PCB & Transmission Line Calculators

Where PCB & Transmission Line Calculators Fit

A trace behaves like a wire until the signal’s rise time gets short compared with the trip down the board. After that it is a transmission line, with its own impedance, delay, and coupling. PCB & Transmission Line Calculators give those numbers from the stackup.

Two jobs come up most. One is hitting a target impedance, usually 50 ohms single-ended. The other is carrying current without cooking the copper.

Microstrip, Stripline, and Impedance

The Pcb Impedance Calculator covers both structures in one place. The Microstrip Impedance Calculator, Embedded Microstrip Impedance Calculator, and Asymmetric Stripline Impedance Calculator handle each geometry in detail.

Microstrip runs on an outer layer with one plane below. Part of its field sits in the board and part in the air. Stripline runs on an inner layer between two planes, so the whole field stays in the laminate.

That difference drives the results. Stripline sees the full dielectric constant, so it needs a narrower trace for the same impedance and its signals travel slower. Microstrip is easier to route and probe, but it radiates more and reacts to whatever sits above it.

Impedance depends on four things: trace width, height to the reference plane, copper thickness, and the dielectric constant. The height that counts is trace to plane, not overall board thickness, and getting that wrong is the usual reason a calculated value misses the board.

A useful starting point on FR-4, where the dielectric constant is about 4.2 to 4.6: a 50 ohm microstrip needs a width near twice the dielectric height. A 6 mil dielectric with 1 oz copper is a common combination because it lands on a width that fabs can hold.

Two limits are worth knowing. The IPC-2141 microstrip equation holds for width-to-height ratios between 0.1 and 2.0, and drifts outside that range. Solder mask over a microstrip lowers impedance by roughly 2 to 5 ohms, which is the difference between bare and coated microstrip.

Closed-form results land within a few percent of a field solver for ordinary FR-4 geometry. Fabrication tolerance is the wider variable, commonly ±10% on controlled impedance, so specify impedance control on the fab drawing and use test coupons when it matters.

Trace Width and Resistance

The Microstrip Trace Width Calculator answers the current question, using the IPC-2221 thermal model. Width follows from the current and the temperature rise you accept.

Layer position changes the answer sharply. Outer-layer traces shed heat to air and use a constant of 0.048 in that model, while inner-layer traces sit between dielectric and use 0.024. An inner trace therefore needs to be much wider for the same current.

The Pcb Trace Resistance Calculator gives the resistance of a finished trace from length, width, copper thickness, and temperature. That resistance sets voltage drop on power traces and heat in the copper.

Copper weight is a thickness in disguise. One ounce per square foot is about 1.4 mils, or 35 µm, so a 2 oz board has half the resistance per square.

Delay, Wavelength, and Inductance

The Microstrip Wavelength Calculator gives guided wavelength, effective dielectric constant, and propagation delay for a trace.

Delay follows the square root of the effective dielectric constant. In air a signal takes about 85 ps per inch. On FR-4 microstrip, with an effective constant near 2.8, that becomes roughly 140 to 150 ps per inch. Stripline sees the full constant near 4.5, so it runs about 170 to 180 ps per inch.

A handy check: signals cover about 6 inches per nanosecond on inner layers. A 6 inch trace adds roughly 1 ns, which matters once rise times drop to a few hundred picoseconds.

The Microstrip Inductance Calculator returns inductance in nanohenries, reactance, and inductance per inch. Short traces still have inductance, and at high frequency that is what turns a decoupling path or a ground connection into a problem.

Crosstalk Between Traces

The Microstrip Crosstalk Calculator and Stripline Crosstalk Calculator estimate the voltage a switching trace induces on its neighbour.

Four inputs drive the result: spacing between traces, height to the plane, dielectric constant, and the driver’s rise time. Faster edges couple harder, which is why a design can pass at one clock speed and fail with a faster part in the same footprint.

Spacing relative to plane height matters more than raw spacing. Bringing the reference plane closer ties the field down and cuts coupling, so a thin dielectric helps as much as extra gap.

Stripline couples less than microstrip at the same geometry, because both planes confine the field. Routing sensitive signals on inner layers is the usual fix when spacing alone is not enough.

Coax and Twisted Pair

Off the board, the same theory applies to cable. The Coax Impedance Calculator works from inner conductor diameter, shield diameter, and dielectric constant, which is why 50 ohm and 75 ohm cables have different inner conductors at the same outside size.

The Coax Inductance Calculator adds inductance, capacitance, and velocity factor for the same cable. Velocity factor is what turns electrical length into physical length when cutting a cable to a quarter wavelength.

The Twisted Pair Impedance Calculator takes wire diameter, spacing, and insulation to give characteristic impedance. It covers the differential pairs used in Ethernet, RS-485, and sensor wiring.

Common Mistakes

Using board thickness as dielectric height. Only the distance from trace to its reference plane sets impedance.

Assuming one dielectric constant. FR-4 ranges from about 4.2 to 4.6 and shifts with frequency, so use the laminate datasheet.

Sizing an inner trace with outer-layer rules. Buried copper cannot shed heat, and needs far more width for the same current.

Ignoring solder mask on RF traces. The coating pulls microstrip impedance down by a few ohms.

PCB and Cable FAQs

What trace width gives 50 ohms on FR-4?

For microstrip, start with a width about twice the dielectric height. On a 6 mil dielectric with 1 oz copper, that lands near 11 to 12 mils. Stripline needs a narrower trace for the same 50 ohms, since its field stays in the laminate. Always check the number against your fab’s own stackup, because plating and etch tolerance shift it.

Microstrip or stripline for high-speed signals?

Stripline gives better shielding, lower crosstalk, and steadier impedance, so it suits critical clocks and long buses. Microstrip is easier to route, easier to probe, and faster by about 30 ps per inch. Many boards use both: microstrip for connector breakout, stripline for the sensitive runs.

How wide does a trace need to be for 3 amps?

It depends on copper weight, allowed temperature rise, and layer. The IPC-2221 model uses a constant of 0.048 for outer layers and 0.024 for inner ones, so an inner trace is roughly twice as wide for the same current and rise. Enter 10 °C rise for conservative power traces, and 20 °C where a warmer trace is acceptable.

How much delay does a PCB trace add?

On FR-4, about 140 to 150 ps per inch for microstrip and 170 to 180 ps for stripline. The underlying figure is 85 ps per inch in air, multiplied by the square root of the effective dielectric constant. So a 6 inch inner-layer trace adds roughly 1 ns, which is why length matching exists on DDR and other parallel buses.

How far apart should traces be to avoid crosstalk?

Judge spacing against the height to the reference plane, not on its own. A common starting rule keeps edge-to-edge spacing at least three times the trace width for sensitive signals, then the calculator checks the actual coupled voltage for your rise time. Moving the plane closer or routing as stripline cuts coupling further.

Why is coax 50 ohms or 75 ohms?

Impedance comes from the ratio of shield diameter to inner conductor diameter, with the dielectric constant scaling the result. 50 ohms is a compromise between low loss and power handling, so it dominates RF and test gear. 75 ohms gives slightly lower loss for signal work, which is why video and broadcast use it.