Calculate characteristic impedance for outer layer PCB traces using IPC-2141 closed-form equations. Includes effective Dk, propagation delay, and design optimization recommendations.
Surface Microstrip Cross-Section
IPC-2141A closed-form equations for surface microstrip impedance calculation
Valid for W/H ratio between 0.1 and 3.0
Typically εeff ≈ 0.6 × εr to 0.8 × εr for FR-4
Microstrip signals travel faster than stripline because of lower effective Dk
Standard for RF and high-speed digital signals. Typical geometry:
100Ω differential impedance for USB, HDMI, Ethernet:
Solder mask coating affects impedance:
For signals >1 Gbps:
Microstrip radiates more than stripline:
For production success:
| Property | Microstrip | Stripline | Coplanar Waveguide |
|---|---|---|---|
| Location | Outer layer | Inner layer | Outer layer |
| Reference Planes | 1 (below) | 2 (above & below) | 1 + coplanar grounds |
| Propagation Delay | ~145 ps/in | ~175 ps/in | ~130 ps/in |
| EMI Radiation | Moderate | Low | Moderate |
| Impedance Control | Good (±10%) | Excellent (±5%) | Good (±10%) |
| Manufacturing | Easy | Requires multilayer | Moderate |
| Best For | RF, High-speed digital | Clocks, sensitive signals | mmWave, RF transitions |
A microstrip is a type of transmission line consisting of a conducting strip separated from a ground plane by a dielectric substrate. It's located on the outer layers of a PCB with air above the trace and dielectric below. This asymmetric structure results in a quasi-TEM mode of propagation.
Effective Dk is the weighted average of the dielectric constants seen by the electric field. Since part of the field passes through air (Er=1) and part through the substrate (Er=4.0-4.5 for FR-4), the effective Dk is lower than the substrate Dk, typically around 3.0-3.5 for FR-4 microstrips.
Microstrip impedance is affected by solder mask thickness, humidity, and nearby components because the electric field extends into the air above the trace. Stripline is fully enclosed by dielectric, providing more consistent impedance. Manufacturing variations in outer layer plating also affect microstrip more.
For standard FR-4 PCBs, microstrip impedance typically ranges from 30Ω to 120Ω. Common targets are 50Ω for single-ended RF/high-speed, 75Ω for video, and 85-100Ω for differential pairs. Going below 30Ω requires very wide traces; above 120Ω requires extremely narrow traces that are hard to manufacture.
Solder mask (typically Er=3.5-4.0, thickness 0.5-1.5mil) covers the microstrip trace and slightly lowers the impedance by 2-5Ω. This is called 'coated microstrip'. For precise impedance control, specify solder mask openings over controlled impedance traces or account for the coating in calculations.
Inner layer traces with dual reference planes.
100Ω pairs for USB, HDMI, PCIe.
RF and mmWave applications.
All impedance equations explained.
90Ω differential
PCIe85Ω differential
Ethernet100Ω differential
HDMI100Ω TMDS
DDR540Ω single-ended