The Molex 0559090474 is a 40-position plug from the SlimStack 55909 series, designed as the board-to-board mezzanine interconnect for compact portable devices. The mated stack height lands at 1.5mm — a specific z-height that suits thin enclosures where the board pair sits close together. The solder retention feature anchors the plug to the board during reflow, preventing the part from floating off the pads as the paste wets out. This is a surface-mount plug with outer shroud contacts, meaning the mating receptacle slides inside the shroud for alignment.
The 1.5mm mated stacking height is the single dimension that governs the mechanical fit in a mezzanine stack-up. It defines the z-height from the bottom of the plug board to the top of the receptacle board. If the enclosure has a 2.0mm gap allowance, this connector fits with 0.5mm clearance above the top board. If the gap is 1.2mm, it does not fit — no shimming, no spacer. The 0.40mm pitch drives the routing density on the board. Each signal pair occupies 0.80mm center-to-center across two rows, which forces fine-line PCB traces — typically 0.15mm line/space or tighter — between the pads. A standard 0.50mm pitch connector would use wider traces, so swapping this part into an existing layout without re-spinning the board is not possible.