DF40HC(3.5)-40DS-0.4V(51) from Hirose Electric Co Ltd is a fine-pitch board-to-board rectangular connector designed for compact electronic assemblies that need reliable mezzanine interconnection in limited space. As part of the DF40 series, it is suited to applications such as portable devices, industrial control modules, embedded systems, display assemblies, communication hardware, and other dense PCB designs where 0.40 mm pitch and a 3.5 mm stacking height are important. Engineers typically choose this type of connector for high-density signal routing, low-profile packaging, and repeatable SMT assembly. The provided specifications list the product status as Active, which is favorable for ongoing design use, although the separate lifecycle field is marked Unknown and should be verified with Hirose or the latest authorized channel data before long-term commitment. RoHS compliance information is not explicitly provided in the input, so compliance status should be treated as information not available pending official manufacturer confirmation.