PCB Impedance Calculator

Calculate the characteristic impedance of PCB transmission lines for microstrip, embedded microstrip, stripline and edge-coupled differential pair configurations. Enter the dielectric thickness, dielectric constant (Dk), trace width, copper thickness and (for differential pairs) trace separation. The calculator returns the characteristic impedance using standard closed-form equations.

Transmission line configurations

Microstrip: a trace on an outer layer with a reference plane on the adjacent inner layer. The electromagnetic field extends through both the dielectric and the air above the trace, resulting in an effective dielectric constant lower than the substrate Dk. Microstrip is the most common controlled-impedance configuration for outer-layer routing.

Embedded microstrip (coated microstrip): a microstrip trace covered by solder mask or prepreg. The coating increases the effective dielectric constant, reducing the impedance compared to an uncoated microstrip with the same trace width. Account for solder mask Dk (typically 3.5-4.5) when calculating outer-layer impedance.

Stripline: a trace on an inner layer between two reference planes. The electromagnetic field is entirely within the dielectric, making the effective Dk equal to the substrate Dk. Stripline provides better shielding and lower radiation than microstrip but requires two reference planes, using more layers.

Differential pair: two traces routed in parallel with controlled spacing. The differential impedance depends on the individual trace impedance and the coupling between traces, which is determined by the trace separation relative to the dielectric height. Tighter coupling (closer spacing) reduces the differential impedance.

Accuracy considerations

Closed-form equations provide engineering estimates within approximately ±5-10% of measured values. For production boards with impedance tolerance specifications (typically ±8-10%), AstroPCB uses a 2D field solver that models the actual trace cross-section (trapezoidal due to etching), copper roughness, etch factor, and the specific Dk of the laminate lot being used. The field solver provides the accuracy needed to hit impedance targets within the specified tolerance on the first fabrication run.

Common impedance targets

50 ohm single-ended: general-purpose RF, clock distribution, standard interfaces. 75 ohm: video, cable TV. 85 ohm differential: PCIe. 90 ohm differential: USB 3.x/4, SATA. 100 ohm differential: Ethernet, LVDS, DDR. The stackup must be designed with these targets in mind before layout begins — trace widths are determined by the stackup geometry, and changing the stackup after routing requires re-routing.

For production impedance control, request a quote and include your impedance requirements. The engineering team will design a stackup that achieves your targets with verified accuracy.