PCB Stackup & Impedance Calculator
Design PCB stackups and calculate controlled impedance for high-speed signals. Interactive stackup builder with microstrip, stripline, and differential pair calculations.
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Impedance Calculator
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How This Calculator Works
Impedance Calculation Methods
Microstrip (Surface Traces)
Uses Wheeler's approximation with modifications:
Where: W = trace width, h = dielectric height, t = copper thickness, εr = dielectric constant
Stripline (Embedded Traces)
For traces between two ground planes:
Differential Pairs
Approximation from single-ended impedance:
⚠️ This is a simplified model. For critical designs, use a 2D field solver (like Saturn PCB, Polar Si9000, or free tools like TNT, AppCAD).
Current Carrying Capacity
Based on IPC-2221 guidelines (simplified):
This gives a rough estimate for external layers at 10°C rise. For precise current requirements, consult IPC-2221A or use dedicated trace width calculators.
Note on Accuracy: These formulas are industry-standard approximations accurate to ±5-10% for most practical PCB geometries. For mission-critical designs (aerospace, medical), always validate with your fab's impedance modeling service or commercial field solvers.
Understanding PCB Stackups
💡 What is Controlled Impedance?
Controlled impedance ensures signal integrity for high-speed digital and RF signals. By carefully designing trace geometry and stackup, you match the characteristic impedance (Z₀) to your system's requirements (typically 50Ω for single-ended or 90-100Ω for differential).
⚡ When Do You Need It?
Any signal faster than ~50MHz or with rise times under ~1ns needs impedance control. This includes USB 3.0+, HDMI, DisplayPort, Ethernet (1Gbps+), DDR memory, PCIe, and most RF applications.
📐 Microstrip vs Stripline
Microstrip: Trace on outer layer with one reference plane. Easier to route, more susceptible to EMI.
Stripline: Trace sandwiched between two reference planes. Better signal integrity, lower EMI, but harder to route.
🎯 Typical Tolerances
Standard PCB fabrication achieves ±10% impedance tolerance. For tighter control (±5%), request a controlled-impedance service from your fab. They'll provide test coupons and adjust trace widths based on their process.
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Frequently Asked Questions
How do I choose between 2-layer and 4-layer PCB?
Use 2-layer for simple, low-speed designs (<50MHz). Go 4-layer when you need: continuous ground plane for return currents, better EMI performance, or controlled impedance for high-speed signals (USB 3.0, HDMI, Ethernet). The cost difference is often worth it for signal integrity.
What dielectric constant (Er) should I use for FR4?
Standard FR4 has Er ≈ 4.2-4.5 at DC, but it drops to ~4.0 at 1GHz. For most designs, use Er = 4.2 as a starting point. Your fab will provide the exact value for their laminate. For critical designs, request the fab's impedance calculation or use a field solver.
How does copper weight affect impedance?
Heavier copper (2oz vs 1oz) increases trace thickness, which slightly lowers impedance. Always specify copper weight to your fab when requesting controlled impedance. Standard is 1oz (35µm) for signal layers, but power layers often use 2oz (70µm).
Why 90Ω or 100Ω for differential pairs?
Differential impedance is typically 2× the single-ended impedance (with some coupling factor). USB 3.0 uses 90Ω, PCIe and HDMI use 85-100Ω, Ethernet uses 100Ω. Check your interface spec. The calculator helps you find the trace width and spacing to achieve the target.