RO3000 With Rolled Copper: What Smooth Foil Actually Buys

The phrase “laminated copper” in the title is a literal translation of the Chinese term for rolled copper, and it has nothing to do with lamination — rolled-annealed foil is wrought, not deposited. The distinction matters because the standard RO3000 configuration table offers it on exactly one grade: RO3003, in 1/2 oz and 1 oz, with the codes AH and A1. Everything else in the family is electrodeposited. This article prices the difference: what roughness does to conductor loss at 10, 24, 39 and 77 GHz, why the benefit is set by the copper share of the budget rather than by the dielectric constant, and what the foil costs in peel strength, availability and process route.

“Laminated copper” is a translation artefact. The material being described is rolled-annealed copper foil — metal that is cast, rolled and annealed, with its grains elongated in the rolling direction and a surface that is smooth by construction. The alternative, electrodeposited foil, is grown onto a rotating drum from a sulphate bath; it is cheaper, it can be made very thin, and it has a nodular treatment on the bond side to give the resin something to grip.

The two are not interchangeable in an RF stack, and the reason is one number: surface roughness against skin depth. At 10 GHz current in copper occupies about 0.66 µm; at 77 GHz it occupies 0.24 µm. A rolled foil sits at 0.2–0.4 µm RMS, which is at or under the skin depth across the whole band. A standard electrodeposited foil sits at 1.5–3 µm, which is two to five times the skin depth at 10 GHz and six to twelve times at 77 GHz. Every excursion of the current over a peak lengthens its path, and the loss rises with the square root of that lengthening.

What the foil does not do is change the dielectric. A reader who expects a rolled-copper option to rescue a loss budget on a 0.0037 Df laminate will not find the rescue here: section 4 puts copper at 69 % of the loss on RO3003 and only 39 % on 4350B, so the same foil buys half as much where the dielectric is doing the damage. The foil is a tool for the last part of a budget on an already-low-loss material, not a substitute for choosing the material. The full loss hierarchy across families is worked through in ultra-low-loss PCB materials.

The circulating description says rolled copper has been incorporated into RO3003, RO3035 and RO3203, and that it is available from 1/2 oz up to 0.040 inch. Both statements are wrong. The RO3000 standard configuration table lists rolled copper against RO3003 only, in two weights, and 0.040 inch is a dielectric thickness in the core list, not a foil weight.

That single row is the whole rolled-copper story in the RO3000 family, and it explains why millimetre-wave work concentrates on RO3003 rather than on the higher-Dk grades: a designer who wants a 10.2 Dk laminate and a smooth foil cannot have both from stock. It is worth noting what 0.040 inch refers to, because the confusion is common — it is 1.02 mm, a dielectric thickness available as a non-standard build, and the non-standard core range for these grades runs from 0.005 to 0.195 inch in 0.005 inch increments.

The other half of the configuration is the dielectric itself. RO3003 is 3.00 ±0.04 with 0.0010 dissipation factor at 10 GHz, CTE 17/16/25 and thermal conductivity 0.50 W/m·K — ceramic-filled PTFE with no woven glass, which is why its Dk holds to ±0.04 across lots and why the 77 GHz radar industry settled on it. The grade itself is introduced in the RO3003 introduction and its core list in RO3003 thickness options.

Copper’s skin depth falls as the square root of frequency, so a roughness that is tolerable at 2 GHz becomes the dominant feature at 77 GHz. The table gives the depth, the ratio of three foil classes to it, and what that ratio does to conductor loss.

The last column is the one that matters, and the number to hold on to is 8.3. At 77 GHz a standard electrodeposited foil presents the current with a surface whose features are eight times the depth the current penetrates, which is why foil choice moves from a rounding error at 2.4 GHz to a first-order term at 77 GHz. Rolled foil stays at or below 1.25× across the whole band, which is the definition of “electrically smooth” rather than merely “visually smooth”.

Converting the ratios into decibels needs a roughness model, and models disagree in detail but not in direction. The Hammerstad correction — multiply conductor loss by 1 + (2/π)·arctan[1.4·(Δ/δ)²] — gives a multiplier of about 1.18 for 0.3 µm rolled foil and 1.91 for a 2 µm electrodeposited foil at 10 GHz, and 1.73 against 1.99 at 77 GHz. The two multipliers converge, but the loss they multiply does not: conductor loss rises with the square root of frequency, so the absolute difference grows from about 0.07 dB/inch at 10 GHz to 0.2–0.3 dB/inch at 77 GHz on the same 50 Ω geometry. Measured comparisons of the two foils on the same laminate tend to show a slightly wider gap than the model, because a standard foil’s nodular treatment adds effective roughness beyond its RMS figure. That is the range worth designing to, and it is worth building a coupon before committing to a number.

Here is the part the marketing leaves out. A foil change only reduces the conductor term, so its value depends on how much of the total loss is conductor. The intuition is that a higher-Dk laminate has more dielectric loss and therefore less to gain from a smooth foil; the arithmetic says the opposite, because raising Dk narrows the 50 Ω line and a narrow line has higher conductor loss.

Read the last column and the premise dissolves. Copper is 58 to 69 % of the loss on every grade in this family, including the 10.2 Dk one, because the line has narrowed from 1.29 mm to 0.43 mm and conductor loss varies as 1/w. The smooth-foil premium is not diluted by high Dk; it is concentrated by narrow lines. What does dilute it is a laminate whose dielectric term is already dominant — 4350B at 39 % is a marginal case, and FR-4 at 11 % is not a case at all.

That leads to a rule with a sharper edge than the usual advice: a smooth foil is worth the premium on any controlled-Dk laminate, and worth nothing on an epoxy one. It also explains an asymmetry that buyers notice and engineers rarely do — the option does not exist on the RO4000 platform, so making rolled foil a requirement removes 4350B and 4003C from the candidate list before any electrical comparison happens. The platform-wide view is in the RO4000 platform overview.

A second-order consequence compounds it. Conductor loss varies as 1/w, so it grows as the substrate gets thinner at fixed impedance: on a 0.127 mm core the 50 Ω line is 0.32 mm wide and the smooth-copper conductor term is about 0.40 dB/inch at 10 GHz against 0.045 for the dielectric — nine tenths of the loss. Thin laminates are exactly where a smooth foil matters most, which happens to be where millimetre-wave designs already are.

A 77 GHz radar front end is the application the foil was developed for, so it is worth pricing the change on a realistic geometry rather than on a per-inch figure. Take a 0.127 mm RO3003 core, a 50 Ω microstrip 0.32 mm wide, and a 50 mm feed run from the transceiver to the antenna.

Half a decibel on a 50 mm feed is not a headline, and it is the honest answer for a single-chip radar with the antenna beside the transceiver. It becomes a decision on a 200 mm corporate feed, where two decibels come off the transmit chain and the same again off the receive chain — on a cascaded noise figure that is a range change, not a measurement curiosity. The lesson is that the foil is a distribution-network purchase, not a per-trace one, and phased arrays with long corporate feeds are where it earns the premium.

There is a second, quieter argument for the same choice at this band: consistency. A rolled foil’s roughness is set by the rolling mill and varies little across a panel, while an electrodeposited foil’s nodular treatment varies with bath condition and position on the drum. At 77 GHz a 0.3 µm change in effective roughness is a measurable change in loss, which is why phase- and amplitude-matched channels are easier to build on the smoother foil. The phase side of the same problem — what temperature does to a 77 GHz channel — is quantified in 77 GHz phase coherence in radar PCB design.

Rolled foil is not free, and three of its costs are process rather than price. They are also the reason two shops can quote the same stack 40 % apart.

The fourth row is the one that surprises buyers. Choosing rolled copper does not add an option to an FR-4-style build; it selects a material family whose process is different in four places at once. A quotation that adds the foil premium but keeps an FR-4 process assumption will not survive the first build, and the four differences — bonding temperature, hole activation, drilling behaviour and finish — are set out grade by grade in PCB surface treatment processes and in the process notes for the family in the RO3000 model matrix.

In practice the choice is not between two foils but between three classes, and the middle one is the one most projects should examine first. Very-low-profile electrodeposited foil sits between standard ED and rolled: smoother than standard, cheaper than rolled, and available in more weights and on more laminates.

The roughness figures are typical values from foil suppliers rather than laminate specifications, and that distinction is itself a procurement problem: a laminate data sheet does not publish the foil’s roughness, so a requirement written as “smooth foil” cannot be verified on delivery. The way to make it verifiable is to name the foil code — AH or A1 — on the order, which ties the build to a specific product rather than to an adjective. The same logic applied to foil types across the whole catalogue is in PCB copper foils.

The description this article replaces contains six statements worth correcting, because four of them would change a purchase order.

The last row deserves a sentence more. At 77 GHz the losses that decide whether a radar module works are, in order, the feed network, the transitions from package to board, and the laminate. A rolled foil addresses the first and, through consistency, part of the second; it does nothing for the third, which is already the smallest term on a 0.0010 Df material. A design that buys the foil without fixing the launch geometry has bought the cheapest improvement available.

Rolled copper is one of the few material choices that can be specified unambiguously, because the data sheet carries foil codes. Six lines make the order executable: the grade as RO3003 with Dk 3.00 ±0.04 and Df 0.0010 at 10 GHz; the cladding as “rolled, 1/2 oz, code AH” or “rolled, 1 oz, code A1”, not “smooth foil”; the core thickness in millimetres with a tolerance quoted as core; the bond ply by name, since a 0.10 mm bond layer that is left out of the model moves a 50 Ω line to 61 Ω; the finish from the PTFE-compatible list, with no HASL; and the process notes that follow from the substrate — sodium-naphthalene or plasma activation for plated holes, and no aqueous reflow.

XCE PCB presses and plates RO3003 with rolled foil in-house: 2 to 72 layers, impedance to ±10 % on coupons cut from the panel centre and edge, panels to 600×1200 mm, PTFE hole activation on site, samples in as fast as 24 hours, inspected to IPC-A-600G and IPC-6015. Send the stack-up for a quote or WhatsApp +86 180 3341 4657.

Rolled copper in the RO3000 family means one grade and two weights: RO3003 with 1/2 oz (AH) or 1 oz (A1). It is not a laminate option that can be added to RO3035 or RO3203 from the standard table, and the “0.040 inch” figure that circulates is a dielectric thickness, not a foil.

The physics is a ratio, not an adjective. At 77 GHz skin depth in copper is 0.24 µm; a rolled foil’s 0.2–0.4 µm sits at the depth, while a standard electrodeposited foil’s 1.5–3 µm sits six to twelve times above it. That is worth 0.2–0.3 dB per inch of conductor loss at that band — 0.5 dB over a 50 mm feed and 2 dB over a 200 mm corporate feed, one way.

And the arithmetic that decides whether to pay for it is in section 4, which is not the arithmetic most people expect: copper is 58 to 69 % of the loss on every grade in this family, including the 10.2 Dk one, because a narrow 50 Ω line on a high-Dk material has more conductor loss than a wide one on a low-Dk material. A higher Dk does not dilute the value of a smooth foil. What dilutes it is a laminate whose dielectric term already dominates: 39 % copper share on 4350B, 11 % on FR-4.