Dissociative scattering of molecular SiF₃⁺ and SiF⁺ ions from a Cu (100) single crystal surface has been investigated in the incident energy range from 5 eV to 200 eV with a scattering angle of 77°. The ion intensities of dissociatively scattered ions and parent molecular ions were measured as a function of incident ion energy. The observation showed that the onset energies for the dissociation of SiF₃⁺ and SiF⁺ ions are 30 eV and 40 eV, respectively, within a precision of ±2 eV. The obtained values are consistent with an impulsive collision model where dissociation of incident ion is caused by vibrational excitation during the collision.