Millimeter Wave Specific Challenges mmWave Design Challenges Informational

How does surface finish on PCB traces affect performance at millimeter wave frequencies?

Q: Is ENIG always bad for mmWave?

ENIG is problematic at mmWave because the nickel layer (3-5 um thick, resistivity = 7e-8 ohm·m vs copper = 1.7e-8) carries most of the RF current (the skin depth at 28 GHz is only 0.39 um, and the nickel is on the surface). The higher resistivity of nickel increases the conductor loss by 50-200% compared to bare copper or silver. However: if the microstrip traces use a different finish (bare copper, OSP, or silver) and only the component pads use ENIG: the impact is minimized (the current flows on the trace surface, not on the pad surface). This selective-finish approach allows ENIG pad solderability while preserving trace performance.

Q: How do I specify copper roughness on my PCB order?

In your PCB fabrication documentation: (1) Specify the copper foil type: "Use VLP (very low profile) electrodeposited copper foil, Rq < 1.0 um, per IPC-4562 Class 5 or equivalent." (2) Specify the surface finish: "Immersion silver per IPC-4553, or OSP per IPC-4555." (3) Be specific about which layers need low-profile foil (at minimum: the signal layers carrying mmWave traces). Ground planes can use standard foil (the return current roughness loss is less critical than the signal trace roughness because the current spreads over a wider area on the ground plane). (4) Request a test coupon: include a microstrip transmission line coupon on the fabrication panel. Measure the insertion loss of the coupon at 28 GHz to verify the fabricated loss meets your design expectations.

Q: What about surface roughness on the laminate side?

The copper has two surfaces: the trace side (faces air or solder mask in microstrip) and the laminate side (bonded to the PCB substrate). The laminate side roughness is determined by the bonding treatment, not the surface finish. For embedded stripline traces: both sides of the copper are bonded to laminate. The roughness on both sides contributes to loss. The laminate-side roughness is typically the drum-side roughness (3-5 um for standard ED). For the lowest stripline loss: use VLP or HVLP foil with profile-controlled bonding treatment on both sides. This requires a dual-treated foil (smooth on both sides), which is available from copper foil manufacturers (Mitsui, Circuit Foil, Furukawa).

PCB trace surface finish significantly affects performance at millimeter-wave frequencies because the skin depth becomes comparable to the surface roughness. At 28 GHz on copper: skin depth = 0.39 um. At 77 GHz: skin depth = 0.24 um. Standard PCB copper has surface roughness of 1-5 um (RMS). This roughness is 3-20× larger than the skin depth, forcing the current to follow the rough surface contour. Effects: (1) Increased conductor loss: the rough surface creates a longer effective path for the current (like walking over hills instead of a flat road). The additional loss: alpha_rough = alpha_smooth × (1 + (2/pi) × arctan(1.4×(R_q/delta)^2)), where R_q is the RMS roughness and delta is the skin depth (Hammerstad-Jensen model). For R_q = 2 um at 28 GHz (delta = 0.39 um): alpha_rough = alpha_smooth × (1 + (2/pi) × arctan(1.4×(5.13)^2)) = alpha_smooth × (1 + 0.64 × arctan(36.8)) = alpha_smooth × (1 + 0.64 × 1.54) = alpha_smooth × 1.99. The conductor loss nearly doubles at 28 GHz due to roughness alone. At 77 GHz: the effect is even stronger (loss multiplier ≈ 2.5-3×). (2) Surface finish options: HASL (Hot Air Solder Leveling): uneven surface (2-5 um roughness). Poor for mmWave. ENIG (Electroless Nickel Immersion Gold): smoother (1-2 um roughness). The nickel layer (3-5 um) is lossy (resistivity 3.6× copper). At mmWave: the current flows primarily in the nickel layer, increasing loss significantly (nickel loss is 2-3× copper loss). ENIG is poor for mmWave despite its smooth surface. ENEPIG: similar to ENIG but with an additional palladium layer. Same nickel loss problem. OSP (Organic Solderability Preservative): preserves the bare copper surface. No additional lossy metal layer. Roughness depends on the copper foil type. Good for mmWave if low-roughness copper is used. Immersion Silver: silver is more conductive than copper (1.05× conductivity). Thin layer (0.1-0.3 um) over smooth copper. Silver roughness: < 0.5 um. Excellent for mmWave (lowest loss of standard finishes). Immersion Tin: thin tin layer over copper. Moderate roughness. Acceptable for mmWave but not as good as silver.

Category: Millimeter Wave Specific Challenges

Updated: April 2026

Product Tie-In: mmWave Components, Substrates, Packaging

Surface Finish at mmWave

Surface finish selection is a critical but often overlooked factor in mmWave PCB design. The wrong finish can add 1-3 dB of loss per centimeter to microstrip traces at 28-77 GHz.

Copper Foil Types

(1) Standard electrodeposited (ED) copper: the most common copper foil type. The drum side (facing the laminate) has a dendritic treatment (rough nubs that bond to the resin). Roughness on the drum side: 3-5 um RMS. The foil side (facing outward): 1-2 um RMS. For microstrip: the signal is on the foil side (smoother), but the ground plane (drum side) also contributes to loss through the return current. (2) Reverse-treated (RT) foil: the smooth side is bonded to the laminate (using an adhesion promoter). The rough drum side faces outward. This is worse for microstrip because the signal now runs on the rough side. Avoid RT foil for mmWave. (3) Low-profile (LP) and very-low-profile (VLP) foil: specially manufactured copper with reduced roughness: LP: R_q = 1-2 um (30-50% smoother than standard ED). VLP: R_q = 0.5-1 um (60-80% smoother). HVLP (hyper VLP): R_q = 0.3-0.5 um (nearly smooth). Cost: LP is 10-30% more than standard. VLP/HVLP: 50-100% more. For 28 GHz: LP or VLP foil reduces the conductor loss by 20-40% compared to standard foil. For 77 GHz: VLP or HVLP is essential (standard foil nearly triples the conductor loss). (4) Rolled annealed (RA) copper: formed by rolling (not electrodeposition). Inherently smooth (R_q = 0.3-0.5 um). Used in some high-frequency laminates (Rogers Cuclad, Taconic TLY). Excellent for mmWave.

Loss Comparison at 28 GHz

For a 50-ohm microstrip on Rogers RO4003C (Dk = 3.55, 5 mil dielectric): (1) Standard ED foil + ENIG finish: trace width = 0.28 mm. Conductor loss = 0.35 dB/cm (smooth) × 2.0 (roughness factor) × 1.5 (nickel loss factor) = 1.05 dB/cm. For a 3 cm trace: 3.15 dB loss. (2) VLP foil + immersion silver: conductor loss = 0.35 × 1.3 (VLP roughness) × 1.0 (no nickel) = 0.46 dB/cm. For a 3 cm trace: 1.37 dB. The VLP + silver combination saves 1.78 dB (versus ED + ENIG) per 3 cm trace. In a mmWave system: this could mean the difference between meeting and failing the link budget. (3) For comparison: smooth copper (ideal, R_q = 0): conductor loss = 0.35 dB/cm. This is the theoretical minimum for the given geometry. VLP + silver achieves 0.46/0.35 = 1.31× (31% above theoretical). Standard ED + ENIG: 1.05/0.35 = 3.0× (200% above theoretical).

Design Recommendations

(1) At 28 GHz: use VLP or HVLP copper foil. Use immersion silver or OSP surface finish (avoid ENIG). Specify the foil type in the PCB fabrication notes (the fabricator may default to standard ED if not specified). (2) At 77 GHz: use HVLP or RA copper foil. Use immersion silver surface finish. Consider thin-film processes (photolithographic metallization on ceramic substrates) for the lowest loss. (3) Solder joint considerations: OSP provides good solderability but degrades with storage (the organics oxidize). Immersion silver provides excellent solderability and long shelf life. ENIG provides excellent solderability and corrosion resistance but the nickel layer is lossy at mmWave. Compromise: use ENIG on component pads (where solderability is important) and bare copper or immersion silver on trace surfaces (where loss is critical). This requires selective surface finish, which is available from some high-end fabricators.

Surface Roughness Loss

Skin depth: δ = √(ρ/(πfμ))
At 28GHz Cu: δ = 0.39 μm
Loss multiplier ≈ 1 + (2/π)arctan(1.4(Rq/δ)²)
Standard ED: Rq = 3-5μm → 2-3× loss
VLP: Rq = 0.5-1μm → 1.3× loss

Common Questions

Frequently Asked Questions

Is ENIG always bad for mmWave?

ENIG is problematic at mmWave because the nickel layer (3-5 um thick, resistivity = 7e-8 ohm·m vs copper = 1.7e-8) carries most of the RF current (the skin depth at 28 GHz is only 0.39 um, and the nickel is on the surface). The higher resistivity of nickel increases the conductor loss by 50-200% compared to bare copper or silver. However: if the microstrip traces use a different finish (bare copper, OSP, or silver) and only the component pads use ENIG: the impact is minimized (the current flows on the trace surface, not on the pad surface). This selective-finish approach allows ENIG pad solderability while preserving trace performance.

How do I specify copper roughness on my PCB order?

In your PCB fabrication documentation: (1) Specify the copper foil type: "Use VLP (very low profile) electrodeposited copper foil, Rq < 1.0 um, per IPC-4562 Class 5 or equivalent." (2) Specify the surface finish: "Immersion silver per IPC-4553, or OSP per IPC-4555." (3) Be specific about which layers need low-profile foil (at minimum: the signal layers carrying mmWave traces). Ground planes can use standard foil (the return current roughness loss is less critical than the signal trace roughness because the current spreads over a wider area on the ground plane). (4) Request a test coupon: include a microstrip transmission line coupon on the fabrication panel. Measure the insertion loss of the coupon at 28 GHz to verify the fabricated loss meets your design expectations.

What about surface roughness on the laminate side?

The copper has two surfaces: the trace side (faces air or solder mask in microstrip) and the laminate side (bonded to the PCB substrate). The laminate side roughness is determined by the bonding treatment, not the surface finish. For embedded stripline traces: both sides of the copper are bonded to laminate. The roughness on both sides contributes to loss. The laminate-side roughness is typically the drum-side roughness (3-5 um for standard ED). For the lowest stripline loss: use VLP or HVLP foil with profile-controlled bonding treatment on both sides. This requires a dual-treated foil (smooth on both sides), which is available from copper foil manufacturers (Mitsui, Circuit Foil, Furukawa).

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