What this 0.40 mm pitch mezzanine receptacle does that a wider-pitch part cannot
The Hirose DF40TC(3.0)-40DS-0.4V(51) is a 40-position, 2-row receptacle from the DF40T series, built for board-to-board mezzanine stacking at a mated height of 3 mm. At 0.016" (0.40 mm) pitch, it packs 40 I/Os into a footprint that a 0.50 mm or 0.80 mm pitch part would need nearly double the board real estate to deliver. The gold finish is there to keep contact resistance stable through a handful of mates, not to survive hundreds of reconnects. Surface-mount termination with pick-and-place and solder retention features means it runs through a standard reflow line without special handling.
The 3 mm stack height sets the board gap — measure your enclosure before you commit
The mated stacking height of 3 mm is the single dimension that decides whether this connector fits your board stack. That is the distance between the two PCBs when the receptacle is mated with its matching header. If your enclosure or mezzanine card gap is tighter than 3 mm, this part will not fit. If it is wider, the connector may not maintain contact beam pressure. The DF40T series includes variants with other stacking heights — this one is fixed at 3 mm. The receptacle itself sits 0.114" (2.90 mm) above the board surface before mating, so the header side contributes the remaining 0.10 mm to reach the 3 mm total.
Gold flash plating — what it means for mating cycle life
That is flash plating — the thinnest gold layer in common use. It prevents oxidation during storage and initial mating, but it wears through after a few dozen cycles. For comparison, a connector rated for 500+ mating cycles typically carries 30 µin or more of gold. The gold is there for corrosion protection during the product life, not for wear life.
SMT assembly features that keep the line moving
The DF40TC(3.0)-40DS-0.4V(51) includes pick-and-place vacuum pick area and solder retention features. The pick-and-place surface lets the placement nozzle handle the part without a custom nozzle. Without them, a 40-position, 0.40 mm pitch part would be prone to tombstoning or shifting during reflow. The part ships in Tape & Reel packaging, which is the standard feed format for high-speed SMT lines.