Manufacturing and Production PCB Fabrication for RF Informational

What is the effect of etching undercut on microstrip impedance and how do I compensate for it?

Q: Should I specify trace width or impedance?

Always specify impedance (not trace width) in the fabrication notes. Why: the fab house knows their specific etch undercut (which varies with their chemistry and equipment). They will calculate the required design width to achieve your target impedance after accounting for their undercut. If you specify a fixed width: the fab house will produce exactly that width (after their etch), which may not match your intended impedance. Best practice: "50 Ω ±5%, microstrip, reference to Layer 2 ground, Rogers RO4350B, 10 mil dielectric." The fab house takes it from there.

Q: Does copper weight affect RF loss?

Yes, through two mechanisms: (1) Thicker copper (2 oz vs 1 oz) has lower DC resistance but more undercut (wider trace needed, less dense routing). (2) At RF frequencies: the current flows in the skin depth (e.g., 0.66 μm at 10 GHz in copper). The copper thickness beyond a few skin depths does not reduce RF loss. The surface roughness on the bottom of the copper (facing the substrate) determines the conductor loss at RF. Thicker copper often has rougher surfaces (due to the electrodeposition process). For minimum RF loss: use 0.5 oz copper (thinner, less undercut) with very low profile (VLP) surface treatment (smooth bottom surface).

Q: What about laser-etched traces?

Laser ablation of copper: uses a laser (UV excimer or CO₂) to remove copper. Advantages: no chemical undercut (the laser removes copper vertically). Trace width tolerance: ±5-10 μm (much better than chemical etch). Vertical sidewalls (rectangular profile, not trapezoidal). Disadvantages: slow (one trace at a time vs batch chemical etch). Expensive (suitable for prototyping, not mass production). Can damage the underlying dielectric (heat-affected zone). Best for: prototype and small-batch mmWave circuits where tolerances are critical.

What is the effect of etching undercut on microstrip impedance, and how do you compensate for it? Etching undercut occurs during the copper etching process when the etchant removes copper laterally beneath the photoresist, reducing the final trace width below the designed value: (1) Mechanism: the etch process removes copper isotropically (in all directions, not just vertically). The lateral etch beneath the resist is the "undercut." For a 1 oz copper (35 μm thick) trace: undercut per side ≈ 25-35 μm (nearly equal to the copper thickness). Total width reduction: 50-70 μm (2-2.8 mil). The final trace width = designed width - 2 × undercut. For 2 oz copper (70 μm): undercut per side ≈ 50-70 μm. Total width reduction: 100-140 μm (4-5.6 mil). (2) Trapezoidal cross-section: the undercut creates a trapezoidal trace profile (wider at the bottom, narrower at the top). The bottom width is closer to the designed width. The top width is narrower. This trapezoidal shape affects the impedance calculation (the effective width is between the top and bottom widths). (3) Impedance impact: for a 50 Ω microstrip on 10 mil Rogers RO4350B (Dk = 3.48): designed width = 22 mil. After etching (1 oz Cu, 2.5 mil undercut per side): top width = 22 - 5 = 17 mil. Bottom width ≈ 22 mil. Effective width ≈ 19.5 mil. Impedance change: from 50 Ω to approximately 54-55 Ω (8-10% increase). This is a significant shift that must be compensated. (4) Compensation: pre-compensation: increase the designed trace width by the expected undercut: W_design = W_target + 2 × undercut. For the example above: W_design = 22 + 5 = 27 mil. The fab house will etch back to approximately 22 mil effective width. Most impedance calculators and EM simulators can include the trapezoidal profile. Use the trapezoidal model (not the rectangular model) for accurate impedance prediction. (5) Working with the fab house: provide the target impedance (e.g., "50 Ω ± 5%") rather than a fixed trace width. The fab house adjusts the artwork trace width based on their known etch factors for the specific copper weight and etch chemistry. This is the standard practice for impedance-controlled PCBs. Request cross-section analysis of test coupons to verify the actual trace profile.

Category: Manufacturing and Production

Updated: April 2026

Product Tie-In: PCB Substrates, Laminates

Etch Undercut Compensation

Etch undercut is one of the most significant fabrication effects in RF PCB manufacturing, and understanding it is essential for first-pass impedance accuracy.

Advanced Etch Processes

(1) Standard subtractive etch: the most common PCB process. Copper is selectively removed by chemical etching. Undercut is inherent (isotropic etch). Minimum feature size: 3-4 mil for 1 oz Cu, 5-6 mil for 2 oz Cu. (2) Modified subtractive etch: tighter process control using spray etching (more directional than immersion etch). Undercut reduced to 60-80% of standard. Higher cost but better impedance control. (3) Semi-additive process (SAP): start with a thin copper seed layer (< 5 μm). Pattern the photoresist to define the traces. Electroplate copper up to the desired thickness (typically 18-35 μm). Strip the resist and flash-etch the seed layer. The undercut is minimal (only the thin seed layer is etched). Achieves trace width tolerances of ±10-15 μm (vs ±25-35 μm for subtractive). Used for: high-density interconnects (HDI) and mmWave circuits where tight impedance control is critical.

Etch Undercut

1 oz Cu: undercut ≈ 25-35 μm per side
2 oz Cu: undercut ≈ 50-70 μm per side
W_final = W_design - 2×undercut
Pre-compensate: W_design = W_target + 2×undercut
Trapezoidal profile: use effective width for Z

Common Questions

Frequently Asked Questions

Should I specify trace width or impedance?

Always specify impedance (not trace width) in the fabrication notes. Why: the fab house knows their specific etch undercut (which varies with their chemistry and equipment). They will calculate the required design width to achieve your target impedance after accounting for their undercut. If you specify a fixed width: the fab house will produce exactly that width (after their etch), which may not match your intended impedance. Best practice: "50 Ω ±5%, microstrip, reference to Layer 2 ground, Rogers RO4350B, 10 mil dielectric." The fab house takes it from there.

Does copper weight affect RF loss?

Yes, through two mechanisms: (1) Thicker copper (2 oz vs 1 oz) has lower DC resistance but more undercut (wider trace needed, less dense routing). (2) At RF frequencies: the current flows in the skin depth (e.g., 0.66 μm at 10 GHz in copper). The copper thickness beyond a few skin depths does not reduce RF loss. The surface roughness on the bottom of the copper (facing the substrate) determines the conductor loss at RF. Thicker copper often has rougher surfaces (due to the electrodeposition process). For minimum RF loss: use 0.5 oz copper (thinner, less undercut) with very low profile (VLP) surface treatment (smooth bottom surface).

What about laser-etched traces?

Laser ablation of copper: uses a laser (UV excimer or CO₂) to remove copper. Advantages: no chemical undercut (the laser removes copper vertically). Trace width tolerance: ±5-10 μm (much better than chemical etch). Vertical sidewalls (rectangular profile, not trapezoidal). Disadvantages: slow (one trace at a time vs batch chemical etch). Expensive (suitable for prototyping, not mass production). Can damage the underlying dielectric (heat-affected zone). Best for: prototype and small-batch mmWave circuits where tolerances are critical.

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