In the framework of intermediate wave-packets for treating flavor oscillations, we quantify the modi. cations which appear when we assume a strictly peaked momentum distribution and consider the second-order corrections in a power series expansion of the energy. By following a sequence of analytic approximations, we point out that an extra time-dependent phase is merely the residue of second-order corrections. Such phase effects are usually ignored in the relativistic wave-packet treatment, but they do not vanish non-relativistically and can introduce some small modi. cations to the oscillation pattern even in the ultra-relativistic limit.732157163