# Reliability Prediction of Mixed-Signal Module Based on Multi-Stress Field Failure Mechanisms
## 5 Electro-Thermo-Mechanical Multi-Physics Coupling Analysis
### Mechanical Simulation of the Communication Module Based on Temperature Distribution  
Based on the temperature distribution obtained from the thermal simulation, a mechanical simulation model for the communication module is developed using ANSYS Mechanical 2023 to calculate the stress and strain on the FPGA and SRAM solder joints.  
In the ANSYS Workbench, a mechanical simulation model of the communication module is created, including the construction of the 3D model, temperature analysis, boundary conditions and material property settings, and model meshing. Then, thermal loads are applied based on the temperature data. Since thermal simulation significantly impacts the mechanical analysis, the model and temperature results are imported into the transient analysis module, where boundary conditions and material properties, including the density, Young's modulus, Poisson's ratio, etc., are set. The focus is on analyzing the stress and strain on the solder joints, and the Anand model is used to describe the material behavior of SAC305 solder joints.  
After establishing the mechanical simulation model for the communication module, a transient analysis is performed. The focus of the analysis is on the Von-Mises equivalent stress in the FPGA and SRAM solder joints. The results in Figure 18 show that the maximum stress in the FPGA solder joints is 12.737 MPa, with higher stress at the edges than at the center. The maximum stress in the SRAM solder joints is 14 Mpa, with inner stress greater than outer stress. The simulation results, considering the chip power consumption distribution, are more complex and accurate compared to those from standalone mechanical simulations.  
Figure 17: Temperature variation curve of FPGA solder joints and SRAM solder joints.  
Figure 18: Equivalent stress cloud map of FPGA and SRAM solder joints.  
The solder joint with the highest stress in the FPGA and SRAM is selected as the critical solder joint. Assuming that the failure of this solder joint leads to device failure, the failure of the critical solder joint is used to approximate the failure of all solder joints. When evaluating the critical solder joint, the curve of equivalent plastic strain versus time is plotted, as shown in Figure 19. Using the Anand constitutive model, the simulation results for equivalent plastic strain include both plastic strain and creep.  
Figure 19 shows that the equivalent plastic strain of the critical FPGA solder joint changes in a stepwise manner, ultimately accumulating to about 0.00111. The equivalent plastic strain of the critical SRAM solder joint fluctuates periodically, with a variation amplitude of approximately 0.0006106. These trends are consistent with the thermal cycling characteristics of the spacecraft environment, validating the temperature cycle assumptions. The different trends in the equivalent plastic strain of the critical FPGA and SRAM solder joints may be due to the dominance of plastic strain in the FPGA solder joints, while creep dominates in the SRAM solder joints, causing partial strain recovery.  
This demonstrates that, under high dynamic conditions, such as those experienced by the spacecraft, rapid changes in power consumption can cause transient variations in the temperature field and mechanical stress. This model's transient simulation verifies the cumulative effects of stress and plastic strain at different stages. Compared to traditional models that rely on steady-state or quasi-static assumptions, this method provides a more accurate evaluation of transient stress peaks and their cumulative damage.