Manufacturing

Chemical Etching

Q: How does undercut affect RF transmission line dimensions?

Chemical etching dissolves metal isotropically, meaning it removes material laterally under the photoresist mask as well as vertically through the copper thickness. This undercut narrows the final trace width by approximately one copper thickness per side. For a 50-ohm microstrip on 10-mil Rogers RO4003C with 1-oz (35 micrometer) copper, a 0.5:1 etch factor causes 17.5 micrometers of undercut per side, narrowing the trace by 35 micrometers total. At 77 GHz, this shifts the line impedance by 2 to 4 ohms, enough to degrade return loss from 20 dB to 12 dB. RF PCB designers compensate by adding etch compensation (widening the artwork trace) based on the fabricator's characterized etch factor.

Q: What etchant chemistry is used for RF PCB copper?

The two dominant etchants are cupric chloride (CuCl2) for inner layers and alkaline ammoniacal etchant for outer layers with tin-lead or tin etch resist. Cupric chloride operates at 50 to 55 degrees C with an etch rate of 25 to 40 micrometers per minute for 1-oz copper. It produces a more vertical sidewall profile (etch factor 2.5 to 3.5:1) than ferric chloride (etch factor 2:1), which is important for maintaining consistent trace impedance. For PTFE-based RF laminates like Rogers RT/duroid 5880, sodium persulfate is sometimes preferred because it does not attack the PTFE surface, preserving the substrate dielectric constant and loss tangent.

Q: What is the minimum feature size achievable with chemical etching for RF circuits?

Standard spray-etch processes on rigid PCB laminates achieve minimum feature widths of 75 micrometers (3 mil) for 1-oz copper and 50 micrometers (2 mil) for half-oz copper. Advanced photochemical machining (PCM) shops using thin-film photoresist and precision spray nozzles reach 25 micrometers on thin foils (9 to 18 micrometers copper). For MMIC wafers, wet chemical etching of GaAs mesa structures achieves sub-micrometer resolution using phosphoric/hydrogen-peroxide mixtures, though dry plasma etching (RIE/ICP) is preferred above 40 GHz for its anisotropic profile.

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A subtractive fabrication process that uses controlled acid or alkaline solutions to selectively dissolve unprotected metal from copper-clad PCB laminates, waveguide iris plates, and semiconductor wafers. Chemical etching produces microstrip transmission lines, patch antenna elements, coupling slots, and MMIC circuit features with typical resolution of 25 to 75 μm. The etch factor (ratio of vertical depth to lateral undercut) directly determines the dimensional accuracy of RF traces, making etchant selection and process control critical for maintaining 50 Ω impedance tolerance at millimeter-wave frequencies.

Category: Manufacturing

Resolution: 25 to 75 μm

Etch Factor: 2:1 to 3.5:1

Understanding Chemical Etching

Chemical etching begins with a copper-clad laminate coated in photoresist. UV exposure through a phototool mask polymerizes (negative resist) or solubilizes (positive resist) the resist in the pattern of the desired circuit. After developing, the exposed copper is dissolved by a spray or immersion etchant while the resist-protected areas remain intact. The result is a copper trace pattern that, for RF circuits, must hit impedance targets within ±2 to 5 Ω across frequencies up to 110 GHz.

The central challenge is undercut: etchant attacks copper not only downward through the foil thickness but also laterally beneath the resist edge. The etch factor, defined as the ratio of etch depth to lateral undercut, typically ranges from 2:1 (ferric chloride) to 3.5:1 (cupric chloride with optimized spray pressure). For a 1-oz (35 μm) copper layer with an etch factor of 3:1, the undercut per side is ~12 μm, narrowing the final trace by 24 μm total. At 77 GHz on Rogers RO3003 (ε_r = 3.0), this 24 μm narrowing shifts characteristic impedance by 2 to 3 Ω, which is significant for automotive radar front-end matching networks.

Etch Factor and Dimensional Control

Etch Factor:
EF = D / U

Final Trace Width:
W_final = W_artwork − 2U = W_artwork − 2D/EF

Etch Compensation (artwork widening):
W_artwork = W_target + 2D/EF

Where D = copper thickness (etch depth), U = lateral undercut per side, EF = etch factor. Example: 1-oz Cu (D = 35 μm), EF = 3.0 ⇒ U = 11.7 μm/side ⇒ add 23.3 μm to artwork width.

Etchant Comparison for RF PCB Fabrication

Etchant	Chemistry	Etch Rate (μm/min)	Etch Factor	Best For
Cupric Chloride	CuCl₂ + HCl	25 to 40	2.5 to 3.5	Inner layers, fine traces
Alkaline Ammoniacal	Cu(NH₃)₄Cl₂	20 to 35	2.0 to 3.0	Outer layers with Sn resist
Ferric Chloride	FeCl₃	20 to 30	1.5 to 2.5	Prototype, thick copper
Sodium Persulfate	Na₂S₂O₈	15 to 25	2.0 to 2.5	PTFE substrates (no attack)
H₃PO₄/H₂O₂	Phosphoric/peroxide	0.5 to 2	1:1 (isotropic)	GaAs mesa etch (MMIC)

Common Questions

Frequently Asked Questions

How does undercut affect RF transmission line dimensions?

Chemical etching dissolves metal isotropically, removing material laterally under the photoresist as well as vertically. For 1-oz copper (35 μm) with an etch factor of 2.5:1, undercut is ~14 μm per side, narrowing the trace by 28 μm total. At 77 GHz on 10-mil RO4003C, this shifts impedance by 2 to 4 Ω, degrading return loss from 20 dB to 12 dB. Designers compensate by widening artwork traces by the predicted undercut amount based on the fab shop's characterized etch factor.

What etchant chemistry is used for RF PCB copper?

Cupric chloride (CuCl₂) is dominant for inner layers, operating at 50 to 55 °C with 25 to 40 μm/min etch rate and a vertical sidewall profile (EF 2.5 to 3.5). Alkaline ammoniacal etchant is used for outer layers with tin resist. For PTFE laminates like RT/duroid 5880, sodium persulfate is preferred because it does not attack PTFE, preserving the substrate's dielectric constant and loss tangent.

What is the minimum feature size achievable with chemical etching?

Standard spray-etch processes reach 75 μm (3 mil) for 1-oz copper and 50 μm (2 mil) for half-oz copper. Advanced photochemical machining with thin-film resist achieves 25 μm on 9 to 18 μm foils. For MMIC wafers, wet etching of GaAs achieves sub-micrometer features, though dry plasma etching (RIE/ICP) is preferred above 40 GHz for its anisotropic profile and tighter dimensional control.

Related Terms

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