Board-Stacking Fit for a 5.00 mm Gap
The Hirose FX4B1-20S-1.27SV(71) is a 20-position receptacle from the FX4 series of mezzanine board-to-board connectors. It mates with the corresponding FX4 header (outer shroud contacts) to complete the board-to-board interconnect.
Cross-Shop: FX4A1-20S vs FX4B1-20S
The FX4A1-20S-1.27SV(71) is a direct functional peer — same 20 positions, same 0.050" (1.27 mm) pitch, same 0.197" (5.00 mm) stack height, same gold plating thickness. The only difference is the base product number (FX4A vs FX4B). Both are active, both are receptacles with center strip contacts. If one is on allocation or has a lead-time gap, the other is a drop-in alternative for the same board footprint and mating header. Confirm the specific part marking on your BOM before swapping, but the mechanical and electrical interface is identical.