The breakdown of oxide films with thickness ranging from 7 to 4 nm is studied as a function of the stress current and of the capacitor area. The tests were carried out using Hg- and poly-Si-gated capacitors. It is found that the probability of soft breakdown (SBD) decreases if the capacitor area is decreased or if the stress current is increased. It is shown that these experimental observations can be explained if (a) SBD is considered as an incomplete hard breakdown, i.e. a breakdown mode in which the energy dissipated into the percolative path is not enough to trigger its thermal expansion; (b) the SBD probability depends not only on the energy stored in the capacitor and measurement system at the moment of breakdown, but also on the discharge current flowing in the conductive spot.