How do I specify the copper roughness treatment for a low loss RF PCB?

Copper surface roughness directly increases conductor loss at RF and microwave frequencies due to the skin effect, and specifying the correct roughness treatment is critical for low-loss designs: (1) How roughness increases loss: at high frequencies, the current flows in the skin depth (the outermost layer of the conductor). If the surface roughness is comparable to or larger than the skin depth: the current must follow the rough surface contour, traveling a longer path. This increases the effective conductor resistance. The loss increase factor (Hammerstad model): K_rough = 1 + (2/π) × arctan(1.4 × (Rq/δ)²). Where Rq = RMS surface roughness and δ = skin depth. For Rq = δ: K_rough = 1.89 (the conductor loss is 89% higher than a smooth conductor). For Rq = 2δ: K_rough = 1.98 (nearly 2×). (2) Copper roughness grades: standard electrodeposited (STD): Rz = 6-10 μm (Rq ≈ 1.5-2.5 μm). The default for most PCB copper. At 10 GHz (δ = 0.66 μm): K_rough ≈ 1.9 (90% more loss than smooth). Low profile (LP): Rz = 3-5 μm (Rq ≈ 0.8-1.3 μm). At 10 GHz: K_rough ≈ 1.6 (60% more loss). Very low profile (VLP): Rz = 1.5-3 μm (Rq ≈ 0.4-0.8 μm). At 10 GHz: K_rough ≈ 1.3 (30% more loss). Hyper very low profile (HVLP): Rz = 0.5-1.5 μm (Rq ≈ 0.1-0.4 μm). At 10 GHz: K_rough ≈ 1.1 (10% more loss). Reversed treated (RT) or rolled annealed (RA) copper: Rz = 0.3-1.0 μm (Rq ≈ 0.1-0.3 μm). Near-ideal smooth surface. At 10 GHz: K_rough ≈ 1.05 (5% more loss). (3) Specifying roughness: in the fabrication drawing, specify: copper foil manufacturer and grade (e.g., "Mitsui MicroThin VLP" or "Oak-Mitsui HLP"). Roughness parameter: Rz ≤ X μm or Rq ≤ Y μm (on the treated side facing the substrate). Include a requirement for the copper to be laminated with the smooth (drum) side facing outward (toward the signal). (4) Cost impact: VLP copper adds approximately 10-30% to the laminate cost. HVLP and RA copper are less widely available and may add 30-50% to the laminate cost. The additional cost is justified for: circuits above 20 GHz, low-noise receiver front-ends, and high-Q filter/resonator designs where every dB of loss matters.