There are primarily two types of phonon-phonon scattering processes:
1. Normal Processes (N-processes): In these interactions, the momentum of the system is conserved, but the individual phonons' momenta change. These processes do not directly contribute to thermal resistance.
2. Umklapp Processes (U-processes): In Umklapp processes, the momentum conservation rule is violated by a reciprocal lattice vector. This type of scattering is significant because it directly contributes to thermal resistance and limits the thermal conductivity of materials.
