PCB Impedance
Calculate the characteristic impedance of PCB traces for microstrip and stripline configurations. Enter trace width, copper thickness, dielectric height, and material εr for instant results with charts and breakdowns.
About This Calculator
The PCB Impedance Calculator helps PCB designers, RF engineers, and electronics hobbyists determine the characteristic impedance of copper traces on a printed circuit board. Whether you are designing high-speed digital interfaces (DDR, HDMI, USB, Ethernet), RF circuits (antennas, filters, amplifiers), or simply want to ensure signal integrity in your design, this calculator provides instant results for both microstrip and symmetric stripline configurations.
The calculator implements the industry-standard formulas from IPC-2141A (Design Guide for High-Speed Controlled Impedance Circuit Boards). For microstrip traces (on outer PCB layers), the formula is Z₀ = 87 / √(εr + 1.41) × ln(5.98 × H / (0.8 × W + T)). For symmetric stripline traces (embedded between two reference planes), the formula is Z₀ = 60 / √εr × ln(4 × H / (0.67 × π × W × (0.8 + T/W))). Both formulas assume all dimensions are in the same unit (mm).
In addition to characteristic impedance (Z₀) in ohms, the calculator computes the effective dielectric constant (εreff), which accounts for the mixed dielectric (substrate + air for microstrip) surrounding the trace, the propagation delay in ps/mm, and the per-unit-length capacitance and inductance. The impedance vs trace width chart helps you quickly determine what trace width yields your target impedance — a critical step in PCB stackup design.
Regional Notes
Global (All Regions): PCB impedance design follows universal engineering principles defined by IPC standards. PCB manufacturers worldwide use the same formulas, though they may specify trace dimensions in either mm or mils (1 mil = 0.0254 mm). This calculator accepts mm input. The most common target impedance values are 50 Ω single-ended and 100 Ω differential, used globally for RF, high-speed digital, and telecommunications. Dielectric constant values for common substrates like FR4 (εr ≈ 4.2-4.8) do not vary by region. Always consult your PCB fabricator for their specific stackup capabilities and impedance tolerance (±10% is standard, ±5% is achievable with tighter process control).
Frequently Asked Questions
What is PCB characteristic impedance?
PCB characteristic impedance is the ratio of voltage to current in a transmission line formed by a copper trace and its reference plane. It depends on the trace width, copper thickness, dielectric height to the reference plane, and the dielectric constant of the PCB substrate material. For high-speed digital and RF designs, controlled impedance (typically 50 Ω, 75 Ω, or 100 Ω differential) is critical to prevent signal reflections and ensure signal integrity.
What is the difference between microstrip and stripline?
A microstrip trace is on the outer layer of a PCB with a single reference plane below it, exposing one side to air. A stripline trace is sandwiched between two reference planes within the PCB, providing better shielding and electromagnetic interference (EMI) performance. Microstrip generally offers higher impedance for the same trace width compared to stripline, but stripline provides better isolation between adjacent traces and reduced radiation.
What is the typical impedance for PCB traces?
50 Ω is the most common single-ended impedance standard for RF, high-speed digital, and general-purpose PCB design. 75 Ω is used for video and broadcast applications. Differential pairs commonly target 90 Ω (for USB) or 100 Ω (for HDMI, Ethernet, LVDS). The actual impedance achieved depends on your PCB stackup geometry — a typical 50 Ω microstrip on FR4 with 1 oz copper uses a trace width roughly twice the dielectric height.
How do I choose the PCB substrate dielectric constant?
Standard FR-4 has a dielectric constant (εr) of approximately 4.2 to 4.8 depending on the resin content and frequency. For high-frequency designs above 1 GHz, consider low-loss materials like Rogers RO4003C (εr ≈ 3.38), Isola FR408 (εr ≈ 3.65), or PTFE-based laminates (εr ≈ 2.2). Always use the manufacturer's specified εr at your operating frequency, as dielectric constant varies with frequency.
What happens if the PCB trace impedance is mismatched?
Impedance mismatch causes signal reflections at the discontinuity, leading to ringing, overshoot, reduced noise margins, and electromagnetic interference (EMI). In digital circuits, this degrades the eye diagram and can cause logic errors at high speeds. In RF circuits, mismatch reduces power transfer (measured by return loss / VSWR) and can damage sensitive transmitter components. Controlled impedance design with a continuous return path is essential above 50 MHz or for rise times under 1 ns.
How does copper thickness affect trace impedance?
Thicker copper reduces the characteristic impedance slightly because it increases the trace cross-section, lowering the inductance-to-capacitance ratio. Standard 1 oz copper (35 µm) is the baseline — switching to 2 oz copper (70 µm) typically reduces impedance by 2-3 Ω for a 50 Ω microstrip design. To maintain the same target impedance with thicker copper, you must increase the trace width slightly or adjust the dielectric height.
What is propagation delay in PCB traces?
Propagation delay is the time a signal takes to travel the length of a PCB trace. It depends on the effective dielectric constant of the substrate material and is approximately 3.3356 × √εr_eff ps/mm. For FR4 (εr_eff ≈ 3.3), the delay is about 6 ps/mm (152 ps/inch). This delay must be matched within differential pairs and across parallel data buses (e.g., DDR memory) to ensure all signals arrive simultaneously at the receiver.
How is PCB impedance calculated?
PCB impedance is calculated using formulas from the IPC-2141 standard. For microstrip, Z₀ = 87 / √(εr + 1.41) × ln(5.98 × H / (0.8 × W + T)), where W is trace width, T is copper thickness, H is dielectric height, and εr is the dielectric constant. For symmetric stripline, Z₀ = 60 / √εr × ln(4 × H / (0.67 × π × W × (0.8 + T/W))). These formulas assume trace dimensions are in consistent units.