I am redesigning my prototype PCB that uses USB 2.0 (for an STM32L4).

How critical is impedance control, considering the track lenghts are quite short e.g. 30 to 40mm

The prototype board was a standard 4 layer PCB from PCB Way (I don't know the stackup for this board) The layers are….

Layer 1: Signal
Layer 2: GND
Layer 3: 3.3V
Layer 4: Signal

The differential pair had the following contraints (in KiCad)....



USB works on this board, I use it to flash a W25Q external flash and when flashing the data rates are around 200kbs. Does the data rate adjust automatically to account for different impedances? ie if I had used impedance control would the data rate be faster?

When redesigning the board, should I consider impedance control or can I get away with a standard PCB ?

 

Both the rising and falling edges are important; data rates come later.

Here is a video that demonstrates the robustness of USB 2.0: USB 2.0 real-world robustness test.

However, it is always a good practice to do impedance control and to have a clear reference GND plane.