100Ω Microstrip Calculator

Understanding 100Ω Microstrip Impedance

100Ω differential impedance is standard for Ethernet, DDR memory interfaces, and HDMI signals. To achieve 100Ω characteristic impedance using a microstriptrace on standard FR-4 material (Er=4.5) with 0.2mm dielectric height, you need a trace width of approximately 0.050mm (2.0 mil). This calculation follows the IPC-2141 standard and Hammerstad equations for accurate impedance control.

Key Parameters for 100Ω Design

• Target Impedance: 100Ω (characteristic impedance Z₀)
• Trace Type: Microstrip (surface layer with air above)
• Required Width: 0.050mm (2.0 mil)
• Dielectric Constant: 4.5 (FR-4 standard)
• Dielectric Height: 0.2mm (layer spacing)
• Copper Thickness: 35µm (1oz copper)
• Effective Er: 3.033
• Propagation Delay: 5.809 ps/mm

Microstrip vs Stripline for 100Ω

Microstrip traces are routed on the outer layers of the PCB with air above the trace. This results in a lower effective dielectric constant (3.03 vs 4.5 for stripline), requiring a different trace width to achieve 100Ω. Microstrip is preferred for high-frequency designs due to lower propagation delay and easier access for probing.

Common 100Ω Applications

100Ω impedance is required by the following standards and interfaces:

IPC-2141 Impedance Formula

For microstrip, the characteristic impedance is calculated using the Hammerstad & Jensen formula, which accounts for the effective dielectric constant created by the air-dielectric interface. The formula considers trace width (W), dielectric height (H), copper thickness (T), and material properties to accurately predict impedance within ±5% tolerance.