Atmega32u4 USB 2.0 Routing Check

According to this article avoid routing the differential USB signal over the BGA pins

• Do not place probe or test points on any high-speed differential signal.

• Do not route high-speed traces under or near crystals, oscillators, clock signal generators, switching power regulators, mounting holes, magnetic devices, or ICs that use or duplicate clock signals.

• After BGA breakout, keep high-speed differential signals clear of the SoC because high current transients produced during internal state transitions can be difficult to filter out.

• When possible, route high-speed differential pair signals on the top or bottom layer of the PCB with an adjacent GND layer. TI does not recommend stripline routing of the high-speed differential signals.

• Ensure that high-speed differential signals are routed ≥ 90 mils from the edge of the reference plane.

• Ensure that high-speed differential signals are routed at least 1.5 W (calculated trace-width × 1.5) away from voids in the reference plane. This rule does not apply where SMD pads on high-speed differential signals are voided.

• Maintain constant trace width after the SoC BGA escape to avoid impedance mismatches in the transmission lines.

• Maximize differential pair-to-pair spacing when possible.