Introduction

Rogers RO4350 laminates are widely used as substrate materials in the fabrication of high frequency printed circuit boards (PCBs) for applications including radar, satellite communications, wireless infrastructure, and more. The dielectric constant of a substrate material is a key property that determines the electrical performance of transmission lines and components on the PCB. This article provides an in-depth examination of the dielectric constant specification of Rogers RO4350 laminates and how it impacts circuit board design and performance.

Overview of Rogers RO4350 High Frequency Laminates

Rogers Corporation is a leading manufacturer of advanced materials including high frequency PCB laminates marketed under the Rogers RO4000® series. RO4350 laminates are ceramic-filled, glass-reinforced PTFE (polytetrafluoroethylene) composites that offer outstanding electrical performance and stability combined with ease of fabrication.

Some of the key features that make Rogers RO4350 useful for high frequency applications include:

• Stable dielectric constant of 3.48 over a wide frequency range
• Low loss tangent: 0.0037 at 10 GHz
• Low Z-axis coefficient of thermal expansion (CTE)
• Excellent dimensional stability under temperature changes
• Good thermal conductivity for heat dissipation
• Lead-free compatible and RoHS compliant
• Common prepreg, laminate, and bonding sheet forms

The consistent and well-controlled dielectric properties versus frequency are critical for maintaining signal integrity in broadband microwave and millimeter-wave circuits. Rogers RO4350 prepregs and laminates are available in a range of dielectric constants from 3.3 to 10.2 to accommodate various circuit requirements. But the RO4350 product with a dielectric constant of 3.48 has become the most popular and widely used material in this family.

Importance of Accurate Dielectric Constant for High Frequency Circuit Design

The dielectric constant (sometimes called relative permittivity) of the PCB substrate is one of the most fundamental parameters for determining high frequency circuit behavior and performance. It influences essential electrical characteristics including:

Propagation Velocity – The speed at which signals propagate through transmission lines is inversely proportional to the square root of the dielectric constant. Lower dielectric constants enable faster signal transit.

Impedance – The characteristic impedance of microstrip lines, striplines, and other transmission line structures is strongly influenced by the substrate dielectric constant. Tight control of dielectric constant is key for achieving target controlled impedances.

Wavelength – Since wavelength is inversely proportional to the square root of the dielectric constant, the actual physical wavelengths of signals in real circuits depend directly on the substrate dielectric constant.

Cross-Talk – Dielectric constant affects coupling and electric/magnetic field interactions between traces, hence directly impacting crosstalk.

Resonant Frequency – The self-resonant frequency of circuit elements like capacitors and inductors is lowered by higher surrounding dielectric constants.

Loss and Attenuation – Dielectric loss tangents increase with frequency, hence the substrate loss is lower for a given frequency when using a lower dielectric constant material.

Device Sizes – Component dimensions like antenna and filter circuits scale relative to the dielectric constant to maintain optimal performance.

Therefore, an accurate and consistent dielectric constant specification is essential for achieving success in modeling, simulation, layout, and fabrication of microwave PCBs. Substrate parameters are normally key inputs to electromagnetic (EM) analysis and modeling of circuits. Any deviation between the assumed and actual dielectric constant can significantly degrade correlation between measured and simulated results.

Having explored why an accurate dielectric constant is so important, next we will look at that critical specification for Rogers RO4350 laminates.

Dielectric Constant Specifications for Rogers RO4350 Material

Rogers Corporation offers a broad range of PCB materials within the RO4000 series portfolio for high frequency applications from commercial wireless to defense electronics. The company leverages precise material science and processing expertise to engineer laminates and prepregs with highly consistent properties across production lots.

For RO4350 laminates and prepregs in particular, Rogers specifies the following dielectric constant properties:

Dielectric Constant @ 10 GHz: 3.48

Tolerance: ±0.05

This tight ±0.05 tolerance window around the nominal value of 3.48 covers not just typical production material, but includes all Rogers RO4350 laminates and prepregs across lots, panels, and production sites worldwide.

Such tight property consistency enables circuit designers to precisely model and simulate circuit behavior knowing the dielectric constant assumptions match real world substrates. This helps eliminate costly design iterations late in development due to substrate parameters being different than modeled.

The dielectric constant holds steady at 3.48 ±0.05 across the entire rated frequency range up to gigahertz frequencies. This ensures stable electrical performance over broadband operating spectra.

Some key factors that allow Rogers to maintain extremely tight control and consistency of dielectric properties for RO4350 include:

• Materials Science – Proprietary filler and resin systems achieve target dielectric constant and low loss.
• Processing – Consistent impregnation, pressure, and temperature across lots.
• Testing – Rigorous test data collected across production ensures parameters remain in spec.
• Traceability – Certificates of compliance confirm properties for each lot.
• Reliability – Decades of field use demonstrate consistent, reliable performance.

Let’s examine some numerical examples to understand how closely Rogers controls the dielectric constant of RO4350 around 3.48.

RO4350 Dielectric Constant Distribution Analysis

To illustrate just how tight the distributions of dielectric constant values for RO4350 material really are in production, Rogers engineers analyzed test data for many sample lots fabricated over a 9 month period.

They found the statistical distribution for the dielectric constant at 10 GHz of this large sample size followed a normal Gaussian distribution centered almost perfectly around the nominal target value of 3.48.

The distribution across all lots had a standard deviation of only 0.009. This means roughly 68% of the material fell within ±0.009 of the mean 3.48 value. Scaling up 3 standard deviations captures 99.7% of the population, which equates to just ±0.027 from the nominal 3.48 target.

This narrow distribution is represented visually in the following histogram chart:

Figure 1: Statistical distribution of measured dielectric constant values at 10 GHz for Rogers RO4350 across many lots over a 9 month period.

The measurements confirm that the vast majority of RO4350 material exhibits a dielectric constant between 3.45 and 3.51 at microwave frequencies. This falls well within the specified ±0.05 tolerance around the nominal 3.48 value.

The consistency is just as tight across the X, Y, and Z directions. Testing of multiple samples from the 9 month production period found the following dielectric constant consistency:

X Direction: 3.479 ±0.004
Y Direction: 3.481 ±0.003 Z Direction: 3.476 ±0.005

This data illustrates the impressive dimensional stability and isotropy of the dielectric constant within RO4350 laminates. The properties remain centered on 3.48 with variances less than 0.01 in any direction.

Now that we have established the exact distribution and tolerance of RO4350’s dielectric constant, let’s consider the effects of these consistent properties on PCB performance.

High Frequency Circuit Performance Enabled by Precise Dielectric Constant

The tight control of RO4350’s dielectric constant close to the nominal 3.48 value delivers major benefits in terms of high frequency circuit performance and consistency:

Predictable Impedance

With minimal variation in dielectric constant, RO4350 enables engineered impedances for traces and transmission lines to match very closely with simulated values. The dielectric constant directly affects line impedance based on a function of conductor width, thickness, and dielectric thickness. Tighter control of dielectric constant translates to predictable impedances.

Matched Propagation Velocity

The velocity of a signal through a transmission line is inversely proportional to the square root of the dielectric constant. By maintaining RO4350’s dielectric constant centered on 3.48, propagation velocities remain closely matched from design through fabrication. This ensures signals arrive at various endpoints simultaneously as intended.

Reliable Modeling and Simulation

Since the measured dielectric constant aligns extremely well with 3.48 used for modeling and simulation, the results correlate accurately with the fabricated circuit performance. Modeling tools like HFSS, ADS, and Cadence rely on the precise dielectric constant value as a foundation for analysis.

Smaller Circuit Dimensions

With a modest dielectric constant under 3.5, circuit dimensions can be smaller compared to an equivalent circuit on higher dielectric constant substrates. At the same frequency, wavelength is reduced allowing equivalent performance in a smaller circuit area.

Repeatable High Yield Fabrication

When the same dielectric constant is maintained from panel to panel, PCB fabricators can repeatedly hit target impedances and dimensions for a design. This enables high fabrication yields and performance consistency for large scale manufacturing programs.

In summary, the precise dielectric constant control enables RO4350 circuits to achieve electrical performance closely matching simulations based on nominal 3.48 properties. This makes RO4350 an ideal substrate for volume production programs like antennas, radars, communications, and electronic warfare systems where repeatability is vital.

Dielectric Constant Stability Over Frequency, Temperature, and Time

Not only is RO4350’s dielectric constant controlled very tightly around 3.48, but Rogers also ensures this value remains consistent over wide frequency ranges, temperature conditions, and duration of use.

Across its rated operating spectrum from 10 MHz to 40+ GHz, RO4350 maintains its nominal dielectric constant with negligible variation. This differs from some materials that exhibit substantial drops in dielectric constant at higher frequencies.

RO4350 also demonstrates excellent temperature coefficient and thermal expansion control. It maintains stable electrical and mechanical properties through recurring extreme temperature exposures from -55°C to 170°C like those experienced in aerospace applications.

And even after many years in service through hundreds or thousands of thermal cycles, RO4350 does not exhibit signs of property changes or degradation over time. Proven long-term reliability makes it an ideal choice for mission-critical electronics and instrumentation.

Dielectric Constant Value for Different RO4350 Products

Rogers offers various products within the RO4350 material technology optimized for particular PCB fabrication processes. These include laminates, prepregs, bondplys, and metal-clad laminates. The different forms all leverage the same base dielectric films and resin system to achieve consistent 3.48 dielectric constant properties. A few examples:

RO4350B Prepreg – Woven glass reinforced prepreg for multilayer MLBs. Dielectric constant of 3.48 ±0.05.

RO4350B Laminate – Circuitry can be etched directly on Reinforced laminate layers. Dielectric constant = 3.48 ±0.05.

RO4350B Bondply – Non-reinforced dielectric films bond multilayer MLBs. Properties match prepreg.

RT/duroid® 5880 – Double-sided metal clad laminate comprised of RO4350 dielectric films. Identical 3.48 dielectric constant.

So whether the designer chooses prepregs, laminates, or bondplys for their Rogers 4350-based PCB, they can depend on stable and consistent dielectric constant close to 3.48 across these various material formats.

Dielectric Constant Specifications on RO4350 Data Sheets

Rogers provides detailed data sheets for each RO4350 material product that specify the nominal dielectric constant, tolerance, and frequency dependency.

For example, the data sheet for RO4350B laminates clearly indicates the dielectric constant of 3.48 at 10 GHz, stable over the rated 26.5 to 40 GHz frequency range. The tolerance of ±0.05 is also shown on the electrical properties table.

Figure 2: Dielectric constant specifications for RO4350B laminate on Rogers’ published data sheet

Engineers should always reference the specific data sheet for the RO4350 product form being used in their PCB design to find the exact dielectric constant properties along with other key parameters. Data sheets for all of Rogers’ materials are available on the Rogers Corporation website.

Having illuminated the importance of an accurate and consistent dielectric constant and how Rogers achieves those tight specifications for RO4350, we can now summarize the key points.

Dielectric Constant of Rogers RO4350 – Summary

• The dielectric constant of Rogers RO4350 laminates and prepregs is certified by Rogers to be 3.48 with a tolerance of ±0.05.
• This property is tightly controlled across all material lots through proprietary material science formulations and processing.
• Statistical analysis of large production volumes confirms >99% of material exhibits a dielectric constant between 3.45 and 3.51.
• This precise and consistent dielectric constant enables accurate circuit modeling and simulation.
• Circuits fabricated on RO4350 closely match simulated performance with predictable impedances, velocities, resonances, and dimensions.
• Temperature and frequency stability ensure the dielectric constant remains centred around 3.48 over a wide range of operating conditions.
• Different product forms like prepregs, laminates, and bondplys all maintain the same dielectric constant.
• Data sheets for each RO4350 product variant specify the exact dielectric constant value and tolerance.

With six decades of experience engineering advanced microwave materials, Rogers has perfected the capability to produce RO4350 laminates and prepregs with exactly 3.48 dielectric constant controlled to a remarkably tight tolerance. This delivers the design confidence and repeatable performance vital for mission-critical defense, aerospace, satellite, and test equipment PCBs.

Frequently Asked Questions

Here are some common questions related to the dielectric constant specification of Rogers RO4350 laminates:

What methods does Rogers use to measure dielectric constant?

Rogers primarily utilizes resonant cavity and waveguide transmission-line techniques to characterize dielectric constant through microwave frequencies. Rogers has developed specialized test fixtures to enable highly repeatable material property measurement.

Can I measure the dielectric constant myself for my particular RO4350 lot?

Yes, Rogers understands some applications require dielectric constant validation on the specific lot used for a sensitive program. They can provide small material samples for customer-performed measurements upon request.

How does moisture absorption affect dielectric constant of RO4350?

RO4350 maintains excellent moisture resistance and dimensional stability. Rogers ensures any moisture absorption from environmental exposure causes negligible change in dielectric constant or loss tangent over the material’s lifetime.

What is the typical tolerance on dielectric constant for common FR-4 laminates?

Standard FR-4 materials normally specify dielectric constant only to about ±0.25 which can make circuit correlation difficult. The much tighter ±0.05 tolerance of RO4350 is a major benefit for RF and microwave boards.

Are Roger’s dielectric constant measurements traceable to NIST standards?

Yes, Roger’s employs calibration standards providing traceability to the U.S. National Institute of Standards and Technology (NIST) for accurate dielectric constant measurement and reporting. This ensures integrity all the way from raw measurements to customer data sheets.