0.40mm Pitch Mezzanine Receptacle — 40 Circuits, 0.8mm Stack
The Hirose BM20B(0.8)-40DS-0.4V(51) is a 40-position receptacle from the BM20 series, built for board-to-board stacking at a tight 0.016" (0.40mm) pitch. The 0.40mm pitch means the board footprint is dense: you'll need fine trace and via routing to escape the pads, especially for inner-row signals. Gold contact finish at 2.00µin (0.051µm) thickness handles moderate mating cycles — enough for one assembly pass in production, but not a field-service reconnect every week. Solder retention features hold the connector flat during reflow, which matters on a part this small to prevent the connector from lifting off the pads.
SMT Termination and Solder Retention
Surface mount termination is the only option here — no through-hole variant in this order code. The solder retention feature is a set of metal tabs that reflow to pads on the board, locking the connector in place during the solder process. Without them, a 0.40mm-pitch part can shift from paste surface tension alone.
This receptacle mates with the BM20 series header, specifically the BM20B(0.8)-40DP-0.4V(51) — same 40 positions and 0.40mm pitch, but the header uses outer shroud contacts and has a 0.026" (0.65mm) height above board. When mated, the total stack height is 0.8mm. That 0.8mm is the distance between the two PCBs, not the component height on either side. If your design needs a different gap, check the BM20 series for other header heights — the receptacle height is fixed at 0.80mm. The 34-position version BM20B(0.8)-34DS-0.4V(51) shares the same pitch and height but fewer circuits, for when the BOM calls for a lower signal count.