The Hirose DF40HC(3.5)-40DS-0.4V(51) is a 40-position receptacle with center strip contacts from the DF40 series, designed for board-to-board mezzanine stacking at a 3.5mm mated height.
Pitch and Footprint — the Board Layout Decision
At 0.40mm pitch, this is a fine-pitch connector. The 3.5mm stacking height means the mating header sits low, so keep-out zones on the opposing board must account for the receptacle body height above the PCB at 0.134" (3.40mm).
The DF40HC(3.5)-40DS-0.4V(51) carries 40 positions. The same footprint and 3.5mm stack height is shared with the 30-position DF40HC(3.5)-30DS-0.4V(51) and the 50-position DF40HC(3.5)-50DS-0.4V(51).
Gold contact finish at 2.00µin is a light flash. It handles moderate mating cycles — think assembly and occasional field service — but for high-cycle applications like battery swap or frequent reconnection, a thicker gold layer would be preferred. The gold prevents oxidation on the mating surface, keeping contact resistance stable through the connector's service life in a controlled indoor environment.