How does Arlon 25N compare to Rogers RO4003C?

Both Arlon 25N and Rogers RO4003C are hydrocarbon ceramic/glass thermosets designed as cost-effective, easily processed alternatives to pure PTFE (Teflon). Arlon 25N has a lower dielectric constant (3.38 vs. 3.55 for RO4003C). Their loss tangents are very similar (0.0025 for 25N vs. 0.0027 for RO4003C at 10 GHz). Both can be processed using standard FR-4 fabrication techniques without the specialized plasma treatments required for PTFE via plating. While Rogers commands more market share in general RF design, Arlon 25N remains a critical alternative frequently specified in legacy military/aerospace prints and certain base station power amplifier designs.

Why is Thermal Coefficient of Er (TcEr) important?

When a high-power amplifier operates, it generates massive amounts of heat, causing the PCB temperature to fluctuate rapidly. In standard FR-4, the dielectric constant (Er) changes significantly as temperature rises. Because the characteristic impedance and electrical length of microstrip traces depend on Er, the impedance matching networks will drift out of tune as the board heats up, causing return loss degradation and power reflections. Arlon 25N has an exceptionally low Thermal Coefficient of Dielectric Constant (TcEr). Its Er remains almost perfectly flat across a wide temperature range, ensuring that filters, impedance transformers, and phase shifters perform exactly the same at cold start as they do at maximum thermal load.

What is the difference between Arlon 25N and Arlon 25FR?

Arlon 25N and 25FR share the exact same resin system and RF electrical properties (Er = 3.58 for the FR variant). The difference is flammability certification. The 'FR' in 25FR stands for Flame Retardant. Brominated flame retardants are added to the 25FR resin to achieve a UL 94 V-0 flammability rating, which is a mandatory safety requirement for commercial consumer electronics and indoor base stations. The standard 25N does not have these retardants, which gives it a slightly lower loss tangent and dielectric constant, making it preferred for pure performance applications where UL 94 V-0 certification is not legally required.

PCB Design

Arlon 25N

A 100-Watt power amplifier board experiences massive thermal swings. When the transistor reaches 85°C, standard FR-4 substrate material expands, causing plated through-holes to crack, while its dielectric constant drifts, detuning the carefully designed matching networks and plummeting the output power. Pure PTFE (Teflon) substrates solve the electrical drift, but they are soft, dimensionally unstable, and extremely difficult to process. Arlon 25N represents the industrial compromise: a woven glass-reinforced hydrocarbon thermoset. It processes easily like FR-4, meaning via holes plate reliably without plasma preparation. Yet electrically, its dielectric constant is locked down. It refuses to drift under thermal stress, ensuring that the phase delays and impedance matches designed at room temperature remain perfectly stable when the amplifier is running red-hot.

Category: PCB Design

Dielectric Constant (ε_r): 3.38

Loss Tangent (tanδ): 0.0025 @ 10 GHz

RF Substrate Property Comparison

Property	Arlon 25N	Rogers RO4003C	PTFE (RT/duroid 5880)	Standard FR-4
Dk (ε_r)	3.38	3.55	2.20	4.2 to 4.7
Df (Loss Tangent)	0.0025	0.0027	0.0009	0.020
Resin System	Hydrocarbon Thermoset	Hydrocarbon Ceramic	PTFE (Teflon)	Epoxy
Processing	Standard FR-4 flow	Standard FR-4 flow	Complex (Plasma etch required)	Standard
UL 94 V-0 Rated?	No (Use 25FR variant)	No (Use RO4350B)	Yes	Yes

Why thermal stability matters:
Electrical Length (θ) = (360 · f · L · √ε_eff) / c
If the dielectric constant (ε_r) changes with temperature, the electrical length (θ) of every trace changes. A 90-degree Doherty combiner might become 95 degrees, destroying efficiency.

Coefficient of Thermal Expansion (CTE):
Z-axis CTE of Arlon 25N is highly matched to copper, minimizing stress on plated through-holes during thermal cycling.

Common Questions

Frequently Asked Questions

How does it compare to Rogers?

Arlon 25N is a direct competitor to Rogers RO4003C. Both are glass-reinforced hydrocarbon thermosets designed to bridge the gap between FR-4 processing ease and PTFE electrical performance. 25N has a slightly lower Dk (3.38 vs 3.55) and near-identical loss.

What does the "N" mean?

The standard Arlon 25N is not flame retardant. The "FR" variant (Arlon 25FR) adds brominated flame retardants to achieve a UL 94 V-0 rating, which is required for most consumer and commercial indoor electronics. The FR variant has a slightly higher Dk of 3.58.

Why use it instead of PTFE?

Pure PTFE (like RT/duroid 5880) has lower loss (0.0009) but is extremely soft and dimensionally unstable during PCB pressing. Plating vias in PTFE requires nasty sodium etchants or plasma chambers. Arlon 25N is rigid and processes in standard chemical baths exactly like cheap FR-4.

Related Terms

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Material Selection

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