Solver for viscoelastic problems

Hello,
Does any one have information about what is the solver used for viscoelastic problems? Since it involves time, is it explicit, implicit or something like coupled explicit - implicit (e.g. Newmark + Newton)… Or can one choose?.
Cheers,
/J.

Maybe someone will still find this useful. According to Dr. Dhondt’s book “The Finite Element Method for Three-Dimensional Thermomechanical Applications”, which is basically a theory guide for CalculiX, it seems that this software uses implicit time integration (backward Euler scheme) along with Newton-Raphson method for *VISCO procedure.