The Hirose DF12NB(3.5)-20DP-0.5V(51) is a 20-position header with outer shroud contacts from the DF12 series, designed for board-to-board mezzanine stacking at a 3.5mm mated height. Gold-plated contacts with 8.00µin thickness handle moderate mating cycles in production and field service, though the thin plating means you'll want to keep the mate count under a few dozen for reliable contact resistance. That matters when the board goes through a wave or reflow oven and the connector sees thermal expansion stress.
Stacking Height and Board Spacing
The mated stacking height is 3.5mm, which is the dimension from the top of the bottom board to the bottom of the top board when the connector pair is mated. That's a tight sandwich — you're looking at a board-to-board gap that fits thin-profile enclosures like handheld devices, sensor modules, or mezzanine memory cards. The 0.110" (2.80mm) height above the board gives the pick-and-place nozzle enough clearance during assembly.
At 20 positions, this header sits below the 36-position DF12NB(3.5)-36DP-0.5V(51) and the 40-position DF12NB(3.5)-40DP-0.5V(51) in the same series. If your BOM calls for 20 signal lines, this is the one; if you need more, step up to the 36 or 40. The DF12NB(4.0)-20DP-0.5V(51) matches the 20 positions but uses a 4.0mm stacking height — choose that one when your board gap needs an extra 0.5mm of clearance.
No official second-source cross-reference exists, but the DF12 series is a Hirose proprietary footprint — the mating receptacle is the DF12C series receptacle housing with the same 0.50mm pitch and 3.5mm stack height.