Unless a boundary condition is specified on interior boundaries (such as a contact resistance condition), COMSOL ensures continuity in the field variables across interior boundaries. For assembly geometries with identity pairs, select a Continuity node on the Pairs menu in the boundary part of the context menu for most physics interfaces. The Continuity condition is only suitable for pairs where the boundaries match.

Select the pairs where you want to impose continuity across the pair boundaries. Select the pairs from the Pairs list (Ctrl-click to deselect).