ENIG vs Bare Copper: Why Nickel Adds Microstrip Loss

ENIG is the usual outer-layer finish on RF boards because it solders and does not tarnish. It is also a nickel barrier with a gold flash. At microwave frequencies that nickel sits where the current lives.

On the stackup below — 50 Ω microstrip, 6.6 mil Rogers RO4350B, ½ oz reverse-treated foil, relieved mask, 10 GHz — Transmission Line Calculator reports 1.22 dB/in with ENIG and 0.91 dB/in with bare copper. The extra 0.31 dB/in is the plating. On a 4 inch feed that is 1.2 dB you will not recover by tweaking the width.

The gold is not the problem. The nickel is.

Current hugs the surface

Skin depth is the distance the current penetrates a metal before it has mostly died away:

$$\delta = \frac{1}{\sqrt{\pi f \mu_0 \mu_r \sigma}}$$

At 10 GHz in copper, $\delta \approx 0.66\,\mu\mathrm{m}$. Half-ounce foil is $17.5\,\mu\mathrm{m}$ thick, so the RF current is a thin sheet on the surface. Coat that surface and the current sees the coating, not the copper under it.

Nickel is thick compared with a skin depth

IPC-4552B ENIG is a thin gold flash on electroless nickel. Transmission Line Calculator uses $0.05\,\mu\mathrm{m}$ Au on $4.5\,\mu\mathrm{m}$ Ni.

Metal	Role	Thickness	Skin depth at 10 GHz
Gold	oxidation barrier	$0.05\,\mu\mathrm{m}$	$0.79\,\mu\mathrm{m}$
Nickel	solder-diffusion barrier	$4.5\,\mu\mathrm{m}$	$0.72\,\mu\mathrm{m}$
Copper	the foil you paid for	$17.5\,\mu\mathrm{m}$	$0.66\,\mu\mathrm{m}$

The gold flash is a small fraction of a skin depth, so it is almost transparent. The nickel is about six skin depths. Current on a plated face dies in the nickel and never reaches the copper.

Nickel also conducts worse than copper ($\sigma \approx 1.4\times10^7$ versus $5.8\times10^7\,\mathrm{S/m}$) and it is ferromagnetic. This model uses $\mu_r = 3.5$ for plated nickel. Magnetism shrinks skin depth, which raises surface resistance further. In the skin inspector, about 94% of the plated-face current is in the nickel.

Most microstrip current is not on the plated face

ENIG coats the top and the sidewalls. The bottom face sits on the laminate and stays copper. Microstrip carries most of its current on that bottom face, against the dielectric.

Transmission Line Calculator therefore applies the plated surface impedance to 25% of the current on microstrip, not 100%. That is why ENIG raises this line by a third instead of tripling it.

Grounded CPW is less forgiving. More current rides the plated gaps, so the same nickel stack costs more. Switch the topology in the calculator if that is your launch.

ENIG is an outer-layer finish. Inner-layer stripline is usually bare copper and does not pay this tax.

Same board, two finishes

Hold the laminate, thickness, foil, mask, and frequency fixed. Only the finish changes. Loss is locked to 50 Ω, so the width is the same in both widgets.

On ENIG, plating is about a quarter of the 10 GHz loss. On bare copper it is zero. Dielectric and mask barely move. Roughness is still there on both — those are the foil teeth against the laminate, not the plating.

Put your numbers in

The widgets are live. Change height to your core, frequency to your band, and finish to whatever is on the traveler. Leave lock-to-50 Ω on so the width follows.

A few finishes worth trying on this same line at 10 GHz:

Finish	What the current sees	Loss on this stackup
Bare copper	Copper	$0.91\,\mathrm{dB/in}$
OSP	Copper (the organic is electrically transparent)	$0.91\,\mathrm{dB/in}$
Immersion silver	Thin Ag, then copper	$0.91\,\mathrm{dB/in}$
ENIG	Ni, then almost no copper	$1.22\,\mathrm{dB/in}$
ENEPIG	Pd + Ni; nickel still dominates	$1.21\,\mathrm{dB/in}$

Immersion silver is a better conductor than copper and it is thin, so it does not wall off the foil. ENEPIG still has $4.5\,\mu\mathrm{m}$ of nickel, so it tracks ENIG. Hard gold is a connector stack (thicker Au-Co on nickel), not an RF improvement.

For a sweep to 20 GHz and side-by-side captures, open Transmission Line Calculator. Save the ENIG line to memory, switch to bare copper or OSP, and read $\Delta\alpha$ at 10 GHz and 28 GHz.

When ENIG is still the right call

The extra 0.3 dB/in is a tax, not a veto. I still spec ENIG when the board has to be assembled later, sit in a stockroom, or mix RF with parts that want a gold pad. OSP fingerprints and has a short shelf life. Immersion silver has its own tarnish and creep rules.

The engineering move is to spend the tax where it is cheap. Keep long RF runs on inner-layer copper. Keep ENIG on short launches, connectors, and mixed-signal outer layers. If the outer layer is the RF path, ask the fab whether OSP or immersion silver is allowed on that construction.

A 0.3 dB/in hit on a 0.4 inch launch is noise. The same hit on a 6 inch 10 GHz feed is 1.8 dB in the cascade before the first amplifier.

What this number is not

Transmission Line Calculator is a closed-form quasi-TEM model, not full-wave EM. Trust it in the usual $0.1 < W/h < 10$ region and below the TE1 estimate.

The nickel $\mu_r = 3.5$ is a typical plated-Ni modeling value. Electroless Ni-P is less magnetic than bulk nickel, and fab thickness wanders around the IPC-4552B window. Foil roughness (Huray on RTF here) is a separate, often larger, term.

If 0.3 dB/in is your margin, measure a coupon on the actual stackup. Use the widget to decide what to put on that coupon.