The Hirose DF12NB(4.0)-20DP-0.5V(51) is a 20-position header with outer shroud contacts from the DF12 series, designed for board-to-board mezzanine stacking. Typical applications include portable electronics, handheld devices, and any stacked-PCB design where the 4mm mated stack height fits the enclosure's z-height budget.
The mated stacking height is 4mm, with the header itself standing 3.30mm above the board surface. That 4mm gap between parallel PCBs is the dimension you design the enclosure around — tight enough for slim handheld gear but tall enough to route traces under the connector body. The 0.50mm pitch means the board footprint needs fine-line capability; expect 0.25mm trace/space minimum to route the inner rows out. The outer shroud contacts on this header mate with the corresponding DF12 receptacle, which has the contacts recessed inside the shroud for alignment during blind mating.
Gold contact finish at 8.00µin thickness is the standard for this series — enough for occasional mating cycles during assembly and field service, but not a high-cycle-duty connector. The gold does protect against corrosion in non-sealed environments, so it's fine for consumer electronics interiors.