All About FR4 Dielectric Constant and Material Properties

 FR4 stands as the most common PCB substrate material used in electronics manufacturing offering proven performance across countless devices and markets. But what exactly is FR4 and why does it gain such ubiquitous adoption in the fabrication of printed circuit boards?

This articles provides an in-depth reference on FR4 laminates covering dielectric properties impacting PCB performance, variations in formulations, comparative strengths and weaknesses to alternative high frequency circuit substrates, considerations when selecting FR4 materials, and guidance using FR4 effectively in designs. After reading, engineers and fabricators gain expert insights to apply this versatile material optimally across applications.

What is FR4 Material?

FR4 constitutes a broad classification for glass reinforced epoxy laminate sheets serving as reliable, cost-effective foundations upon which to manufacture printed circuit boards. The designation “FR” denotes flame resistance while “4” signals woven fiberglass reinforcement for mechanical rigidity.

Typical FR4 sheet composition showing copper, epoxy, and fiberglass

FR4 sheets typically consist of cotton or paper reinforcement fabrics embedded within an epoxy resin binder formulated to achieve target flame resistance, processing, and electrical characteristics. Copper foil layers get laminated onto one or both sides to create a basic PCB substrate primed for circuit patterning.

Dielectric Constant

The dielectric constant constitutes one of the most important electrical properties governing FR4 PCB performance, indicated as:

$D_{k}$ (pronounced “D kay”)

Dk Definition: Ratio of electrical permittivity of the insulating material to permittivity of a vacuum. Directly impacts propagation velocity through a substrate.

Typical FR4 formulations exhibit dielectric constants between 3.9 and 4.8 depending on glass styles and resin chemistry over a frequency range spanning hundreds of megahertz suitable for many digital circuits.

Key FR4 Material Properties

Myriad factors characterize FR4 substrates—here are primary attributes impacting PCB fabrication and circuit designs:

Property	Typical Range	Description
Dielectric Constant	3.9 - 4.8	Ratio of substrate vs air permittivity
Loss Tangent	0.015 - 0.035 @ 10 GHz	Dielectric losses increasing with frequency
Breakdown Voltage	12 - 60 kV/mm	Maximum sustainable electric field strength
Water Absorption	0.1 - 0.4 %	Tendency to absorb ambient moisture
Copper Peel Strength	3.5 - 10 lbf/in	Adhesion between copper foil and resin
Glass Transition (Tg)	130 - 180 °C	Temperature marking epoxy transition from glassy to rubbery state
Coefficient of Thermal Expansion (CTE)	12 - 20 ppm/°C below Tg <br> 200-400 ppm/°C above Tg	Amount substrate dimensions change per degree temperature
Shelf Life	1 year @ <30°C/ 60% RH	Duration substrate retains properties during storage
Operating Temperature Range	-65 to 150°C	Sustainable ambient temperature extremes

These interdependent electrical, mechanical, chemical, and thermal characteristics guide material selections for target applications based on performance requirements, operating conditions, and fabrication processes.

FR4 Formulation Variants

Under the FR4 umbrella exist a spectrum of epoxy blends and fiberglass styles tuned towards specific mechanical needs and electrical properties:

Types

• Standard FR4 – Well proven epoxy resin systems using traditional fiberglass reinforcement
• High Tg FR4 - Modified formulations with higher glass transition temps for enhanced thermal capability
• Low Loss FR4 – Tuned dielectrics targeting lower signal loss in high frequency applications
• Halogen Free FR4 – Environmental friendly variants without brominated flame retardants
• High Frequency FR4 – Cutting edge low loss polymer resins for microwave circuits

Reinforcement Fabricators

• Woven fiberglass – Provides dimensional stability minimizes shrinkage
• Non-woven random fiber – Enhances resin filling for solid laminate quality
• Paper fabric – Balances cost while achieving adequate mechanical rigidity

Pros and Cons of FR4 Material

Like any material, pros and cons exist when selecting FR4 laminates:

Advantages

• Well characterized electrical properties
• Multiple vendors provide excellent supply continuity
• Most economical rigid PCB substrate
• Highly reliable across decades of electronics usage
• Compatible with nearly all PCB fabrication processes
• Easy component soldering with standard FR4 glass transitions

Disadvantages

• Electrical losses start increasing above ~500 MHz hindering microwave device integration -Moisture absorption induces small performance drifts over time
• Limited thermomechanical capabilities for extreme temperature and vibration environments compared advanced polymer, ceramic, quartz substrates.

FR4 strikes an ideal balance between electrical characteristics, manufacturability, cost, availability, and reliability that fits many electronics use cases. Each product’s unique constraints and performance goals guide material selections.

FR4 Selection Considerations

Key considerations when specifying FR4 circuit boards:

Design Factor	Material Selection Guidelines
Frequency Range	< 500 MHz: Standard FR4 Variants <br> 500 MHz – 6 GHz: Low Loss FR4 <br> > 6 GHz: RF/MW Specialty Substrates
Operating Temperature Range	-20°C to 125°C: Standard FR4 <br> -65°C to 175°C: High Tg FR4
Environmental Compliance	Lead-free compatible and halogen free formulations to align with legislative directives
Fabrication Process Capability	130°C+ glass transition to withstand lead-free soldering <br> Non-woven fiberglass style for HDI microvias
Signal Integrity	Low loss laminates to minimize crosstalk above 250 Mbps signalling
System Reliability Risks	Moisture resistance treatment in humid deployments <br> Glass reinforcement for shock/vibe resilience

Align intended application conditions and performance goals with FR4 material properties when specifying.

FR4 PCB Design Considerations

Beyond material selection, PCB layouts account for dielectric properties guiding stackups and routing:

Trace Geometry Calculations

Based on ~4.35 Dk, 100 mil FR4 stripline impedance:

Zo = (87/√εr)[ln(5.98h/0.8w + t)]
Zo(100mil) = 50 Ω  given h = 12 mil, t = 1 oz  
Solving: w = 8.8 mils - Choose 10mil trace

Differential Pairs Routing

Reference planes above and below differential pairs controls impedance. Minimum 30 mil separation helps balance leakage inductance in moderate speed links.

Controlled Impedance Rules

Specify controlled impedance layers targeting 50 or 75 ohms with adequate dielectric thicknesses and ground planes. Define width rules per trace geometry calculations.

These techniques deliver stable electrical performance leveraging FR4’s reliable dielectric properties.

Conclusion

FR4 laminates offer optimal balance of capabilities to suit countless electronics applications with a blend of strong electrical properties, exceptional manufacturability, and proven design margins across diverse operating environments. Not only do FR4 substrates empower everyday consumer devices, but also enable cutting edge systems powering emerging 5G communications, autonomous electric vehicles, high efficiency motor drives, and beyond which place immense reliability demands on printed circuit board materials. By leveraging advanced FR4 material formulations tailored to application needs combined with matching stackups and layouts, engineers consistently achieve success developing innovative PCB platforms upon which to build next generation technologies.

FR4 FAQs

Q: Does FR4 absorb a lot of moisture?

FR4 absorbs minor ambient humidity but far less compared to more porous substrates, minimizing detrimental impacts on electrical performance. Careful encapsulation protects boards.

Q: Can FR4 be used for RF microwave circuits?

Standard FR4 supports sub 6 GHz devices. Specialty low loss, high frequency FR4 materials extend the range for executing mmWave products above 30 GHz.

Q: What PCB layer counts typically use FR4?

FR4 appears across all layer complexities—from single sided consumer gadgets to 64+ layer communication systems. FR4 reliably brings multilayer boards to life.

Q: What are key FR4 glass transition temperatures?

Standard ~130°C Tg survives lead-free soldering. High performance variants feature 180°C+ Tg to withstand aggressive lead-free reflow along with enhanced thermal cycling resilience.

Q: Does thickness impact FR4 electrical performance?

Thin laminates down to 2 mils reliably exhibit FR4’s nominal electrical characteristics. Thicker boards above 60 mils often

Should You Use Star Grounding for Analog and Digital Ground Separation?

 When mixing analog and digital circuits on printed circuit boards, managing ground planes appropriately minimizes noise injection ensuring proper function. A common technique is creating separate analog and digital ground domains tied to a central star point or points.

This article provides electronics hardware engineers a comprehensive guide on leveraging star grounding configurations to achieve critical isolation between sensitive analog and noisy digital return paths. By covering fundamental concepts, implementation considerations, printed circuit board layout recommendations, a real-world design example, and helpful FAQs, PCB designers can confidently utilize star grounds to solve mixed-signal stability problems.

What is Star Grounding?

Star grounding involves routing all ground returns from a section of circuitry to a single, localized node point before connecting to the main ground plane. This star point becomes the single tie-in location for that region’s currents flowing to the shared system ground reference.

For analog and digital separation, individual star points are created for groupings of analog and digital circuitry. Traces transport respective return currents from each zone to its localized star point before interconnecting peripherally to the board’s primary ground plane.

Star grounding routes return paths to localized star points

Star grounding enables optimal high frequency isolation between critical circuit zones on dense, mixed signal printed circuit boards.

Why Use Star Grounding?

Star grounding optimizes grounding performance by:

Lowering Inductance:

Shorter traces between components and a single localized star point minimize overall inductive loop area versus sharing long ground lines resulting in lower path inductance.

Containing Noise:

Converging return paths into an star point limits opportunity for shared noisy ground coupling by containing and channeling electrical fluctuations toward the star points instead of peripherally across the entire ground plane.

Improving Filtering:

The discrete inductances and capacitances formed between the star points and main ground provide passive filtering of noise as currents flow through these components.

Star grounding aims to lower inductance while isolating noise

Overall star grounding enhances signal integrity, especially critical for precision analog performance.

Design Considerations

Several key factors guide proper star ground implementation:

Zone Partitioning

Carefully grouping circuits functions into separate analog, digital, or mixed zones facilitates creating effective star grounds for the respective sections.

Star Point Locations

Central points within each zone avoid long return traces reducing inductance. Multiple points per zone can further minimize loop areas.

Return Path Routing

Use fat traces or polygons routed directly between each circuit and corresponding zone star point without extending across zones.

Single Point Interconnections

Tie zones star points together in single locations, often connecting analog zones to cleaner digital star points first before linking to the broader ground plane to avoid injected noise from spreading across domains.

Passive Filtering

Adding capacitors from star points to main ground aids filtering. Ferrites on long domain interconnects also suppress noise leakage.

Implementing Star Grounds

Best practices for executing star grounding include:

1. Partition Circuit Zones

Group analog, digital, and mixed signal functions into separate physical zones. Analog is most sensitive.

2. Place Star Points

Locate singular tie-in points central to each defined zone.

3. Route Return Paths

Use thick traces or polygon pours to route component returns directly to respective nearby star point.

4. Connect Star Points

Join star points together in single locations from cleaner domains progressively towards noisier zones.

This methodology yields low inductance isolated grounds.

PCB Layout Recommendations

Star grounding success depends on printed circuit board layout:

Distribute Ground Vias

Spread ground vias uniformly along the periphery of each zone to enable multiple interconnect locations and lower inductance to the shared ground plane underneath.

Careful Trace Routing

Avoid intertwining ground returns between zones. Interzone traces should optimally cross zones perpendicularly minimizing overlap.

Ground Plane Voids

Punch voids in the primary ground plane underneath analog zones to limit injected noise coupled through a common return path underneath devices. Some digital area voids also help force return currents into intended star points.

Robust Power Distribution

Clean power routing feeding zones prevents unwanted fluctuations passed through ground returns. Strategically placed local decoupling supports robust functionality.

Post Layout Verification

Perform signal integrity analysis and ensure ground noise voltages don’t exceed threshold requirements for proper circuit operation.

Real World Star Grounding Examples

Examining a high speed mixed signal printed circuit board shows star grounding best practices:

Design Goals

• 16 layer board with dedicated power planes
• High speed FPGA, DDR Memory, and PCIE zones dominated left side
• Right side mixed analog, audio, and intermediate frequency (IF) circuits
• Critical RF circuits housed centrally below FPGAs

Implementation

• Careful zonal partitioning on left and right board sides
• Localized star points placed within each zone type
• Short fat traces swiftly route returns to each zone’s star point
• Sparse interconnections tie star points from cleaner domains to noisier ones
• Extensive ground voids underneath analog zones

This combination enables the high performance mixed signal PCB containing sensitive RF, analog and noisy digital circuits in close proximity.

FAQs About Star Grounding

Q: Are star ground points connected to power planes?

No, star points only collect ground returns. Connections to power planes risks unwanted coupling. Separate power returns route to respective power pins or planes.

Q: Is a single star ground point used in small zones?

Yes, when zones host less circuitry, convergence to a single optimized star point is simplest. Larger zones benefit from multiple discrete points targeting device groupings.

Q: How are ground islands used with star grounding?

Raised ground islands underneath devices served by a star point provide contiguous return paths between components and the star point through vias minimizing inductance.

Q: Does adding capacitance impact filtering?

Additional capacitors from zone star points can improve filtering but necessitates considering series resonance. Capacitance between closely spaced stars enables charge transfer equalizing potential differences.

Q: What causes star grounding to be ineffective?

Failure to properly define circuit zones, overlapping space between zones, poor return path routing, inadequate board layer stack planning, and excessive interconnects between star points reduces noise isolation. Careful planning is vital.

Conclusion

In summary, star grounding requiring carefully routing ground returns from circuits to single localized nodes leverages inherent trace inductances to channel electrical fluctuations minimizing injection across sensitive zones. By partitioning mixed signal PCBs into analog, digital and mixed function domains with dedicated star points tied together progressively from cleaner to noisier sections, robust high frequency isolation can be achieved through simple, low-cost layout approaches.

Engineering teams specializing in dense, mixed signal printed circuit boards combining demanding RF, analog, and high speed digital circuits can leverage these proven star grounding best practices to fulfill complex performance requirements critical for cutting edge technologies.