don't panic
Long cables between boards act as antennas. Use common-mode chokes on cable entry points. For high-speed differential pairs (LVDS, USB), choose cables with drain wire connected to chassis ground.
Keep high-aggressor signals (clocks, PWM) physically separate from victim signals (ADC inputs, reset lines) on the connector. Use a 2W spacing rule (twice the trace width) between adjacent connector pins. multi board system design
The plan for how signals and power move between boards using mezzanine connectors , ribbon cables, or rigid-flex structures. Long cables between boards act as antennas