What this 0.40 mm pitch receptacle does for a tight board stack
The Hirose BM20B(0.8)-20DS-0.4V(53) is a 20-position receptacle with center strip contacts, part of the BM20 series. Its 0.016" (0.40mm) pitch and 0.031" (0.80mm) mated stack height put it squarely in the FPC-to-board interconnect space for thin handheld and portable equipment — think display ribbon connections, camera module interfaces, or main-to-sub board links where every tenth of a millimeter of z-height is budgeted.
How the 20-position version compares to the 24- and 30-position siblings
The BM20B(0.8)-20DS-0.4V(53) shares the same 0.016" (0.40mm) pitch, 0.031" (0.80mm) stack height, dual-row layout, and gold finish with the 24-position BM20B(0.8)-24DS-0.4V(53) and the 30-position BM20B(0.8)-30DS-0.4V(53). A board laid out for the 20-position version will not accept the 24-position part without a new layout; the extra four pads simply are not there.
At 0.40 mm pitch, the board footprint requires tight registration between the SMT pads and the solder paste stencil. The solder retention features add mechanical hold during reflow but do not change the pad layout — the standard Hirose-recommended footprint for the BM20 series applies. The 0.80 mm mated stack height means the mating FPC or board must clear any components on the opposing side by at least that height; plan for a keep-out zone under the connector.