In thermal simulation set up, what are parameters 'time step factor' and 'critical time step'?
Have you found an answer to your question already? If not, I would encourage you to read the corresponding sections in Sim4Life's user manual:
2.8.2 Thermal Simulation Settings
126.96.36.199.2 Numerical Stability (this is in the FDTD section, but the same criteria apply in the thermal solver)