Active production — parametric fit for the BOM
The 1N6635US: This is a current-production Zener diode you can spec into a new design without worrying about a last-time-buy window closing before the product ships. Nominal Zener voltage is 4.3 V at 5 W total power dissipation with ±5% tolerance. That tolerance band — 4.085 V to 4.515 V — is tight enough for a rail clamp or a voltage-reference leg in a 5 V supply, but not a precision bandgap substitute. The 500 Ohm maximum Zener impedance (Zzt) means the regulation slope is moderate; expect the voltage to shift with current density through the junction.
SQ-MELF E case — hermetic surface-mount footprint
The SQ-MELF E (D-5B) package is a hermetic, non-cavity, metallurgically sealed surface-mount case. It mates with the standard MELF footprint — no lead frame, no plastic overmold. The hermetic seal and the -65°C to 175°C operating range are the two specs that separate this part from a commercial SOT-23 Zener; this is the junction temperature band for avionics, downhole instrumentation, and satellite power buses. Reverse leakage is 25 µA at 1 V reverse voltage, and forward voltage is 1.5 V max at 1 A forward current. These matter in a clamp circuit: the leakage current at the knee determines how much the protected rail droops before the Zener starts conducting.