90Ω Stripline Calculator
Controlled Impedance | FR-4 (Er=4.5) | IPC-2141
For 90Ω stripline on FR-4 with 0.2mm dielectric:
Impedance Parameters
MOD: IMP_CAL_V1Impedance Check
Understanding 90Ω Stripline Impedance
90Ω differential impedance is required for USB 3.0, SATA, and PCIe high-speed interfaces. To achieve 90Ω characteristic impedance using a striplinetrace on standard FR-4 material (Er=4.5) with 0.2mm dielectric height, you need a trace width of approximately 0.050mm (2.0 mil). This calculation follows the IPC-2141 standard and Hammerstad equations for accurate impedance control.
Key Parameters for 90Ω Design
- Target Impedance: 90Ω (characteristic impedance Z₀)
- Trace Type: Stripline (centered between two ground planes)
- Required Width: 0.050mm (2.0 mil)
- Dielectric Constant: 4.5 (FR-4 standard)
- Dielectric Height: 0.2mm (layer spacing)
- Copper Thickness: 35µm (1oz copper)
- Effective Er: 4.500
- Propagation Delay: 7.076 ps/mm
Stripline vs Microstrip for 90Ω
Stripline traces are embedded between two ground planes, completely surrounded by dielectric material. This provides better EMI shielding and more consistent impedance but requires wider traces for the same impedance value. Stripline is ideal for sensitive signals requiring maximum isolation.
Common 90Ω Applications
90Ω impedance is required by the following standards and interfaces:
90Ω Standard Applications
IPC-2141 Impedance Formula
For stripline, the impedance calculation is simpler as the trace is fully embedded in the dielectric. The formula uses: Z₀ = (60/√Er) × ln(4b / (0.67π(0.8W + T))) where b is the total dielectric thickness and W is the trace width. This provides better impedance consistency compared to microstrip.
Frequently Asked Questions
How tight should my 90Ω trace width tolerance be?
Most high-speed interfaces require ±10% impedance tolerance. For a 90Ω target, this means81Ω to 99.00000000000001Ω is acceptable. Specify controlled impedance to your PCB manufacturer and expect trace width tolerance of ±0.5-1 mil to achieve this.
Does copper thickness affect 90Ω impedance?
Yes, thicker copper slightly lowers impedance due to increased effective trace width. For precision designs, always include copper thickness in your impedance calculations. Our calculator uses35µm (1oz) as the standard.
What if my PCB uses different dielectric than FR-4?
High-speed designs often use low-loss materials like Rogers 4350B (Er=3.48) or Megtron 6 (Er=3.4). Lower Er requires wider traces for the same impedance. Use our full calculator above to input your specific material properties.
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Complete Your PCB Design
After calculating 90Ω stripline trace width, use our other free tools to complete your high-speed PCB design. Calculate trace current capacity and via sizing for your power delivery network.