How does the plating finish on a PCB trace affect RF performance at frequencies above 10 GHz?
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.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
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).