Issue link: https://resources.randsim.com/i/1504833
10 Electrothermal Mechanical Stress Reference Design Flow for Printed Circuit Boards and Electronic Packages // / Stage 10: Transfer Temperatures from Icepak to the Board in Mechanical Essentially, at this point we can transfer temperatures for the entire board as well as its components and heat sinks. ANSYS Mechanical also allows you to choose exclusively only those objects of interest for which you want temperatures to be transferred through the built-in Icepak subsystem within Workbench. However, in our multiphysics problem, we included all the components and their heat sinks, as well as the board. The temperatures for the entire board and all its components and heat sinks are contained in the .loads file written out f rom Icepak. By way of this .loads file, all the temperature values are mapped onto the geometry in Mechanical. Figure 20 shows the temperature map once applied on the model in Mechanical. This temperature map is due to the natural convection (still air) thermal co-analysis f rom within SIwave and Icepak. So, in this way, the temperature field f rom the CFD analysis is applied on the model. See the temperature field mapped onto the board in Mechanical in Figure 20. / Stage 11: Simulate the Board for Thermal Stress, Deformation and Elastic Strain The heat generated by ICs and calculated f rom Joule heating will cause structural deformation. Equivalent stress, deformation and elastic strain due to Joule heating computed in Icepak, induced by the power loss in SIwave, can now be easily analyzed for the entire model in Mechanical. Shown below are figures illustrating the stress and deformation (exaggerated for visualization purposes) for the PCB model. Observe that the board tends to bow downward. Recall the principle of the bimetallic strip — a higher concentration of copper on the top layer causes the board to bow downward due to copper's higher coefficient of thermal expansion compared to FR4. See Figures 21 and 22 for the mechanical model solution producing warping shown by the deformation contours. Stress contours for the overall models are also shown. The elastic strain plot (Figure 23) helps us determine possible locations of delamination and fracture. For our board, maximum strain is under the microprocessor U100; the region is depicted by the max marker. This is where the IC may tend to pull away from the PCB. The heat sink along with the IC is treated as one lumped metal of aluminum which is fastened to the PCB underneath it. The PCB has two different materials (copper and FR4), which give rise to the high strain shown in the Figure 24. This is an approximation in which we have simplified the interface between the package and the board — the interface in actuality is achieved by solder balls and bumps. For this simplified analysis we used a basic cuboid to model the IC, and assigned it the same material (aluminum) as the heat sink. / Conclusion In reality the IC package has a more complicated shape made of plastic or ceramic, and is connected to the board with solder balls. You could perform a more detailed analysis including more realistic package geometry and materials along with models of the solder balls to further assess the robustness of the board. One possible way to perform a more detailed analysis would be to utilize sub modeling. This will help you understand whether the board might fail with additional results analyses, such as fatigue and f racture processing. For example, ANSYS solutions can simulate the influence and underlying physics of the flip-chip attachment process commonly used in the semiconductor industry; after the process is complete the IC assembly is subjected to thermal cycling to assess the cumulative damage in individual components. Stress, strain and deformation due to CTE mismatch can be determined at both the component and assembly level for solder bumps, solder balls, die, underfill and for the PCB. Figure 20. Icepak temperatures mapped onto the board in Mechanical Figure 21. Thermal-induced deformation in Mechanical Figure 22. Thermal-induced stress Figure 23. Elastic strain