  We study the effect of charge and spin noise on singlet-triplet qubits in Si
quantum dots. We set up a theoretical framework aimed at enabling experiment to
efficiently identify the most deleterious defects, and complement it with the
knowledge of defects gained in decades of industrial and academic work. We
relate the dephasing rates $\Gamma_\phi$ due to various classes of defects to
experimentally measurable parameters such as charge dipole moment, spin dipole
moment and fluctuator switching times. We find that charge fluctuators are more
efficient in causing dephasing than spin fluctuators.
