What this counter does on the board
The SNJ54390J is a decade counter — it counts input clock pulses from 0 through 9 and resets on the tenth pulse, delivering a BCD output at the Q pins. Asynchronous means the ripple carry propagates through the flip-flops one stage at a time, so the output settling time equals the sum of the propagation delays through all four stages. For a 5 V system clocking at a few megahertz the ripple delay is manageable; above 10 MHz the glitch on the count rollover becomes wide enough to clock downstream logic, and you either gate the output or switch to a synchronous counter.
Lifecycle and BOM fit
Texas Instruments lists the SNJ54390J as Active. The part is RoHS non-compliant — the lead finish on the DIP package exceeds the 1000 ppm threshold, so this part cannot be used in a product shipped into EU RoHS or China RoHS jurisdictions without an exemption. For a military or aerospace BOM that has a leaded-solder exemption, the non-compliance is irrelevant; for a commercial design it is a showstopper.
Sourcing desk reality
No pin-compatible second-source or cross-reference is recorded for this order code. For a BOM line that needs a guaranteed alternate, the only route is a board spin to a synchronous counter in a RoHS-compliant package.