The Hirose BM20B(0.8)-40DP-0.4V(51) is a 40-position header from the BM20 series, designed for board-to-board mezzanine stacking. The mated stack height is 0.8mm, and the header sits 0.026" (0.65mm) above the board surface — a tight z-budget for compact handheld or portable electronics where every tenth of a millimeter counts. The contacts are finished with 2.00µin (0.051µm) gold, which is a light flash plating. That's adequate for the limited mating cycles typical of internal board stacks in consumer devices — think a display-to-mainboard interconnect that mates once during assembly and stays put. For applications requiring frequent reconnection or field service, a heavier gold layer would be the better call.
The 0.40mm pitch is the primary footprint constraint — it demands a PCB fab capable of fine traces and tight registration between the two rows. The header's own height above board is 0.65mm, leaving just 0.15mm of clearance between the top of the header body and the mating receptacle — essentially a zero-gap stack.