Many people are familiar with FR4 laminates and their use in electronics and other industries. But what does FR4 stand for, and what properties make the FR4 material so helpful, especially in PCB fabrication? Below, we explain everything about it, including its many uses.
What is FR4 Material?
FR4 is an insulating composite material created out of resin-bound glass fibers. It comes in sheets ready for use or further processing. The image above is an example of this product.
The material has good electrical, thermal, mechanical, and other qualities. These make it suit many uses, from structural to insulation. It also costs less to produce than many materials of similar characteristics and use.
Before delving into its composition and structure, let’s see why we call it FR4. The definition will help you understand the material and its specifications better.
FR4 Material in Full
FR stands for Flame Retardant, defining the material’s self-extinguishing ability, which stops flames from spreading. In other words, it will not support burning in a fire accident but suppresses it.
The number 4 represents a specific grade of the flame-retardant epoxy as expressed by its qualities. Other versions include FR 1, 3, 5, and 6. FR1 has the lowest qualities. The highest grade (FR6) has the most superior characteristics.
What is FR4 Material Made Of?
The typical FR4 sheet comprises woven glass fibers, epoxy resin, and various additives. Each constituent material has a specific function, as explained below.
- Glass Fiber Cloth – acts as reinforcement, giving the material its characteristic strength and dimensional stability.
- Epoxy Resin – a thermoset plastic that permanently binds the fiber strands. It also provides electrical insulation and thermal resistance properties.
- Additives – they include curing agents and fillers to achieve desired characteristics, including flame retardation.
Traditionally, FR4 manufacturers used halogen compounds like bromine to make the resin resist flames. Today, they use safer materials with little to no halogen compounds.
FR4 Material Properties
The material has excellent characteristics. These suit a broad range of applications, from low to high-end electronics, and include the following.
Flame Retardation
As mentioned earlier, it subdues flames. This quality makes it a safe electrical insulator. It also makes it compliant with various standards, such as UL 94V-0. The UL standard defines flame-resistant material requirements.
Electrical Characteristics
Various electrical properties define it. The most critical are dielectric constant, electrical strength, dissipation factor, and surface resistivity. Their values may vary slightly, depending on the specific structure and composition.
The material has a reasonably consistent Dk across the RF spectrum, exhibiting low signal loss at high frequencies. It also has exceptional resistance in dry and humid conditions.
Moisture Absorption
It has good moisture-resistance characteristics, absorbing negligible amounts to remain functional in humid environments. Tests show a water absorption rate of 0.10% when immersed in water over 24 hours.
Thermal Performance
It thermally performs well. It has a high glass transition rating, at above 130°C. High Tg variants reach up to 180° C. These laminates can also operate in temperatures of up to 260°C.
Mechanical Properties
In addition to the mentioned quality, various mechanical properties describe FR4 laminates. For instance, it has high flexure strength that makes it withstand strength, vibrations, and shock.
FR4 Material Datasheet
FR4 materials undergo various tests to determine how well they fit usage needs, including environmental conditions. The two tables below show values for PCB FR4 material.
Physical Properties
Property | Value and Range |
Tensile strength | 65,000 psi |
Compressive strength | 38,000 psi |
Bond strength | 2,300 psi |
Flexural strength | LW -A 0.125″: 60,000 psi |
Shear strength | 21,500 psi |
Hardness, M scale | M scale: 115 |
Izod impact strength at 49°C | LW: 14.00 |
Flammability rating | 94V-0 |
Maximum temperature | 140° Celsius or 284° Fahrenheit |
Water absorption rate | 24 hours: 0.10 |
Specific gravity | 1.85 |
Electrical Properties
Property | Value |
Dissipation Factor | 0.0180.009 at 1MHz |
Dielectric Constant | 4.80 at 1MHz |
Electric Strength | 800-1800 Volts/MIL |
Dielectric Breakdown | > 50 kV |
Permittivity (A) | 4.4 |
What is the Difference Between FR4 and FR5 Material?
While both are glass fiber-epoxy laminates, FR5 is a higher grade with better characteristics. It uses superior materials with low dissipation factor, high glass transition temperature, and other properties.
These qualities make it best for high-end applications, including aerospace, military, and similar uses. However, that comes at a price: the material costs more than FR4 and its lower-grade versions.
FR4 Material Benefits and Limitations
FR4 derives its popularity from its many benefits over many other materials. It also has limitations, especially when making PCBs for high-frequency applications or high-power circuits.
Benefits
- Broad operating temperature range
- A high strength-to-weight ratio
- Excellent ability to self-extinguish and suppress flames
- Impressive dimensional stability
- Excellent electrical insulation
- Extremely low moisture absorption
- It’s lighter than most materials, including metals
- It has good chemical resistance characteristics
Limitations
- Low heat dissipation rate
- Its harness makes it challenging to machine
- Limited flexibility
Is FR4 Material RoHS Compliant?
RoHS directives require FR4 laminates for electronic applications to use safe materials. The standard mainly applies to the European market and specifies various hazardous compounds. They include:
- Lead
- Mercury
- Cadmium
- Hexavalent chromium
- Polybrominated biphenyl
- Polybrominated diphenyl ether
FR4 laminates for PCB manufacturing have traditionally included some of these compounds. Modern types, such as lead and halogen-free types, have negligible levels or one.
FR4 Material Dielectric Constant
The dielectric constant is a critical parameter in electronic applications. It determines a PCB substrate’s ability to support high-speed signals or RF applications. The value depends on various factors, such as:
- The fiberglass weave style
- Board’s thickness
- Resin type and content
- Filler type and composition
It also typically varies between 3.8 and 4.8 at frequencies below 1Gz but can be high or low based on material type. Other factors include the laminate thickness and operating conditions.
FR4 Material Applications
FR4 material has many uses, primarily because it has impressive properties and costs less than comparable materials. The following list has these applications, including their specific industries.
- Electronics: PCB fabrication, chip packaging, test board, and testing jig or fixtures
- Electrical: high voltage insulation and arc shields, insulating housings, and bushings or spacers
- Automotive: wire harnesses insulators, electrically insulating brackets, connectors, etc.
- Marine: in ships as insulating housings and structural sheets
- Aerospace: rocket casings and aircraft insulating and structural parts
- Telecommunications: insulating sleeves for high-frequency antennas and other parts
The electronics industry takes the largest share of the FR4 market, with manufacturers using it to fabricate circuit boards and IC chips.
The FR4 in these applications varies, depending on the product requirements. They include standard types for general applications, RF variants, and high Tg versions for extreme heat.
Conclusion
FR4 material is a versatile, high-strength laminate with many uses. It primarily manufactures printed circuit boards but has applications in many other industries. The PCB FR4 offers many benefits, from cost-effectiveness and mechanical strength to impressive electrical and thermal performance.
