Board-stacking selection note
The Hirose DF37NB-20DS-0.4V(51) is a 20-circuit receptacle from the DF37 series, a fine-pitch board-to-board interconnect.
What the 0.40mm pitch and 0.98mm stack height mean for the build
At 0.40mm pitch, the escape routing between pads is tight — a four-layer board with 0.15mm traces and 0.15mm clearances will just fit a single trace between adjacent pads on the outer layer. The 0.98mm mated stack height is a fixed value for this variant; the header counterpart determines the total height, so verify the header part number in the assembly drawing. The 2.00µin gold is a light flash — adequate for the typical 10 to 30 mating cycles in a consumer or handheld device, but if the design calls for frequent reconnects in a test fixture or modular assembly, the DF37NB-20DS-0.4V(75) with 4.00µin gold is the better call.