Power Cycling Life Prediction of 96.5Sn3Ag0.5Cu solder

Hi to all,

I'm trying to set up a FEM method in order to evaluate the number of cycles to failure for thermal fatigue in a power electronic device, composed by IGBTs and diodes.
The steps that I implemented are the following:
1- Input parameters: P_IGBT; P_diode; V_IGBT; V_diode.
2- Heat transfer in solids: Heat source 1: P_IGBT/V_IGBT; Heat source 2: P_diode/V_diode; T=25°C in baseplate of device;
3- Structural mechanics: Thermal expansion in all domains, with input temperature (ht) from the result of Heat transfer in solids; fixed constraint in baseplate;
4- Fatigue: based on strain; Selection of fatigue model: Smith-Watson-Topper criterion, elastic solution with intake restriction; field of solution: solid mechanics; fatigue parameters: sigma'f=276[MPa], b=-0.05; esp'f=0.325; c=-0.57; Young's modulus and Poisson's ratio from material; K'=304 [MPa]; n'= 0.086.
After the computation, I correcty visualize the maximum distribution temperature and von Mises stress, but I can't calculate the number of cycles to failure, in fact the expression ftg.ctf (or log10(ftg.ctf) returns to me the value 0.
Can you help me?
Best regards
Mario Nisi