The Hirose DF40C-12DP-0.4V(51) is the 12-position plug half of the DF40 series board-to-board mezzanine connector system.
Stack height compatibility and board layout
The mated stacking height range of 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, and 4mm is the key mechanical spec to check against your PCB-to-PCB spacing. The plug itself sits 0.045" (1.14mm) above the board — that unmated height matters for clearance under the connector during pick-and-place and for visual inspection of the solder fillets. The 0.40mm pitch is fine enough that the board footprint needs tight routing discipline; the two rows keep the 12 positions in a compact body without pushing the signal density too high for a standard PCB fab.