What the 0.40mm pitch and 0.8mm stack height mean for the board gap
The Hirose BM20B(0.8)-34DP-0.4V(51) is a 34-position header from the BM20 series, built for tight board-to-board stacking. The 0.016" (0.40mm) pitch and 0.8mm mated stacking height define the z-axis budget — the gap between the two PCBs is exactly 0.8mm when mated with the matching BM20 series receptacle. The 2.00µin (0.051µm) gold contact finish is a light flash — adequate for the single mate cycle of a production board stack, but not designed for repeated disconnect/reconnect in the field.
34 positions — matching the BOM line
The 34 circuits match the position count on the schematic. At 0.40mm pitch, the wipe has to seat at the worst case of the stack tolerance; the shroud helps absorb the misalignment so the gold contacts still land on the receptacle pads.
The 34-position count and 0.40mm pitch are the two checks against the BOM; confirm the mating BM20 series receptacle before committing.
The BM20B(0.8)-34DP-0.4V(51) sits between the 30-position BM20B(0.8)-30DP-0.4V(51) and the 40-position BM20B(0.8)-40DP-0.4V(51) in the same BM20 series. All three share the 0.016" (0.40mm) pitch, 0.026" (0.65mm) height above board, 0.8mm mated stacking height, and 2.00µin gold contact finish. There is no 34-position variant with a different plating or stack height in this series; the (51) suffix denotes the standard 0.8mm stack height version.