DF37B-40DP-0.4V(51) from Hirose Electric Co Ltd is a fine-pitch rectangular connector header designed for compact board-to-board interconnect applications where space, profile, and assembly reliability matter. With a 0.40 mm pitch, 40 positions, two-row configuration, SMT mounting, and gold contact finish, it fits densely populated portable, industrial, medical, and consumer electronic designs that need stable signal connection in low-profile assemblies. The listed solder retention feature and reel packaging also support automated surface-mount production. Based on the provided product data, the part is marked Active, which is favorable for ongoing designs, although formal lifecycle confirmation should still be checked with the manufacturer because the separate lifecycle field is listed as unknown. RoHS compliance information is not available in the supplied specifications, so engineers should verify current RoHS and REACH status directly through official Hirose documentation or authorized distribution sources before release to regulated markets.