Noctua NF-A4x20 5V PWM 40mm x 20mm 4-pin PWM Fan. Compared to Noctua's award-winning NF-A4x10, the NF-A4x20 is twice as thick, which allows for increased static pressure performance and makes the NF-A4x20 ideal for demanding applications with high flow resistance.
Generally a good experience as a cooling fan. However , since it was bought to cool a Jetson Nano (suggested on the Nano info pages) the fact that the mounting screws don't fit the Nano's heat sink is a very negative thing.