Thermal-Structural Coupling — CTE Mismatch & Weld Distortion

When is one-way coupling sufficient and when do you need two-way thermal-structural coupling?

One-way coupling (thermal analysis first, then import temperatures as structural loads) is sufficient in the vast majority of cases — deformation does not significantly change the temperature field. Two-way coupling is needed for thermoelastic damping in MEMS, large deformations that change convective areas, or ablation where material removal changes the thermal boundary.

What is CTE mismatch stress and where does it commonly occur in electronics?

When materials with different Coefficients of Thermal Expansion (CTE) are bonded together, temperature changes create differential expansion resisted at the interface, generating stress. Silicon (CTE 2.6 ppm/C) bonded to FR4 PCB (CTE 14 ppm/C) through solder joints is the classic example. The mismatch causes shear stress in the solder, leading to fatigue failure over thermal cycles.