Hirose BM20 Series 30-Position Header
The Hirose BM20B(0.8)-30DP-0.4V(53) is a 30-position header from the BM20 series, designed for board-to-board mezzanine stacking with a mated stack height of 0.8mm. Gold plating on the contact finish handles repeated mate/unmate cycles without the fretting you'd see on tin in low-current signal paths. The 0.026" (0.65mm) height above board keeps the profile low, which helps when the mezzanine gap is tight.
The 0.40mm pitch and 0.8mm mated stack height are the two numbers that drive the board layout. The pitch sets the pad spacing and trace escape width — at 0.40mm you're routing between pads with 0.2mm or finer traces, which is standard for HDI but worth flagging if your fab house runs coarser limits. The 0.8mm stack height means the mating receptacle sits 0.8mm above the board surface; your opposite board's keep-out zone needs to clear that height plus any component height on the underside.