  In this paper we revisit the derivation of boundary conditions for the
Boltzmann Equation. The interaction between the wall atoms and the gas
molecules within a thin surface layer is described by a kinetic equation
introduced in [9] and used in [1]. This equation includes a Vlasov term and a
linear molecule-phonon collision term and is coupled with the Boltzmann
equation describing the evolution of the gas in the bulk flow. Boundary
conditions are formally derived from this model by using classical tools of
kinetic theory such as scaling and systematic asymptotic expansion. In a first
step this method is applied to the simplified case of a flat wall. Then it is
extented to walls with nanoscale roughness allowing to obtain more complex
scattering patterns related to the morphology of the wall. It is proved that
the obtained scattering kernels satisfy the classical imposed properties of
non-negativeness, normalization and reciprocity introduced by Cercignani [11].
