Sintering is a complex process involving multiple physical processes, including mass transport, grain coalescence and growth, and rigid-body motion. Phase-field modeling has proven to be an effective framework for capturing microstructural evolution across a wide range of sintering processes, from conventional isothermal sintering to advanced techniques such as selective laser sintering, field-assisted sintering, and phononic sintering. Conventionally, sintering has been interpreted primarily as a diffusion-dominated process, where distinct mass transport paths, such as bulk diffusion, surface diffusion, and grain-boundary--surface diffusion, govern the kinetics of microstructural evolution. This perspective leads to the well-established power-law relationship, $\chi^n$, describing sintering neck growth. In contrast, the role of rigid-body motion and its influence on neck evolution has received comparatively little attention. In this work, we investigate the effect of rigid-body motion on neck growth during phase-field sintering by introducing a generic phase-field framework that explicitly accounts for rigid-body motion via the momentum balance [1]. Onsager phenomenological relations for non-equilibrium processes are embedded directly into the formulation of a non-isothermal phase-field model, resulting in additional kinetic couplings that clarify the interactions among diffusion, mechanical motion, and microstructural evolution [2,3]. The variational framework is further extended to include trapping effects and surface diffusion mechanisms [4]. Using the analytical $\chi^n$ law as a reference, neck growth predicted by different phase-field formulations is systematically simulated and benchmarked. Comparisons between models with and without rigid-body motion are performed to quantify their contribution to neck growth kinetics and to elucidate their role relative to diffusion-dominated mechanisms.