4mm Stack, 40 Circuits — DF12 Mezzanine Header
The Hirose DF12NC(4.0)-40DP-0.5V(51) is a 40-position header from the DF12 series of board-to-board mezzanine connectors.
0.50mm Pitch and 4mm Stack — the Board Layout Numbers
The 4mm mated stacking height is the critical z-height dimension: the space between the two parallel boards after mating is exactly 4mm, so the keep-out zone on both boards must clear that gap. The 0.130" (3.30mm) height above the mounting board gives the clearance under the header on the bottom side.
40 Positions Against the 20 and 50 — Position Count Drives the BOM
The DF12NC(4.0)-40DP-0.5V(51) carries 40 positions. The 50-position version (DF12NC(4.0)-50DP-0.5V(51)) adds solder retention features, which the 40-position lacks.