Picking the right stack height before you route the board
The Hirose DF40C-44DP-0.4V(51) is a 44-position plug from the DF40 series of board-to-board mezzanine connectors. The real selection variable here is the mated stack height: this plug supports six different heights (1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, and 4mm) depending on which receptacle height variant you pair it with. That range covers everything from ultra-thin phone-style stacking to slightly taller mezzanine gaps where components sit under the board pair. The plug itself sits 1.14mm above the board — that's the z-height of the component on the top side, before the receptacle adds its own height.