Manufacturing and Production PCB Fabrication for RF Informational

How does the plating finish on a PCB trace affect RF performance at frequencies above 10 GHz?

The PCB surface finish (the metal coating applied over the copper traces) affects RF performance through additional conductor loss, skin effect interaction, and surface roughness: (1) Skin depth context: at frequencies above 10 GHz, the RF current flows in an extremely thin surface layer (the skin depth): at 10 GHz: δ = 0.66 μm in copper. At 40 GHz: δ = 0.33 μm. At 77 GHz: δ = 0.24 μm. The surface finish plating is typically 2-8 μm thick, which is much thicker than the skin depth. At these frequencies, the RF current flows almost entirely in the surface finish, not in the underlying copper. (2) Surface finish options and RF impact: ENIG (Electroless Nickel Immersion Gold): nickel layer: 3-8 μm thick. Gold: 0.05-0.1 μm (flash gold, just for corrosion protection). Problem: nickel has much higher resistivity than copper (6.4 μΩ·cm vs 1.7 μΩ·cm) and is ferromagnetic (additional loss from magnetic effects). At 10 GHz: the RF current flows in the nickel layer (not the copper beneath it). Loss increase: 2-4× higher conductor loss compared to bare copper. At 40+ GHz: even worse (thinner skin depth, more current in the lossy nickel). ENIG is the worst surface finish for RF above 10 GHz. Immersion Silver (ImAg): silver layer: 0.2-0.5 μm thick. Silver has the lowest resistivity of any metal (1.59 μΩ·cm, better than copper). Below 10 GHz: the silver layer is thinner than the skin depth; RF current flows partly in silver, partly in copper. Both have low resistivity. Above 40 GHz: the silver layer is comparable to the skin depth; RF performance is excellent. Loss: comparable to or slightly better than bare copper. Best RF performance of any standard surface finish. Immersion Tin (ImSn): tin: 0.8-1.2 μm. Tin resistivity: 11 μΩ·cm (much higher than copper). Whisker risk (long-term reliability concern). RF performance: moderate (better than ENIG, worse than silver). ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold): nickel: 3-5 μm, palladium: 0.05-0.2 μm, gold: 0.05 μm. Similar RF problem as ENIG (the thick nickel layer dominates the loss). OSP (Organic Solderability Preservative): a thin organic coating over bare copper (does not affect RF performance). Provides solderability protection during assembly. Best RF performance (the trace is essentially bare copper). Disadvantage: limited shelf life (the OSP degrades over months). (3) Recommendation: for RF circuits > 10 GHz: use Immersion Silver (best) or OSP (acceptable if assembled quickly). Avoid ENIG and ENEPIG on RF signal traces. If ENIG is required on the board (for BGA pads, etc.): use selective plating (ENIG on component pads, immersion silver on RF traces).

Category: Manufacturing and Production

Updated: April 2026

Product Tie-In: PCB Substrates, Laminates

PCB Surface Finish for RF

Surface finish selection is one of the most impactful (and often overlooked) decisions for high-frequency PCB performance. Using ENIG on a 40 GHz circuit can add 3-5 dB/inch of unnecessary loss.

Measured Data

Published measurements comparing surface finishes on identical test boards: at 40 GHz (2-inch microstrip on RO4350B): bare copper: 1.5 dB/inch. Immersion silver: 1.6 dB/inch (+0.1 dB). ENIG (5 μm Ni): 3.2 dB/inch (+1.7 dB, more than doubled). At 77 GHz: bare copper: 2.1 dB/inch. Immersion silver: 2.2 dB/inch. ENIG: 4.8 dB/inch (2.3× higher loss). The ENIG loss penalty is dramatic at mmWave frequencies and must be avoided for any performance-sensitive design.

Surface Finish RF Impact

Skin depth at 77 GHz: δ = 0.24 μm
ENIG Ni: 3-8 μm (covers entire skin depth)
Ni resistivity: 6.4 μΩ·cm (4× copper)
ImAg: 1.59 μΩ·cm (better than copper)
ENIG loss at 40 GHz: 2× vs bare copper

Common Questions

Frequently Asked Questions

Can I use ENIG with a thinner nickel layer?

Reducing the nickel thickness helps but does not solve the problem. Even "thin ENIG" (1-2 μm Ni): at 40 GHz: the skin depth is 0.33 μm. A 2 μm Ni layer is still 6× the skin depth. The RF current is still in the nickel. Improvement: modest (maybe 20% less loss than standard ENIG). Conclusion: for > 10 GHz, avoid nickel entirely on RF paths. Use immersion silver or OSP.

Does immersion silver tarnish?

Yes, over time. Silver surface tarnish (Ag₂S formation): occurs in environments with sulfur-containing gases (common in industrial areas). Tarnish increases surface resistance and contact resistance. Mitigation: store finished boards in sealed bags with anti-tarnish paper. Process within 6-12 months of fabrication. Apply conformal coating after assembly (protects the silver from further tarnishing). For long-term storage: immersion tin or OSP may be more practical (despite their inferior RF performance).

What about hard gold plating?

Hard gold (electroplated gold, 1-3 μm thick): excellent conductivity (2.44 μΩ·cm, slightly higher than copper but much better than nickel). No tarnishing (gold is inert). But: hard gold requires a nickel underplate for adhesion (the nickel is beneath the gold). At mmWave: the current may penetrate through the thin gold into the nickel, reintroducing loss. Soft gold (pure gold, wire-bondable): used for MMIC die attach and wire bonding. Excellent RF performance (low resistivity, no magnetic loss). Expensive ($2-10/cm² for thick gold plating).

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