Microstrip Impedance Calculator
Design PCB transmission lines. Calculate Characteristic Impedance (Zo) and Effective Dielectric Constant.
PCB Parameters
PCB Design Quick Reference
Engineering Principles: Microstrip Transmission Line
A microstrip is a type of electrical transmission line which can be fabricated using printed circuit board (PCB) technology. It involves a single conductor trace separated from a ground plane by a dielectric layer.
IPC-2141 simplified formula for surface microstrip.
Accounts for fields existing partially in air and dielectric.
Signal speed is slower than speed of light (c) in vacuum.
Technical Notes
- •Formulas are most accurate for w/h ratios between 0.1 and 3.0.
- •FR-4 (εr ≈ 4.4) is standard for low cost, but Rogers materials are preferred for high frequency (>1GHz) consistency.
- •Controlled impedance routing is critical for high-speed digital lines (USB, PCIe) and RF traces.
Frequently Asked Questions
What is Controlled Impedance?
Controlled impedance is the practice of maintaining a consistent characteristic impedance (Z0) along the entire length of a PCB trace. This is essential for high-speed digital signals (USB, PCIe, DDR) and RF signals to prevent reflections that corrupt data or reduce power transfer.
Why FR-4 dielectic constant varies?
Standard FR-4 is a composite material of woven glass and epoxy resin. The dielectric constant (Er) can vary between 4.2 and 4.8 depending on the resin content, weave style, and frequency. For critical RF designs, consider using materials with tighter tolerances like Rogers or Isola.
Can I use Microstrip for 50 Ohm matching?
Yes, microstrip is the most common transmission line structure for 50Ω matching on PCBs. By selecting the correct trace width (W) for a given dielectric height (H) and constant (Er), you can design a line that looks exactly like a 50Ω resistor to the RF signal.
How does copper thickness affect impedance?
Copper thickness (e.g., 1oz vs 0.5oz) has a secondary effect on impedance. Thicker copper slightly reduces the impedance (Z0) and lowers resistive losses. However, the effect is small compared to Trace Width or Dielectric Height changes.
What is the difference between Microstrip and Stripline?
Microstrip is an outer-layer trace with a single ground plane beneath it. Stripline is an inner-layer trace sandwiched between two ground planes. Stripline is fully shielded but requires different design formulas and has slower signal propagation.
When should I use Rogers instead of FR-4?
Use Rogers (or other high-frequency laminates) when: 1) Frequency > 2 GHz, 2) You need very stable impedance control, 3) You need low dielectric loss (tan δ) for long traces. For basic <1GHz designs, FR-4 is usually sufficient.