  We use ab-initio calculations to predict the thermal conductivity of cubic
SiC with different types of defects. An excellent quantitative agreement with
previous experimental measurements is found. The results unveil that
B$_\mathrm{C}$ substitution has a much stronger effect than any of the other
defect types in 3C-SiC, including vacancies. This finding contradicts the
prediction of the classical mass-difference model of impurity scattering,
according to which the effects of B$_\mathrm{C}$ and N$_\mathrm{C}$ would be
similar and much smaller than that of the C vacancy. The strikingly different
behavior of the B$_\mathrm{C}$ defect arises from a unique pattern of resonant
phonon scattering caused by the broken structural symmetry around the B
impurity.
