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6 Electrothermal Mechanical Stress Reference Design Flow for Printed Circuit Boards and Electronic Packages // You cannot tell f rom the geometry alone where the current is going to concentrate, so the adaptive meshing allows the field solver to determine that and improve its accuracy. More accurate fields translate to more accurate DC resistance values, power dissipation and temperatures. The adaptive mesh refinement process in SIwave is iterative and based on a convergence criterion that monitors the change in the total power loss f rom one adaptive pass to the next. So, in each pass, the solver computes the total Ohmic power loss and compares it to the power loss in the previous pass. The difference is converted to a percentage and compared with the user-specified error tolerance. If it is smaller than the specified value, the solution is deemed to have reached convergence. This automated adaptive refinement technique guarantees an accurate solution with the fastest speed and minimum use of computational resources. To give you a sense of how fast our board is solved in SIwave, the DCIR simulation of our PCB converges in just 30 seconds. Note: For a quick thermal analysis, without detailed CFD modeling in Icepak, you can export the Joule power losses in the metal directly f rom SIwave to ANSYS Mechanical. This type of analysis does not consider the power losses of attached circuit components, such as ICs and resistors. In this case ANSYS Mechanical computes a solution to the heat conduction equations in the board using a simplified convection boundary condition at the exterior surfaces of the board. We recommend using our reference design flow when the greatest accuracy is required as it incorporates CFD analysis in Icepak f rom where the resulting temperatures are transferred to the board in Mechanical as well as for the components and the heat sinks. For the purposes of the multiphysics problem of this PCB, the authors considered the board as well as all the components (plus the heat sinks) in the analysis. We will configure a CFD analysis of the board and the ICs by coupling SIwave and Icepak in the next stage of our reference design flow. / Stage 4: Automated and Iterative Thermal Analysis Based on the DCIR analysis, SIwave also generates Icepak power maps for the board. The I 2 R power losses f rom SIwave serve as the starting point for the thermal analysis. During the Icepak Simulation Setup in SIwave, you can define the worst-case thermal design power for each component. The power dissipation values come f rom the manufacturer's data sheets. Enter these values for the ICs and voltage regulators. Optionally, include heat sinks for these components. Then choose the type of thermal simulation: forced convection or natural convection (still air). For our board, both types of thermal analyses were performed. When you launch this simulation, it invokes the Icepak solver directly f rom SIwave. The Icepak solver is launched in non-graphical mode. The PCB design and setup information gets transferred into Icepak along with DC power loss maps f rom the SIwave analysis. The Icepak analysis calculates the heat flow and temperatures across the board and for all the objects that were included in the simulation. For convenience, basic thermal analysis results can be displayed on SIwave as shown by the thermal data plots in Figure 11. On our board, hot spots develop around voltage regulators, and the temperature near the microprocessor increases. Figure 8. Current density plot Figure 9. Final mesh of the PCB

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