3.5 mOhm at 10V — the on-resistance that decides the thermal budget
The BSZ035N03MSGATMA1 is an Infineon OptiMOS N-channel MOSFET rated for 30V drain-source and 40A continuous drain at the case (Tc), with a maximum on-resistance of 3.5 mOhm at 20A and 10V gate drive. That Rds(on) figure is the spec that sets the conduction loss floor for a 12V or 24V rail switch — at 20A the dissipation is just 1.4W in the channel, before package thermal resistance enters the calculation.
18A at ambient vs 40A at case — reading the dual current rating
The datasheet lists 18A continuous drain at 25°C ambient (Ta) and 40A at the case (Tc). That 2.2x spread tells you the PG-TSDSON-8 package's thermal limit, not the silicon — the 2.1W (Ta) vs 69W (Tc) power dissipation figures confirm the package is the bottleneck for board-level designs. For a load switch on a standard 2-layer PCB with minimal copper, budget around the 18A Ta rating; the 40A Tc figure applies only with substantial heatsinking or forced airflow.
The ±20V maximum gate-source rating gives comfortable margin for 10V or 12V gate drive designs without worrying about oxide breakdown during transients.
Package realities for the rework bench and the pick-and-place line
The 8-pin body is small enough that a standard hot-air rework station can handle removal, but the pad solder paste volume needs to be controlled to avoid voiding under the die attach area.
Gate charge and switching — 74 nC at 10V
The 5700 pF input capacitance means the driver's peak current capability determines the turn-on and turn-off delay, not the average charge. This part is a solid fit for mid-frequency DC-DC converters and load switches where conduction loss dominates over switching loss.