RO4350B vs. RO4003C: Dk, Loss, UL 94 V-0, and Which Rogers Laminate to Choose

Start from the certification requirement, then the loss budget. If the finished product, or the board as a component, needs UL 94 V-0 recognition, RO4350B is the answer and the loss question only decides whether the design still closes. If flame rating is not required, compare the loss budget at your highest operating frequency. RO4003C earns its place when the extra 0.001 of dissipation factor turns into a meaningful fraction of a dB on your longest RF run.

Plated-through-hole reliability is the third factor, and it usually favors RO4350B. Its Z-axis CTE is 32 ppm/°C against 46 ppm/°C for RO4003C. On thick boards, high-layer-count hybrids, or products that see wide thermal cycling, lower Z-axis expansion reduces barrel stress on plated vias.

Both are glass-reinforced hydrocarbon/ceramic laminates in the Rogers RO4000 series. The values below are typical figures from the Rogers RO4000 data sheet (see sources at the end of this article). Typical values are population averages, not guaranteed limits, so confirm against the current data sheet revision before freezing a stackup.

The numbers most people quote, 3.48 for RO4350B and 3.38 for RO4003C, are process Dk values. Rogers uses them for lot-to-lot quality control, and they are measured with a clamped stripline fixture at 10 GHz. That fixture can trap a thin air gap, which reads lower than the dielectric constant a real etched circuit sees.

For impedance and phase modeling, Rogers publishes a separate design Dk: 3.66 for RO4350B and 3.55 for RO4003C. It is derived from circuit-based measurements over 8–40 GHz and is the value to enter in your field solver. Modeling with process Dk instead of design Dk makes traces come out slightly too wide and tuned structures resonate slightly low.

One exception is worth knowing. The 4 mil (0.10 mm) RO4350B core has a process Dk of 3.33 ± 0.05, not 3.48. If you use the thinnest RO4350B core for a tight-pitch RF layer, check the data sheet note for that thickness rather than the headline value. More on how fabricators handle this in how to specify a controlled impedance PCB.

RO4350B is UL 94 V-0 rated. Rogers designed it as the flame-retardant drop-in for RO4003C, using the same processing method. RO4003C is non-brominated and is not V-0 rated. For products that go through safety certification that checks board flammability, such as many consumer, telecom, and industrial products, that single difference usually decides the material before any electrical comparison starts.

If you need V-0 and RO4350B-like electrical properties but the board runs hot for long periods, look at RO4835. It shares the 3.48 process Dk and V-0 rating, and Rogers formulated it for improved resistance to oxidation-driven property drift at elevated temperature.

A quick way to size dielectric loss is the rule-of-thumb estimate: loss (dB/inch) ≈ 2.3 × frequency (GHz) × Df × √Dk. Using design Dk and data sheet Df for a stripline trace gives the figures below. They cover dielectric loss only, and conductor loss from copper roughness usually adds as much or more at these frequencies.

At 2.5 GHz the gap is about 0.011 dB per inch, which disappears next to connector launches and via transitions on most boards. At 10 GHz it grows to about 0.046 dB per inch, close to half a dB over a 10-inch run. That is when RO4003C starts to pay for giving up the flame rating. Smoother copper, such as Rogers LoPro reverse-treated foil, lowers conductor loss on either laminate, so compare foil options before switching materials for loss alone.

If you are replacing PTFE to save cost, the step from PTFE down to either RO4000 laminate is much larger than the step between RO4350B and RO4003C. Re-run the full loss budget instead of treating either one as a drop-in for PTFE. Rogers PCB vs. FR-4 covers the earlier question of whether you need Rogers at all.

Both laminates keep their Tg above 280 °C, so neither passes through a glass transition during lead-free reflow. That is a real advantage over standard FR-4, whose Z-axis expansion rises sharply above its Tg. Between the two Rogers materials, though, the gap is not trivial. RO4350B expands 32 ppm/°C in Z against 46 ppm/°C for RO4003C, and copper sits near 17 ppm/°C.

The closer the laminate is to copper, the less strain each thermal cycle puts on via barrels. For thin two-layer RF boards the difference rarely matters. For thicker multilayer or hybrid boards, many reflow passes, or products qualified to wide temperature cycling, RO4350B carries the lower risk.

Take a 50 Ω microstrip on a 0.508 mm (20 mil) core with 1 oz copper. With design Dk, the closed-form solution gives about 1.08 mm trace width on RO4350B and 1.10 mm on RO4003C. If a board laid out for RO4003C is built on RO4350B without re-solving width, impedance drops to roughly 49.3 Ω. That is well inside a normal ±10% tolerance, and often inside ±5%.

Tuned structures are less forgiving. Effective Dk rises by about 2.5%, so filters, couplers, and resonant antenna elements shift down by about 1.25% in frequency, roughly 125 MHz at 10 GHz. For narrowband RF, re-simulate after any laminate change. Confirm these figures in your own field solver, because closed-form estimates ignore solder mask and etch profile. The 50-ohm trace design guide and microstrip vs. stripline vs. coplanar waveguide cover those variables.

Standard thicknesses also differ, which can force a dielectric height change on top of the Dk change. In the Rogers data sheet, RO4003C is listed at 0.008″, 0.012″, 0.016″, 0.020″, 0.032″, and 0.060″. RO4350B is listed at 0.004″, 0.0066″, 0.010″, 0.020″, 0.030″, and 0.060″. Only 0.020″ and 0.060″ overlap, so a 0.012″ RO4003C design has no same-thickness RO4350B core to drop into.

Most laminate problems in RF builds come from an RFQ that says “Rogers” without saying which one. Put these on the fabrication drawing so the quote, CAM review, and build all use the material you simulated:

1. Exact laminate and core thickness, for example “RO4350B, 0.508 mm, 1 oz ED copper”, plus foil type if you require LoPro.

2. Whether substitution is allowed. Write “no substitution” if your filters are tuned, or list approved alternates such as RO4835.

3. The design Dk you modeled with, and the impedance targets with tolerance per layer and structure.

4. The flame-rating requirement, if any, so the fabricator does not propose RO4003C to save lead time.

5. Bonding material for multilayer builds: RO4400-series bondply for all-Rogers stackups, or FR-4 prepreg for hybrids. The high-frequency PCB RFQ checklist covers the rest of the package.