  We study the influence of non-magnetic impurity scatterings on the tunneling
conductance of a junction consisting of a normal metal and a disordered
unconventional superconductor by solving the quasiclassical Eilenberger
equation self-consistently. We find that the impurity scatterings in both the
Born and unitary limits affect the formation of the Andreev bound states and
modify strongly the tunneling spectra around zero bias. Our results are
interpreted well by the appearance of odd-frequency Cooper pairs near the
interface and by the divergent behavior of the impurity self-energy. The
present paper provides a useful tool to identify the pairing symmetry of
unconventional superconductors in experiments.
