  We have studied the effects of phonon-boundary scattering on the thermal
transport in topological Kondo insulator, SmB$_6$. The studies have been
performed by using the $3\omega$ method in the temperature range 300K - 3K. We
show that the observed thermal conductivity of micro-size SmB$_6$ is of the
order of magnitude smaller than for a bulk single-crystal. Using the Callaway
model we analyzed the low-temperature lattice thermal conductivity of the micro
crystal and show that phonon scattering by sample boundaries plays a major role
in the thermal resistance in this topological material. In addition, we show
that the temperature dependence of the lattice thermal conductivity shows a
double peak structure that suggests complex phonon-phonon or phonon-defects
interactions in SmB$_6$. These findings provide guidance for the understanding
of the thermal transport of advanced materials and devices at a micro-scale.
