Trace Impedance Calculator
Microstrip and stripline baseline impedance
Required Parameters
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Quick Answer
Microstrip impedance depends heavily on the trace width, dielectric height (prepreg/core thickness), and the substrate's dielectric constant (Er).
Trace Impedance Calculator
Calculate the characteristic impedance of microstrip and stripline PCB traces for high-speed digital and RF designs.
Microstrip Impedance
For a trace on the outer layer of a PCB:
Z0 = (87 / sqrt(Er + 1.41)) x ln(5.98 x H / (0.8 x W + T))
Where Z0 is the impedance in Ohms, Er is the dielectric constant, H is the dielectric thickness, W is the trace width, and T is the copper thickness.
Design Tips
- FR-4 dielectric constant is typically 4.2 - 4.6
- Use controlled impedance PCB stackups from your fab
- Differential pairs need consistent spacing
- Route impedance-controlled traces on reference planes
Related Tools
- PCB Trace Width Calculator
- Reactance Calculator
- Frequency to Wavelength Calculator
Design Notes
This calculator uses IPC-2141 analytic approximations, which are excellent for sub-GHz designs. However, for precise multi-gigabit routing (PCIe, USB 3.0, DDR4), you must rely on 2D/3D field solvers (like Si9000 or built-in Altium/KiCad solvers) and work directly with your PCB manufacturer's controlled-impedance stackup. Prepreg thickness often shrinks during pressing; always let the fab house tweak your final trace width.
Common Mistakes
- 1
Not referencing a solid, continuous ground plane directly beneath the trace (routing over splits causes huge impedance discontinuities).
- 2
Ignoring the solder mask effect: Solder mask slightly lowers the impedance of an external microstrip by 2-3 ohms.
- 3
Using generic FR4 formulas for precise 5GHz+ RF feeds without accounting for the varying Er of woven glass weaves.
Engineering Handbox
1. Apply Microstrip: Z0 = (87 / √(4.4 + 1.41)) × ln((5.98 × 0.18) / (0.8 × 0.3 + 0.035)) 2. Z0 = 36.12 × ln(1.0764 / 0.275) 3. Z0 = 36.12 × 1.364 = 49.2 Ω
Knowledge Base
What is the typical impedance for a PCB trace?
50Ω is the standard for most single-ended RF, digital, and general-purpose signals. 75Ω is used for video (HDTV, cable TV). 90Ω or 100Ω differential is common for USB, HDMI, and Ethernet pairs.
What is the difference between microstrip and stripline?
A microstrip is a trace on an outer layer with one reference ground plane below it. A stripline is an inner-layer trace sandwiched between two ground planes. Stripline offers better shielding but tighter coupling.
What is the dielectric constant (Er) of FR4?
Standard FR4 has an Er of approximately 4.2-4.8 at 1 MHz. At higher frequencies (1-10 GHz), Er drops to roughly 3.8-4.2. For precise impedance control, always use your PCB manufacturer's measured Er value.
How does trace width affect impedance?
Wider traces have lower impedance. For a 50Ω microstrip on standard FR4 (Er=4.3) with 8-mil dielectric height, the trace width is approximately 14-16 mils (0.36-0.40 mm) using 1oz copper.
Why does my PCB manufacturer adjust my trace widths?
PCB manufacturers routinely adjust trace widths for controlled-impedance boards because prepreg thickness changes during lamination pressing (typically 10-15% thinner). They compensate by slightly widening or narrowing traces to hit the target impedance.
What is controlled impedance PCB?
A controlled impedance PCB is one where specific traces are manufactured to a target impedance (e.g., 50Ω ±10%) through precise control of trace width, dielectric height, and copper thickness. Required for USB, PCIe, DDR, HDMI, and any high-speed interface.
Do I need a field solver for impedance calculations?
For frequencies below 1 GHz and standard stackups, analytic formulas (like those in this calculator) are sufficient. For multi-gigabit signals (PCIe Gen3+, USB 3.0, DDR4), use a 2D field solver like Polar Si9000 or the built-in solvers in Altium/KiCad.
How does solder mask affect trace impedance?
Solder mask adds a dielectric layer on top of external traces, lowering the impedance by approximately 2-3 ohms. For precision impedance control, specify solder mask opening (exposed copper) or provide your solder mask Er to the fab house.
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