Board-stacking selection: 70-position mezzanine receptacle at 0.40mm pitch
The Hirose DF40HC(3.0)-70DS-0.4V(51) is a 70-position receptacle from the DF40 series, designed for board-to-board mezzanine stacking. It uses a 0.016" (0.40mm) pitch in a dual-row layout, with center strip contacts that mate to the matching DF40 header. The connector is specified for a 3mm mated stacking height, which sets the board-to-board spacing in the assembly. Surface-mount termination and gold contact finish are standard for this member of the DF40 family.
What the 0.40mm pitch and 3mm stack height mean for your board
You'll need fine-line PCB capability to route traces between pads, and the via placement under the connector body must be planned early. The 70 positions on a dual-row layout give a high I/O count in a compact footprint, suited for dense signal interconnect between processor and memory or sensor boards.