Once the stackup, laminate and copper foil of a high frequency board are fixed, two outer-layer steps still act directly on the RF traces: solder mask and surface finish. A microstrip sits half in dielectric and half in air, so whatever covers the line and whatever plates its surface both enter impedance and loss. On an ordinary board these steps are about solderability and appearance. On an RF board they become electrical parameters.
For laminate choice see High Frequency PCB Material Selection; for foil roughness see Copper Foil Roughness and Insertion Loss. This article covers only outer-layer coverage and finish.
Three outer-layer states
| Outer-layer state | What changes | Source of variation in fabrication | What the drawing must state |
|---|---|---|---|
| Mask over microstrip | An extra dielectric above the line; effective Dk rises, impedance drops, dielectric loss increases | Mask thickness differs on top of and beside the line; varies lot to lot | Covered or not; whether simulation includes mask |
| Mask opening | Air above the line, close to the ideal simulated microstrip | Mask-to-copper registration at the opening edge; bare copper needs a finish | Opening area and edge clearance |
| Opening + surface finish | The outermost conductor layer becomes another metal, where high frequency current concentrates | Plating thickness and uniformity | Finish type; whether zoned |
Mask over the line: an uncontrolled dielectric
Solder mask is not an RF material. Its Dk and Df are usually not specified at RF frequencies, and its coating thickness has no tolerance class like a laminate. Over a microstrip it adds a layer of uneven thickness: thinner on top of the line, thicker in the low areas between lines.
Impedance ends up below the bare-line simulation and loss above it, and the offset moves with coating thickness from lot to lot. If the simulation models the mask and the coupon is masked too, impedance stays controllable. If the simulation assumes bare lines and the board is fully masked, the gap shows up at acceptance.
Mask opening: edge clearance and copper protection
An opened RF line has air above it, which is closest to the model. Fabrication has two things to handle:
- Opening edge: mask and copper images have a registration offset, so the opening needs clearance or the mask edge lands on the trace edge. Give the opening area in the mask data, not just a note saying "no mask on RF lines".
- Copper protection: opened copper must be finished or it oxidizes. The finish then becomes the outer surface of the RF conductor.
Surface finish: a different metal on the conductor surface
The higher the frequency, the more current crowds into a thin surface layer of the conductor. The finish occupies exactly that layer, so it matters more on opened RF lines than on ordinary pads.
- ENIG: nickel conducts worse than copper and is ferromagnetic. Current flowing through it adds loss, more so at higher frequency. Nickel and gold control is covered in ENIG Nickel-Gold Thickness and Black Pad Control.
- Immersion silver: good conductivity and small conductor-loss impact, but silver tarnishes; packaging, storage and assembly windows matter more.
- OSP: an organic film that leaves the copper unchanged, the smallest loss impact; shorter solderable shelf life and limited reflow cycles.
- Immersion tin / HASL: HASL is not flat enough for fine RF structures; immersion tin has shelf-life limits.
- Hard gold: for contact or mating surfaces, not usually a whole-board finish for RF traces.
A general comparison is in PCB Surface Finish Comparison. SKPCB offers HASL, ENIG, OSP, immersion silver/tin and hard gold. Two finishes on one board (for example OSP on the RF area, ENIG on pads) adds process steps and needs engineering review at quotation.
The impedance coupon must match the RF line
The coupon's outer-layer state must match the real RF line: opened if the line is opened, masked if the line is masked, same finish. Otherwise a passing coupon says nothing about the traces. See Impedance Test Coupon Verification. SKPCB's standard impedance tolerance is ±10%; ±5% requires engineering review, see Impedance Control PCB.
Drawing checklist
- Mark RF openings as a separate area in the solder mask data, with edge clearance;
- State whether the simulation includes mask, and the coupon's mask state;
- Specify the finish type; if zoned, supply the zoning layer;
- Put impedance target, tolerance, coupon location and outer-layer state in one note;
- If a finish substitute is possible, state whether it is allowed and who approves it.
Summary
On a high frequency board, solder mask and surface finish are not cosmetic options; they are part of the RF trace. Mask coverage decides what dielectric sits above the line, and the finish decides which metal forms its outer surface. Write both into the drawing together with the coupon state, and impedance and loss can be verified on the finished board. See High Frequency PCB for materials and capabilities, or submit your drawing via the quote page for review.
FAQ
Q: Must RF traces never have solder mask?
A: No. Masked lines can be impedance controlled if the simulation models the mask and the coupon matches. Mask thickness and dielectric properties vary more, so lines sensitive to loss or phase matching are usually opened.
Q: Why apply a finish after opening the mask?
A: Bare copper oxidizes, which hurts solderability and long-term stability. The opened copper needs a finish, and that layer becomes the RF conductor's outer surface.
Q: Why does ENIG increase high frequency loss?
A: High frequency current crowds into the conductor surface. ENIG's nickel layer conducts worse than copper and is ferromagnetic, so current flowing through it adds loss.
Q: Can the RF area and pad area use different finishes?
A: Selective finishing is possible but adds steps and registration requirements. Submit the zoning layer at quotation for engineering review.
Q: Should the impedance coupon be opened?
A: It should match the RF line's outer-layer state, both mask and finish, or the result does not represent the traces.
Q: Which laminates does SKPCB use for high frequency boards?
A: Rogers 4350B/4003C/3003, Arlon 85N and Taconic RF-35; see the High Frequency PCB page.