Board-fit reality: 0.40mm pitch, SMT, 40 positions
The Hirose DF37NB-40DS-0.4V(51) is a 40-position, dual-row receptacle from the DF37 series, designed for board-to-board mezzanine stacking. It uses a 0.016" (0.40mm) pitch — that is the tightest in the DF37 line — and mounts via surface-mount soldering with a solder retention feature to hold it in place during reflow. The connector stands 0.038" (0.97mm) above the board and mates to a stacking height of 0.98mm, which sets the z-gap between parallel PCBs.
What 40 positions at 0.40mm pitch means for your layout
The 0.40mm pitch is the dominant selection factor here. It drives the PCB footprint — you need tight trace-and-space routing between pads, and the stencil aperture for the SMT paste must be sized to avoid bridging. The 40 positions in two rows give you a dense I/O or mezzanine interconnect, but the fine pitch limits current per contact well below what a 0.50mm or 0.80mm connector would carry.
The DF37 series offers the same 0.40mm pitch and 0.97mm height in 34-position (DF37NB-34DS-0.4V(51)) and 50-position (DF37NB-50DS-0.4V(53)) variants. The 40-position version here sits between them — if your BOM calls for 34 or 50 circuits, the footprint and stack height are identical, so the board layout transfers.