FR-4 glass epoxy is the most common PCB substrate material used today. Over 90% of all manufactured PCBs are built using some variant of FR-4 laminate. This ubiquitous material enables cost-effective fabrication of consumer electronics, industrial control boards, instrumentation circuits and a myriad of other electronic systems.
This article provides an in-depth examination of FR-4 materials and the key reasons behind their widespread use for PCB fabrication across diverse applications and industries.
Overview of FR-4 PCB Substrates
FR-4 is a composite thermosetting material composed of woven fiberglass cloth fabric embedded in an epoxy resin binder. The FR designation refers to flame resistant grade 4 material.
The glass cloth provides mechanical strength and stability to the substrate while the epoxy resin binds everything together into a rigid sheet. FR-4 laminates have a typical thickness between 0.2mm to 3mm. Copper foils are bonded to the top and bottom surfaces through a hot roll or press lamination process for constructing the PCB.
The composition of FR-4 can be tuned by factors like:
- Type of glass weave – plain, satin or leno
- Density of glass weave – higher density equals better dimensional stability
- Flow characteristics of epoxy resin system
- Fillers used – typically silica for improved thermal performance
- Copper foil type – rolled or electrodeposited; thickness varies with application
This versatility of formulation options enables extensive customization of FR-4 properties. Multiple grades of FR-4 materials catering to specific applications and fabrication requirements are offered by leading manufacturers.
Key Electrical Properties
The electrical characteristics of FR-4 substrates include:
- Relative Dielectric Constant (Dk): Typically between 4.2 to 4.6 at 1 MHz
- Dissipation Factor (Df): 0.02 or less at 1 MHz
- Dielectric Strength: Around 15MV/m
- Volume Resistivity: > 1 X 10^13 ohm-cm
The relatively high dielectric constant of FR-4 arises from polarization of the glass fibers and epoxy medium under an applied electric field. This enables good energy storage per unit volume.
The low loss tangent indicates minimal electrical power loss in the material enabling FR-4’s usage for frequencies up to Gigahertz ranges. The excellent resistivity confines the electrical conduction through the intended copper traces.
Key Mechanical Properties
Important mechanical properties of FR-4 substrates:
- Flexural strength: > 20 Kpsi per IPC 4101C
- Tensile strength: > 7 Kpsi per IPC 4101C
- Decomposition temperature: > 340°C per UL 746E
- Glass transition temperature: > 130°C per IPC 4101C
- Coefficient of thermal expansion (CTE): 13-18 ppm/°C along X and Y axes
- Water absorption: 0.15% max when immersed for 24 hrs at 23°C as per IPC 4101C
- Flammability rating: Typically V-0 per UL 94 standard
The high flexural and tensile strength arises from the fiber weave reinforcement within the substrate. The decomposition temperature rating qualifies FR-4 for long term operational reliability under electrical loads. Low water absorption ensures stable electrical performance in humid ambients.
Key Fabrication Benefits
FR-4 offers several advantages that make it conducive for efficient PCB fabrication:
- Dimensional stability – Low Z-axis CTE and robust construction ensures stable board dimensions during fabrication and operation. This enables high yields.
- Etchability – The epoxy resin provides good etchability characteristics for creating finefeatured copper traces using subtractive processes.
- Bondability – FR-4’s surface readily bonds with common laminates like copper foils for multilayer boards. This allows low cost buildup of complex PCBs.
- Drillability – The reinforcement weave enables clean punch-through during drilling of holes. This allows plated-through holes for interconnections between layers.
- Solderability – FR-4’s surface absorbs solder well, enabling reliable attachment of components to the board using wave soldering or reflow methods.
- Reworkability – Damaged surface mount components can be removed and replaced without excessive damage to the PCB pad surfaces.
Primary Applications
Some major application areas where FR-4 PCBs dominate include:
- Consumer electronics – Cellphones, laptops, TVs, home appliances. FR-4 provides optimal cost-performance tradeoff.
- Industrial controls – PLCs, motor drives, process controllers. FR-4’s electrical capabilities suffice for sensing and control needs.
- Instrumentation – Oscilloscopes, signal analyzers, function generators. FR-4’s stability supports precision signal measurements.
- IT/networking – Servers, routers, switches, base stations. High speed FR-4 grades support backplane and backhaul wiring.
- Automotive – Infotainment systems, dashboards clusters, telematics controllers. Withstands thermal cycling in automotive environments.
- Aerospace/Defense – Avionics equipment, ground radar systems, communications electronics. Flame resistant grades are available.
This ubiquity across nearly all electronics sectors is enabled by the versatility of FR-4 to be customized into grades addressing the cost, performance and reliability needs of diverse applications.
Grades of FR-4 Materials
While FR-4 refers to the broad family of glass-epoxy composites, there are several different grades tailored to specific PCB fabrication and performance requirements:
- Standard FR-4 – For double sided and multilayer boards without special needs. Offers lowest cost.
- High Tg FR-4 – With glass transition temperature >170°C. Prevents delamination during lead-free soldering.
- Low loss FR-4 – Reduced resin filler enhances high frequency response. Used for RF circuits.
- High temp FR-4 – Decomposition temp >340°C. Withstands soldering heat up to 280°C.
- Halogen free FR-4 – Non-toxic with reduced smoke emissions. Used in human-facing devices.
- High CTE FR-4 – CTE matched to copper for reduced thermal stress. Improves reliability.
- High flex FR-4 – Withstands flexing stresses without fracturing. Used in dynamic sectors.
- Low Dk FR-4 – Dielectric constant below 4.2. Enhances electrical performance.
This diversification of the basic FR-4 system with additive formulations enables cost-benefit optimizations for an incredibly wide range of end applications and markets.
FR-4 Material Properties Comparison with PTFE and Polyimide
It is insightful to compare the properties of FR-4 against the other two major non-conducting PCB substrate materials – PTFE (Teflon) and Polyimide:
| Parameter | FR-4 | PTFE (Teflon) | Polyimide |
|---|---|---|---|
| Dielectric Constant | 4.2 – 4.6 | 2.1 | 3.2 – 3.5 |
| Dissipation Factor | 0.02 or less | 0.001 | 0.003 |
| Dielectric Strength (MV/m) | 15 | 20 – 50 | 16 – 200 |
| Volume Resistivity (Ω-cm) | >10^13 | >10^18 | >10^15 |
| Tensile Strength (Kpsi) | >7 | >5.5 | >25 |
| Flexural Strength (Kpsi) | >20 | >13 | >35 |
| Decomposition Temp (°C) | 340 | 327 | 400 |
| CTE (ppm/°C) | 13-18 | 17-24 | 3-5 |
| Water Absorption (%) | 0.15 | <0.01 | 0.3 |
| Flammability Rating | V-0 | V-0 | V-0 |
| Cost | Low | High | High |
This illustrates FR-4’s cost advantage coupled with electrical and mechanical properties sufficient for most commercial circuity needs. For extreme temperature, high frequency, space or other challenging environments, designers can take advantage of PTFE or Polyimide’s superior attributes.
Trends in FR-4 Substrates for Modern PCBs
Several FR-4 material innovations are emerging to meet demands of cutting-edge PCB technologies:
- Ultra-low-loss laminates – FR-4 laminates with dielectric loss tangents below 0.005 are enabling higher frequency microwave boards.
- Low Dk materials – FR-4 grades with dielectric constants approaching 3.5 improve impedance control and signal integrity in multi-Gbps boards.
- High reliability formulations – Flame retardant grades with increased thermal rating to 180°C and reduced moisture absorption are being offered.
- Halogen-free eco-friendly grades – Reduced bromine and chloride content improves environmental footprint along with lower smoke emissions.
- Low-CTE formulations – Grades with CTE lowered to 7ppm/°C reduce thermal stresses and via failures in lead-free soldering.
- High-speed digital grades – Fast signal grade FR-4 materials reduce dielectric losses for servers, routers and network switches operating above 25Gbps.
- High-flow prepregs – These enhance impregnation quality for minimizing voids in heavy copper and blind via boards.
- Nano-filled FR-4 – Addition of ceramic nano-particles improves thermal conductivity for power electronics boards.
Cost Considerations
A key factor driving the popularity of FR-4 materials is the low cost compared to exotic substrates:
| Substrate | Indicative Price |
|---|---|
| FR-4 | $5/ft2 |
| PTFE (Teflon) | $100/ft2 |
| Polyimide | $40/ft2 |
| Alumina ceramic | $100/ft2 |
This makes FR-4 ideal for high volume cost-sensitive consumer electronics. Aerospace and industrial electronics can justify the higher costs of specialized substrates needed for extended temperature range, high frequency response or extreme environments.
FR-4 also benefits from economies of scale across the global printed circuit board industry spanning small shops to large-scale manufacturers. Decades worth of process optimization has reduced fabrication costs using FR-4 to very competitive levels.
Key Considerations When Selecting FR-4 Materials
Here are some key considerations when choosing the right FR-4 material grade for a PCB design:
- Frequency range – High speed digital or analog circuits above 500MHz benefit from low loss grades.
- Lead-free soldering – Choose high Tg variants with Tg above 170°C to prevent delamination.
- Number of lamination cycles – More layers need high flow prepreg grades to avoid voids.
- Environmental requirements – Halogen free, low smoke FR-4 grades fit consumer and indoor electronics.
- Thermal loads – High power boards should use high thermal conductivity options to spread heat.
- CTE matching – Match PCB and package CTEs for reliability in temperature cycling conditions.
- Reliability levels – Mission-critical electronics justify premium FR-4 grades and acceptance testing.
- Fabrication factors – Consider drillability, platability, copper peel strength and etchability.
- Cost targets – Weigh cost savings against fabrication yields and potential field failures.
Conclusion
FR-4’s optimal balance of electrical performance, mechanical stability, fabrication characteristics and economical pricing has made it the universal choice for PCB substrates across all industry verticals. While exotic materials exceed FR-4 in specific aspects, most applications can leverage FR-4’s strengths for reliable and cost-effective products.
Continued investments in laminate technology by major FR-4 manufacturers has kept pace with electronics innovation – from revolutionizing consumer devices to enabling mission-critical systems operating under extreme conditions. The versatility of custom FR-4 formulations will ensure the material retains its dominance as the bedrock of printed circuit board fabrication through decades to come.
Frequently Asked Questions
Q: What are the constituent materials in FR-4 substrates?
A: FR-4 is composed of woven fiberglass cloth reinforcement embedded within a flame resistant epoxy resin. This is laminated with copper foil.
Q: What are some key benefits of using FR-4 substrates for PCBs?
A: Benefits include reasonable cost, good electrical properties, dimensional stability, ease of fabrication, availability and reliability.
Q: What are common application areas for FR-4 based PCBs?
A: Nearly all consumer electronics, IT devices, industrial controls, instrumentation, automotive electronics use FR-4 thanks to its cost/performance ratio.
Q: What are some alternatives to FR-4 substrates?
A: Alternatives including PTFE (Teflon), Polyimide, Arlon, Rogers, Isola, ceramic substrates offer higher performance but also substantially higher costs.
Q: How do you select the right grade of FR-4 material?
A: Consider application requirements – frequency, temps, lead-free needs, fabrication complexity, reliability levels, costs. Choose grade accordingly.
