Moisture rarely crosses a connector through a solid wall of molded plastic. It travels along the boundaries between parts, and a compact board-to-board connector holds many of them. Water collects on the mated face and creeps into the narrow gap between the two halves, runs down the outside of the housing and settles where the insert meets the shell, wicks along solder tails on the PCB side, or arrives as condensation inside an otherwise sealed equipment box, which is often the least obvious route of all. Each of these routes is an interface rather than a component, which is why a single well-made part cannot seal a joint by itself. Density makes the job harder. A hybrid layout with 40A power positions and 32 signal pins packs many small openings into a small footprint, so the sealing surface between them stays narrow and every gap is short. Temperature adds a second effect: across -20°C to +105°C, the housing, the insert, and the solder joint expand and contract at different rates. A joint that is tight at room temperature can open a capillary path after a few thermal cycles, and capillary paths move water against gravity. Designers who think in terms of paths instead of barriers usually catch these routes early, before the enclosure drawing is frozen.