What this 0.35mm-pitch header does for the board stack
The Hirose BM28B0.6-40DP/2-0.35V(51) is a 40-position header from the BM28 series, built for board-to-board stacking at a mated height of 0.6mm. That 0.014" (0.35mm) pitch is the tightest constraint in the footprint — it drives the trace escape routing and the via pattern on the mating board.
40 positions vs 36 — the density trade-off in the BM28 family
The BM28 series offers position counts from 34 up to 40 at the same 0.35mm pitch and 0.6mm stack height. The 40-position variant (this one) packs four more signal lines than the BM28B0.6-36DP/2-0.35V(53) in the same board real estate — the footprint outline doesn't change, just the contact count. That extra density matters when routing a parallel bus or a camera sensor interface where every signal needs its own line. The 34-position version (BM28B0.6-34DP/2-0.35V(53)) is the choice when the BOM calls for fewer lines and you want to leave the outer positions unpopulated for mechanical symmetry.
The 0.6mm mated stacking height is fixed by the pair; the header's 0.018" (0.46mm) height above board leaves clearance for the receptacle's body.