40-position, 0.50mm-pitch mezzanine header — 4mm stack height
The DF12E(4.0)-40DP-0.5V(81): This is a Hirose DF12 series header with outer shroud contacts, 40 positions in two rows on a 0.50mm pitch.
What the 4mm stack height and 0.50mm pitch mean for your board
The 0.50mm pitch (0.020") forces a fine-line PCB fan-out — expect at least two signal layers to escape the inner rows cleanly. The two-row arrangement with 40 total positions gives 20 per row, which balances density against routing complexity. Gold flash (8.00µin) is a signal-grade finish, not high-cycle — plan for a few hundred mate/unmate cycles, not thousands.