pcb-toolkit

A Rust library and command-line tool for PCB design calculations — impedance, current capacity, via properties, and more.

Formulas are implemented from the published literature and cited in the source. Saturn PCB Toolkit was the starting point and is still a useful cross-check, but it is not a specification — where it is wrong, we do not match it. The second reference is the set of Mathcad worksheets shipped with Johnson & Graham's High-Speed Digital Design, which is where the non-planar lines, Cohn's stripline, the signal-integrity rules of thumb, loop inductance and plane spreading resistance come from. See Relationship to Saturn PCB Toolkit below, and VALIDATION.md for what is verified against what.

Workspace

Crate	Description
pcb-toolkit	Core library: calculators, material database, unit conversion
pcb-toolkit-cli	Command-line interface wrapping the library
pcb-toolkit-desktop	Tauri desktop GUI (apps/desktop)

Desktop app

A GUI covering every calculator lives in apps/desktop:

cd apps/desktop
npm install
npm run tauri dev

See apps/desktop/README.md for building installers.

Building

Requires Rust 1.85+ (edition 2024).

cargo build --workspace

CLI

The CLI binary is called pcb-toolkit. All commands support --json for machine-readable output. Dimensional inputs accept units: 10mil, 0.254mm, 1GHz, 10nF, etc.

Commands

pcb-toolkit <COMMAND> [OPTIONS]

impedance      Transmission line impedance (microstrip, stripline, embedded, coplanar)
differential   Differential pair impedance (5 topologies)
current        Conductor current capacity (IPC-2221A)
fusing         Fusing current (Onderdonk equation)
via            Via impedance and parasitic properties
inductor       Planar spiral inductor
loop           Single-turn loop inductance (circular or rectangular)
wire           Round wire over a plane, coax, twisted pair
signal         Knee frequency, critical length, reflections, Er from delay
resistance     Plane spreading resistance
reactance      Capacitive/inductive reactance and resonant frequency
wavelength     Wavelength in a dielectric
ohms-law       V=IR, LED bias, attenuators, series/parallel R/C/L
ppm            PPM/Hz conversion, crystal load capacitance
padstack       Padstack geometry (thru-hole, corner-to-corner)
spacing        Conductor spacing (IPC-2221C)
wire-gauge     AWG wire gauge properties
pdn            PDN impedance calculator
thermal        Thermal management (junction temperature)
crosstalk      Crosstalk estimation (NEXT)

Examples

$ pcb-toolkit signal knee -t 0.35

Signal Edge Bandwidth
─────────────────────
  Rise time = 0.3500 ns
  F_knee    = 1428.5714 MHz

A 350 ps edge is a 1.4 GHz design problem whatever the clock rate, and on FR-4 stripline the longest run you can leave unterminated is:

$ pcb-toolkit signal critical-length -t 0.35 --tpd 180

Critical Length
───────────────
  L_crit  = 0.3241 in
  L_crit  = 8.2315 mm
  Flight  = 0.0583 ns  (= Tr/6)
  Longer runs should be treated as transmission lines.

$ pcb-toolkit impedance microstrip -w 10 --height 5 --er 4.6

Microstrip Impedance
────────────────────
  Zo      = 44.8322 Ω
  Er_eff  = 3.3075
  Tpd     = 154.0858 ps/in
  Lo      = 6.9080 nH/in
  Co      = 3.4369 pF/in

$ pcb-toolkit differential edge-coupled-external -w 10 --spacing 10 --height 5 -t 1.4mil --er 4.6

Edge-Coupled External Differential
───────────────────────────────────
  Zdiff    = 76.3503 Ω
  Zo       = 41.0649 Ω
  Zodd     = 38.1751 Ω
  Zeven    = 44.1735 Ω
  Kb       = 0.072841
  Kb       = -22.7525 dB
  Kb_term  = 0.036469
  Kb_term  = -28.7616 dB

$ pcb-toolkit pdn --voltage 5 --current 2 --i-step 50 --v-ripple 5 --area 5 --er 4.6 --distance 2 --freq 1

PDN Impedance
─────────────
  Z target   = 0.250000 Ω
  C plane    = 2587.5000 pF
  Xc         = 61.509157 Ω

$ pcb-toolkit --json impedance microstrip -w 10 --height 5 --er 4.6

{
  "zo": 44.8322367013137,
  "er_eff": 3.307496138618308,
  "tpd_ps_per_in": 154.08582451095563,
  "lo_nh_per_in": 6.908012156792247,
  "co_pf_per_in": 3.4369426075598972
}

Library

Add the dependency:

[dependencies]
pcb-toolkit = "0.1"

use pcb_toolkit::impedance::microstrip::{self, MicrostripInput};

let result = microstrip::calculate(&MicrostripInput {
    width: 10.0,       // mils
    height: 5.0,       // mils
    thickness: 1.4,    // mils (1 oz copper)
    er: 4.6,           // FR-4
    frequency: 0.0,    // Hz (0 = static)
}).unwrap();

println!("Zo = {:.2} Ohms", result.zo);
println!("Er_eff = {:.4}", result.er_eff);

All public functions return Result<T, CalcError>. Inputs are validated at the boundary — negative dimensions, out-of-range dielectric constants, and unknown materials are rejected with descriptive errors.

Materials Database

45 built-in substrate materials with dielectric constant (Er), glass transition temperature (Tg), and surface roughness correction factor. Includes FR-4 variants, Rogers, Isola, Getek, Arlon, Nelco, Ventec, Panasonic, and Teflon.

use pcb_toolkit::materials;

let fr4 = materials::lookup("FR-4 STD").unwrap();
assert_eq!(fr4.er, 4.6);
assert_eq!(fr4.tg, Some(130.0));

Lookup is case-insensitive. Custom Er values can be passed directly to calculator functions.

Relationship to Saturn PCB Toolkit

This project started by reverse-engineering Saturn PCB Toolkit, and Saturn is still a useful source of formulas and test geometries. It is not a compatibility target — it is wrong in places (its crosstalk calculator is high by 48×; its internal differential coefficient is a transposition of the published value; it carries π = 3.14159 and c = 11.8 in/ns). We implement the published model with full-precision constants, and document every divergence with its numerical cause in docs/notes/18-v847-vs-rust-comparison.md.

Practically, that means our impedances run about 0.069% below Saturn's (we use µ₀c = 376.7303 Ω where it uses 120π) and our delays about 0.025% below (exact c, not 11.8 in/ns).

Design Decisions

• f64 everywhere. IEEE 754 double precision for all calculations. No arbitrary-precision or unit-of-measure crates.
• Canonical internal units. Mils for length, Hz for frequency, Farads for capacitance. Conversion happens at the API boundary via the units module.
• Minimal dependencies. thiserror + serde for the library. clap + anyhow + serde_json + toml for the CLI.

Testing

cargo test --workspace

Test a specific calculator:

cargo test -p pcb-toolkit impedance

Float comparisons use the approx crate (assert_relative_eq!). Reference vectors come from the Saturn PCB Toolkit help PDF, from standard tables (AWG, IPC-2221C), and from closed forms worked by hand; VALIDATION.md lists which calculators have which. The CLI has its own integration tests in crates/pcb-toolkit-cli/tests/cli.rs that drive the real binary.

License

Licensed under either of Apache License, Version 2.0 or MIT License at your option.