the geometrical model in SCDM and generate the ECAD data file. They also give you flexibility regarding how these "subsystems" are applied to the PCB in Mechanical for multiphysics analysis. This multiphysics analysis is made possible by using ANSYS Workbench, a f ramework for assembling all the subsystems and linking the temperatures and the ECAD data onto the board in Mechanical. Build the project in Workbench as shown in Figure 17 — the Icepak subsystem in the figure points to the .loads file that contains all the temperature data for the board, components and heat sinks. The External Data component subsystem points to the original .tgz file of the ODB++ directory. The Geometry cell in the Static Structural subsystem is used to import the model in Mechanical f rom SCDM. Each time you use the Edit cell of the model, Workbench re-reads the upstream data and accordingly updates the project in Mechanical. Once the board is imported into Mechanical, ensure that there are as many contacts as there are heat sinks. Gather the required materials f rom the Engineering Data cell; in this case, we add Aluminum Alloy 6061, Copper Alloy and FR4. In Mechanical, the heat sinks are assigned Aluminum Alloy 6061 and the PCB layers are assigned FR4 as their material property. To assign copper for the stackup layers, we need a mesh. This is because we need to know the proportion of metal to non-metal on each layer of the PCB for mapping the layer metallization (ECAD trace) data to the board (which happens by linking the board in Mechanical to the archived .tgz file after generating a mesh). / Stage 9: Import Layer Metallization on PCB with "Trace Mapping Technique" ANSYS Mechanical produces a uniform mesh of hexahedral elements in the board. All the 3-D objects (ICs and components) and heat sinks are represented by a tetrahedral mesh. However, the board is meshed with hexahedral elements to exploit ANSYS Mechanical's metal f raction assignment technique as it works best with a hex mesh. After generating the mesh, material for the metal f raction on each layer is defined as copper. The cross-section of the PCB is also automatically taken into account for calculating the metal f raction. In the metal f raction assignment technique, Mechanical automatically computes the effective material properties on an MCAD mesh based on the spatial distribution of metal and dielectric in the ECAD model.1 In this technique, ANSYS Mechanical generates a point cloud representation of the internal details of the entire board on a rectangular grid. Each cell is divided into sampling points, which represent a mixture of metal and dielectric materials. They are assigned onto the finite element mesh as data points. Using these data points, Mechanical assigns an effective material property to each mesh element by computing a weighted average of the metal and dielectric. As it assigns the property f rom element to element, this eventually develops into a spatially varying metal f raction on a large scale. This metal f raction can be actually seen once it's imported onto the mesh. A metal f raction of 1 means the element is all metal. A metal f raction of 0.5 means the element is half metal and half dielectric. When using actual geometry for traces and vias, computing the thermal induced stress is a daunting task due to the enormous geometrical complexity of ECAD data. This combination of hexahedral mesh with trace mapped metal f raction for the board and tetrahedral mesh with homogenous material for the objects and heat sinks is well-suited for performing structural simulation of ECAD in Mechanical. This is enabled by forming 3-D layered solid parts and mapping the layer metallization data onto the finite element mesh. The imported trace metal on the board is shown in Figure 18. Metal f ractions on the board with and without heat sinks are shown in Figures 18 and 19. Regions in red represent higher copper density while those in blue are considered to be dielectrics. Figure 17. Project in Workbench Figure 18. Imported layer metal without heat sinks Figure 19. Imported layer metal with heat sinks

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