  Non topological solitons, Q-balls can arise in many particle theories with
U(1) global symmetries. As was shown by Cohen et al. \cite{Qballscohen}, if the
corresponding scalar field couples to massless fermions, large Q-balls are
unstable and evaporate, producing a fermion flux proportional to the Q-ball's
surface. In this paper we analyse Q-ball instabilities as a function of Q-ball
size ans fermion mass. In particular, we construct an exact quantum-mechanical
description of the evaporating Q-ball. This new construction provides an
alternative method to compute Q-Ball's evaporation rates. We shall also find
the new expression for the upper bound on evaporation as a function of the
produced fermion mass and study the effects of Q ball's size on particle
production.
