Free PCB Microstrip Impedance Calculator

Q: Why does solder mask affect trace impedance?

Liquid photoimageable (LPI) solder mask has a high dielectric constant ($\varepsilon_r \approx 3.8$ to $4.2$). When coated over a surface microstrip, it pulls fringing fields into the polymer, dropping characteristic impedance ($Z_0$) by approximately 2 to 3 ohms.

Q: How does dielectric thickness (h) influence 50 ohm trace width?

To maintain a 50 ohm impedance on FR-4 ($arepsilon_r approx 4.4$), trace width must roughly equal 1.8 times the dielectric height ($w approx 1.8 cdot h$). If your 4-layer PCB stackup uses a thin 4-mil prepreg, a 50-ohm trace is ~7.2 mils wide; on a thick 2-layer 62-mil board, a 50-ohm trace must be a massive 110 mils wide!

Q: What is the difference between microstrip and stripline?

A microstrip runs on an outer layer exposed to air on one side and a ground plane on the other. A stripline runs on an inner layer sandwiched between two parallel ground planes, fully immersed in dielectric with zero air exposure.

Calculate characteristic trace impedance (Z0), effective dielectric constant, and propagation delay for surface microstrip RF lines.

📡 Substrate & Trace Geometry

PCB Dielectric Substrate Material

Dielectric Const (ε_r)

Dielectric Height h (mils) Prepreg thickness to GND plane

Trace Width w (mils) 0.279 mm

Copper Thickness t (oz)

📊 Transmission Line Characteristics

Characteristic Impedance (Z_0)

50.2 Ω

Excellent 50Ω RF Match (< 1% deviation)

Effective Dielectric (ε_eff)

3.27

Substrate + air fringing

Propagation Delay (T_pd)

153.2 ps / in

6.03 ps / mm (0.55c)

Trace Width / Height (w/h): 1.83 (Wide Microstrip)

Inductance per Inch: 7.69 nH / in

Capacitance per Inch: 3.05 pF / in

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Understanding PCB Microstrip Impedance

A surface microstrip is a flat copper trace fabricated on the outermost layer of a printed circuit board separated from a solid ground reference plane by a dielectric substrate. Because electromagnetic fields travel partly through the dielectric core and partly through ambient air, the line exhibits an effective dielectric constant ($arepsilon_{ ext{eff}}$) that dictates phase velocity and characteristic impedance ($Z_0$).

IPC-2141 Formula (for $w/h ge 1$)

Z_0 = \frac{87}{\sqrt{\varepsilon_r + 1.41}} \cdot \ln\left(\frac{5.98 \cdot h}{0.8 \cdot w + t}\right) \quad \text{(IPC-2141 Simplified)}

\varepsilon_{\text{eff}} \approx \frac{\varepsilon_r + 1}{2} + \frac{\varepsilon_r - 1}{2} \cdot \left(1 + 12 \cdot \frac{h}{w}\right)^{-0.5}

T_{\text{pd}} = \frac{\sqrt{\varepsilon_{\text{eff}}}}{c} = 84.72 \cdot \sqrt{\varepsilon_{\text{eff}}} \quad (\text{ps/inch})

Why 50 Ohms is the Universal RF Standard

Coaxial cables achieve minimum attenuation loss at $77 Omega$ and maximum RF power handling at $30 Omega$. The arithmetic compromise—approximately $50 Omega$—was adopted globally by military and microwave engineers as the ideal universal reference impedance for test equipment, antennas, and transceiver ICs.

Frequently Asked Questions

Why does solder mask affect trace impedance?

Liquid photoimageable (LPI) solder mask has a high dielectric constant ($\varepsilon_r \approx 3.8$ to $4.2$). When coated over a surface microstrip, it pulls fringing fields into the polymer, dropping characteristic impedance ($Z_0$) by approximately 2 to 3 ohms.

How does dielectric thickness (h) influence 50 ohm trace width?

To maintain a 50 ohm impedance on FR-4 ($arepsilon_r approx 4.4$), trace width must roughly equal 1.8 times the dielectric height ($w approx 1.8 cdot h$). If your 4-layer PCB stackup uses a thin 4-mil prepreg, a 50-ohm trace is ~7.2 mils wide; on a thick 2-layer 62-mil board, a 50-ohm trace must be a massive 110 mils wide!

What is the difference between microstrip and stripline?

A microstrip runs on an outer layer exposed to air on one side and a ground plane on the other. A stripline runs on an inner layer sandwiched between two parallel ground planes, fully immersed in dielectric with zero air exposure.