Q: How to design a 4-layer PCB stack-up for optimal signal integrity and cost?
Answer
Recommended 4-layer stack-up: Layer 1 (top): signals; Layer 2: GND plane; Layer 3: power plane; Layer 4 (bottom): signals. This provides 50 Ohm microstrip on top/bottom with consistent reference. For better performance: L1 signals L2 GND L3 signals L4 GND – the L2/L4 dual ground plane reduces crosstalk. Use prepreg thickness to control impedance: 0.25 mm prepreg gives ~50 Ohm with 0.5 mm trace on standard FR-4. Specify core and prepreg types in your fab notes. Avoid FR-4 with Dk >4.6 for >100 MHz signals – use Rogers or Panasonic Megtron. For mixed-signal boards assign power plane on layer 3 with split planes for analog and digital rails.
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