Structural designers working with tight PCBA stacks often meet the word waterproof on a datasheet and treat it as a finished property. In practice, a board-to-board connector keeps moisture out only when the mated face closes evenly, the housing holds the insert without gaps, and the surrounding enclosure limits the water and dust that reach the joint in the first place. That is a different engineering question from choosing a part number. The sections below follow the paths water actually takes, show how mating surfaces and housing fit carry the sealing job, and explain why a waterproof description and an IP rating answer different questions.