Manufacturing and Production PCB Fabrication for RF Informational

What is ENIG versus ENEPIG versus immersion silver for RF PCB surface finish and which performs best?

ENIG, ENEPIG, and immersion silver are the three most common PCB surface finishes for RF applications, each with different trade-offs between RF performance, solderability, and reliability: (1) ENIG (Electroless Nickel Immersion Gold): structure: 3-8 μm electroless nickel + 0.05-0.1 μm immersion gold. The gold prevents nickel oxidation and provides a solderable surface. RF performance: poor above 10 GHz (the thick nickel layer dominates conductor loss, adding 50-100% additional loss). Solderability: excellent (the gold surface wets well with solder). Shelf life: excellent (gold does not tarnish; boards are solderable for years). Wire bondability: good (with proper gold thickness). Cost: moderate ($5-15/panel surcharge). (2) ENEPIG (Electroless Nickel, Electroless Palladium, Immersion Gold): structure: 3-5 μm Ni + 0.05-0.2 μm Pd + 0.05 μm Au. The palladium layer acts as a barrier between the nickel and the gold, preventing nickel diffusion and improving wire bond and solder reliability. RF performance: similar to ENIG (still poor above 10 GHz, due to the thick nickel). Solderability: excellent (better than ENIG for lead-free soldering). Wire bondability: excellent (the palladium/gold surface provides reliable wire bonds). Cost: higher than ENIG ($10-25/panel surcharge). (3) Immersion Silver (ImAg): structure: 0.2-0.5 μm silver deposited directly on copper. RF performance: excellent (silver has the lowest resistivity of any metal; the thin layer does not significantly increase loss above bare copper). Loss at 40 GHz: within 5-10% of bare copper (vs 100%+ for ENIG). Solderability: good (silver surface wets well, but slightly less robust than ENIG). Shelf life: moderate (6-12 months; silver can tarnish in sulfur-containing environments). Wire bondability: poor (not suitable for gold wire bonding). Cost: low ($3-8/panel). (4) Recommendation for RF: below 10 GHz: ENIG is acceptable (the nickel loss penalty is modest). Above 10 GHz: immersion silver is strongly preferred for all RF signal traces. Above 40 GHz: immersion silver is mandatory (ENIG is unacceptable). Mixed requirement (RF traces + BGA pads): use selective plating: immersion silver on RF traces, ENIG or ENEPIG on BGA/QFN pads.

Category: Manufacturing and Production

Updated: April 2026

Product Tie-In: PCB Substrates, Laminates

RF Surface Finish Comparison

The surface finish decision at RF frequencies is not about solderability or shelf life; it is about conductor loss. The wrong choice can add decibels of unnecessary loss to every inch of transmission line.

Selective Plating

The optimal solution for mixed-requirement boards: (1) Process: apply immersion silver to the entire board (excellent RF performance). Mask the RF signal areas with solderable resist. Apply ENIG or ENEPIG to the component pads (excellent solderability and wire bondability). Strip the mask from the RF areas. (2) Cost: adds $10-30/panel to the fabrication cost (for the masking and dual-plating process). (3) Availability: not all fab houses offer selective plating. Specify the requirement early and confirm capability. Advanced RF fabs (TTM, Sanmina, API Technologies) routinely offer this service.

Surface Finish for RF

ENIG: 3-8μm Ni + 0.05μm Au (poor RF > 10 GHz)
ENEPIG: Ni + Pd + Au (poor RF, best solder)
ImAg: 0.2-0.5μm Ag (best RF performance)
40 GHz loss: ImAg ≈ Cu, ENIG ≈ 2× Cu
Selective plating: ImAg on RF, ENIG on pads

Common Questions

Frequently Asked Questions

What about HASL finish?

HASL (Hot Air Solder Leveling): a layer of solder (SnPb or lead-free) is applied to the copper. The solder layer is thick and uneven (10-30 μm), creating impedance variation on RF traces. RF performance: acceptable below 3 GHz (the solder layer is conductive but irregular). Poor above 3 GHz (surface roughness and thickness variation degrade impedance control). Not recommended for any precision RF circuit.

Does the finish affect PIM?

Yes, significant for passive intermodulation: ENIG: the nickel layer is ferromagnetic, which creates nonlinear behavior at high RF power. ENIG finishes can generate PIM levels 10-20 dB worse than silver or gold finishes. Immersion silver: excellent PIM performance (silver is non-magnetic and highly linear). For PIM-sensitive applications (cellular base station filters, diplexers, antenna feeds): avoid ENIG and use immersion silver, immersion tin, or OSP.

What is the minimum silver thickness for RF?

The silver thickness should ideally be at least equal to the skin depth at the operating frequency. At 10 GHz: δ_Ag = 0.64 μm. Standard ImAg: 0.2-0.5 μm (slightly thinner than the skin depth). At these frequencies: the current flows partly in silver, partly in the copper beneath it. Both are excellent conductors, so the RF loss is very low. At 77 GHz: δ_Ag = 0.23 μm. The standard 0.3-0.5 μm ImAg covers the skin depth. RF performance is excellent. Conclusion: the standard immersion silver thickness (0.2-0.5 μm) is adequate for all practical RF frequencies up to 100+ GHz.

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