Non topological solitons, Q balls can arise in many particle theories with U(1) global symmetries. As was shown by Cohen et al. [2], 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 work we analyse Q-ball instabilities as a function of Q-ball size and 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 a new expression for the upper bound on evaporation as a function of the produced fermion mass and study the effects of the size of the Q ball on particle production. We also analyse what happens if external fermion is scattered on a Q ball and demonstrate that it can be converted into antiparticle with a probability of the order of one. This result has important implications for astrophysical applications of dark matter Q balls.