The STFN42 is a high-voltage NPN bipolar junction transistor rated for 400 V collector-emitter breakdown and 1 A continuous collector current, housed in a SOT-89-3 surface-mount package. It is designed for switching and linear applications where the supply rail or flyback voltage exceeds the usual 60–100 V ceiling of general-purpose power transistors — think offline auxiliary supplies, flyback clamp circuits, LED lighting ballasts, and telecom line drivers that need to handle 400 V transients. The 1.4 W maximum power dissipation in the SOT-89-3 footprint means the board copper area and airflow matter; this is not a part you run at full dissipation in still air without a thermal relief plan.
Obsolete — sourcing reality for a BOM line
The STFN42 carries an official product status of Obsolete. That means STMicroelectronics no longer manufactures it, and no last-time-buy window remains open. There is no direct pin-compatible replacement listed by ST in the same SOT-89-3 package; a cross-ship candidate would need to match the 400 V / 1 A / NPN / SOT-89-3 footprint exactly, and those are scarce in current-production portfolios. If you are qualifying a substitute, expect to revalidate the thermal pad layout and the switching performance at your operating point.
The 400 V Vceo is the standout — it gives you a 3x to 4x margin over a 120 V or 200 V part in a 48 V or 120 V DC bus, which matters when inductive kickback or mains-derived transients appear. The 1 A Ic is the continuous current ceiling; pulsed operation can go higher, but the 1.5 V saturation voltage at 250 mA base / 750 mA collector means conduction loss is non-negligible — at 750 mA you drop 1.5 V, so about 1.1 W of dissipation just from Vce(sat). That eats into the 1.4 W package limit fast. The 150°C junction temperature rating is typical for a power transistor, but the SOT-89-3's thermal resistance to ambient (around 55–65°C/W on a standard footprint) means the practical continuous current at 85°C ambient is well below 1 A. The 100 µA collector cutoff at max rating is a leakage floor to watch at high junction temperature.