The Hirose DF40C-100DS-0.4V(51) is a 100-position receptacle with center strip contacts from the DF40 series, designed for board-to-board mezzanine stacking at a tight 1.5mm mated height.
100 Positions vs 90 — When the Circuit Count Matters
The DF40C-100DS-0.4V(51) carries 100 positions against the 90 of the DF40C-90DS-0.4V(51) at the same 0.40mm pitch and 1.5mm stacking height. The decision is strictly a BOM circuit-count match — the footprint, board keep-out zone, and mating half are identical. If the design calls for 90 I/O, the 100-position part still fits physically but leaves ten contacts unterminated; the 90-position version saves a few cents on the BOM line.