  In iron selenide superconductors only electron-like Fermi pockets survive,
challenging the $S^{\pm}$ pairing based on the quasi-nesting between the
electron and hole Fermi pockets (as in iron arsenides). By functional
renormalization group study we show that an in-phase $S$-wave pairing on the
electron pockets ($S^{++}_{ee}$) is realized. The pairing mechanism involves
two competing driving forces: The strong C-type spin fluctuations cause
attractive pair scattering between and within electron pockets via Cooperon
excitations on the virtual hole pockets, while the G-type spin fluctuations
cause repulsive pair scattering. The latter effect is however weakened by the
hybridization splitting of the electron pockets. The resulting
$S^{++}_{ee}$-wave pairing symmetry is consistent with experiments. We further
propose that the quasiparticle interference pattern in scanning tunneling
microscopy and the Andreev reflection in out-of-plane contact tunneling are
efficient probes of in-phase versus anti-phase $S$-wave pairing on the electron
pockets.
