The 6N138SD comes in an 8-SMD gull wing package (supplier device package 8-SMD) — the gull wing leads are hand-solderable under a microscope and rework-friendly with hot air, but the 1.27 mm pitch means the pad-to-pad clearance is tight for a manual iron tip. The 2500Vrms isolation rating between input and output demands a clean creepage gap under the package; a solder bridge across the body is a guaranteed field failure.
The 300% minimum current transfer ratio at 1.6 mA forward current means the output Darlington can sink at least 4.8 mA with only 1.6 mA LED drive — useful for driving a logic input or a low-power relay directly without a buffer stage. The 1.5 µs turn-on and 7 µs turn-off times limit the switching frequency to roughly 50 kHz before the output waveform distorts; this part suits isolated data lines and status feedback, not high-speed comms.
Lifecycle reality — obsolete and sourced per RFQ
onsemi has marked the 6N138SD as Obsolete. No pin-compatible direct replacement is listed; a board spin or a parametric search for a similar Darlington optoisolator in an 8-SMD package is the migration path for new designs.