A description of the bonding front propagation between two adhesive plates is proposed. The model relates the velocity of a bonding front to the adhesion energy, with application to wafer direct bonding. Its derivation is based on a competition between the bonding energy and the viscous drag of the air flow in the gap between the two wafers. The model describes well the experimental data, including the wafer deformation profile during bonding or the dependence of the velocity on the gas viscosity, pressure, and wafer thickness.