Board-stacking selection: 0.40 mm pitch, 0.80 mm stack height
The Hirose BM10NB(0.8)-30DS-0.4V(51) is a 30-position receptacle with center strip contacts from the BM10 series. It's built for parallel board-to-board stacking at a fixed 0.80 mm mated height — the number in the part code after the series name. The 0.40 mm pitch on a dual-row layout packs 30 signal lines into a compact footprint, but that pitch also drives the escape routing: you'll need micro-vias or tight trace-and-space on the inner layers to get all 30 lines out from under the part.
Sizing the footprint: what the pitch and height mean for your board
The 0.40 mm pitch sets the pad layout — the land pattern needs to match the Hirose recommended footprint to keep solder joint reliability across reflow. The 0.80 mm height above board means the opposing header must also be 0.80 mm to hit the mated stack height — no shimming, no variable-gap assembly. If your board stack-up has a different z-height budget, look at other BM10 variants; this one locks you into 0.80 mm.