It has been asked before about the upper limit of surface charge density, at which the electric field starts to drain the charges off the metal surface. The medium between the metal surface is assumed to be vacuum (otherwise the medium might become conductive before the electrons are drained off). Yet there is another effect which would also puts an upper limit on the surface charge density, namely the Schwinger effect . If the electric field is strong enough, a (real) pair of electron and positron may emerge from the vacuum, and then are respectively pulled to the corresponding charged surface, where an annihilation would occur on the negative one, radiating photons. The total charge of the capacitor is thus reduced. My question is, what is this upper limit due to Schwinger effect? (How can it be computed?). It seems that no metal has the work function limit comparable to the Schwinger effect's limit. Is this true?