Controlled Impedance and Layer Stackups in Multilayer Flex Circuits

Balancing Flexibility and Impedance Requirements

Designing multilayer flex circuits involves balancing mechanical flexibility with electrical performance. Adding copper layers improves shielding and signal routing density, but increases mechanical stiffness and risk of mechanical failure.

To build an optimal multilayer flex stackup, engineers must carefully select dielectric thickness and trace geometries.

Microstrip vs. Stripline Configurations in Flex

1. Microstrip Topology: Signal traces are routed on the outer layer over a single internal reference plane. Microstrip offers lower propagation delay and easier routing, but outer signals are exposed to external electromagnetic noise.
2. Stripline Topology: Signal traces are sandwiched between two reference planes inside the stackup. Stripline provides superior shielding and isolation for high-speed differential pairs, but requires additional layers that increase total board thickness.

Cross-Hatching for Flexibility and Impedance Control

Instead of solid copper reference planes, high-speed flex designs often use cross-hatched copper ground patterns.

Cross-hatching removes excess copper mass to maintain bendability while providing an effective electrical ground shield. However, the hatch grid pitch must be kept significantly smaller than the signal's highest operating harmonic to prevent signal leakage and impedance variations.