What is FR-4 material?

FR-4 is not a kind of material but a material grade. It represents the material code of a fire-resistant grade. The meaning it represents is a material property that must be able to extinguish itself after the resin material is burned. There are a few FR-4 materials used in PCB circuit boards, but most are composites from four-function (Tera-Function) epoxy resin plus filler (Filler) and glass fiber.

FR-4 materials have many aliases called in the variant industry and applications: epoxy board, epoxy resin board, glass fiber board, glass fiber board, FR-4, FR-4 Epoxy Glass Cloth, FR-4 light board, FR-4 glass fiber board, FR-4 laminated board, FR-4 reinforcement board, FR-4 epoxy resin board, epoxy board, epoxy glass cloth board, epoxy glass cloth layer Pressure plate, circuit board drilling pad, flame retardant insulation board, etc.

Major FR-4 material features

Vertical Bending Strength A: Normal: E-1/150, 150±5℃≥340Mpa

Parallel layer impact strength (supported beam method): ≥230KJ/m

Insulation resistance after immersion in water (D-24/23): ≥5.0×108Ω

Vertical layer-wise electrical strength (in 90±2℃ transformer oil, plate thickness 1mm): ≥14.2MV/m

Parallel layer breakdown voltage (in 90±2℃ transformer oil): ≥40KV

Relative permittivity (50Hz): ≤5.5

Relative dielectric constant (1MHz): ≤5.5

Dielectric loss factor (50Hz): ≤0.04

Dielectric loss factor (1MHz): ≤0.04

Water absorption (D-24/23, thickness 1.6mm): ≤19mg

Density: 1.70-1.90g/cm³

Flammability: FV0

Color: natural color

Executive standard: GB/T1303.1-1998

FR-4 process performance:

– Melting point of the platen in FR-4 process (203℃)

– High chemical resistance

– Low loss coefficient (Df 0.0025)

– Stable and low dielectric constant (DK 2.35)

– Thermoplastic material

FR-4 PCB Types

FR-4 is one of the substrate materials of the PCB. The surface colors of the FR-4 laminate are yellow FR-4, white FR-4, black FR-4, basket color FR-4, etc. Based on the reinforcing materials used on the PCB, it can be divided into below types:

– FR-1, FR-2, etc.: paper substrate

– FR-4: glass cloth substrate

– CEM series: composite substrate

– Special material substrate (ceramic, metal base, etc.)

FR-4 is a laminate by special electrical film, impregnated in phenolic epoxy resin and other materials, then pressed at high temperature and high pressure. FR-4 board usually has good electrical insulation performance, smooth surface, flatness, and standard thickness tolerance, which is suitable for products with high-performance electronic insulation requirements. It shows high performance on mechanical and dielectric properties and good heat and moisture resistance. PCBs made of FR-4 materials are widely used in insulating and flame-retardant components in motors and electrical equipment.

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​​What is FR-4 CCL (Copper Clad Laminate)?​​

FR-4 CCL is the abbreviation of glass fiber epoxy resin copper clad laminate. It is a flame retardant board made of electronic glass fiber cloth as reinforcement material and epoxy resin as adhesive. FR-4 CCL is one of the most popular types of all copper-clad laminates. It has good heat resistance, high strength, perfect dielectric properties, and is metallizable through holes.

FR-4 CCL classification

FR-4 A CCL:  the A series can be divided into A1, A2, A3, AB1, AB2, AB3, and other types from top to bottom, according to performance. A-grade is widely used in all electronic products, from common consumer electronics to high-end automotive, military, and aerospace products. And the price will reflect the grade change.

FR-4 CCL B:  This grade is a second-grade product, but its performance can meet the bottom needs of electronic products that don’t have a high standard specification. It is only suitable for ordinary double-sided PCB FR- 4 products with low prices.

FR-4 CCL- Halogen Free

This series of products is a product type that conforms to the future environmental protection development trend of CCL. It is more environmentally friendly in raw materials and can be used in high-end electronic products such as military, communications, automotive, and instruments.

CEM-1 CCL: CEM-1 CCL is composited with paper impregnated under epoxy resin, then hot pressed with a double-sided layer of glass fiber cloth and copper foil. CEM-1 CCL shows good mechanical strength, moisture resistance, flatness, heat resistance, and electrical properties due to the additional 2 layers in the structure. It surpasses paper-based CCL in every facet of performance. CEM-1 CCL can be used for high-frequency PCB applications, such as tuners, power switches, ultrasonic equipment, power supplies, etc.

CEM-3 CCL: CEM-3 CCL is improved from FR-4. The structure is made of non-woven fabric impregnated with epoxy resin, and glass fiber cloth is attached on both sides and hot-pressed with copper foil. It differs from FR-4 because CEM-3 replaces most of the glass fiber cloth with a glass mat. This change increases the flexibility of cutting mechanical properties. CEM-3 CCL is usually directly made into the double-sided copper-clad laminate. Because glass mat is looser than glass fiber in structure, its processing convenience is higher than FR-4 in the drilling process, and it is also more efficient than FR-4 during punching. The disadvantage of CEM-3 compared to FR-4 is that the thickness and accuracy of CEM-3 are not as good as FR-4, and the degree of distortion after soldering is also higher than that of FR-4.

Special material base types include BT, PI, PPO, MS, etc. In addition to being classified by resin adhesive used in the board, CCL has a variety of classification methods, such as classification according to flame retardant performance. CCL can be divided into boards: flame retardant (UL94-V0, UL94-V1 grade) and non-flame retardant type (UL94-HB grade); and key bonding processes are divided into two types: direct bonding method and adhesive lamination bonding methods.

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The manufacturing process of FR-4 CCL PCB

Material: The raw materials for making FR-4 CCL mainly include glass fiber cloth, copper foil, and epoxy resin.

Glass fiber cloth： It plays a supporting role in the copper-clad laminate. If there is a need for laser drilling, glass cloth is divided into laser cloth (such as 1080LD) and non-laser cloth.

2. Epoxy resin: It plays an insulating role in the copper-clad laminate.

3. Copper foil： It only plays a conductive role in copper-clad laminates。 Copper foil is divided into electrolytic copper and rolled copper foil according to the production method. At present, CCL mainly uses electrolytic copper foil. Copper foil is usually produced by electroplating on a roller to produce wool foil, which is then subjected to roughening treatment, heat-resistant layer treatment, and passivation treatment. The main colors of copper foil rough surface treatment are: yellow, red, and gray.

Manufacturing process

1. Mixing

Mixing is mixing epoxy resin, curing agent, accelerator, solvent, and other raw materials evenly in a certain ratio. The main purpose of the mixed glue is to impregnate glass fiber cloth and other reinforcing materials. It combines copper foil and glass fiber cloth closely and lets the product owing the features of flame retardant and heat resistance. So it is true that the characteristics of RF-4 CCL ultimately depend on the type of glue.

2. Bonding

Bonding applies glue evenly on the glass cloth and then makes the resin into a semi-cured state by baking the gluing machine. We call it prepreg. In this process, the following indicators need to be controlled.

 – Resin content RC%

– Mobility RF%

– Gel time GT(s)

– Volatile VC%

3. Slicing

Cut out shape according to PCB size requirements

4. Lay up

It is stacked and bonded to the sheet according to the thickness requirements, and make sure the stack is aligned with the configuration structure specified in the design document. Then adhesive suitable cooper foil with corresponding thickness according to the customer’s order, usually in the range of 1/2oz to 2oz. And also, adding a steel plate between each sheet of copper foil and stacking a certain number of plates neatly into a book corresponds to an opening of the press.

5. Lamination

Lamination is to heat and pressurize the material with copper foil in the press to re-melt, flow, and completely solidify the adhesive sheet resin in the semi-cured state so that the adhesive sheet is bonded to the adhesive sheet and the adhesive sheet. The sheet and the copper foil are tightly bonded as a whole.

6. Separating and Cutting

Separating the steel panel from the combination for cleaning and recycling. The FR-4 CCL then flows into the cutting process. In this procedure, the FR-4 CCL is cut into the corresponding size.

8 Inspection

The inspection mainly focuses on the below areas

a. Appearance

 – aesthetic requirements

b. Thickness tolerance

– Meet the installation thickness requirements

– Meets installation strength specifications

– Comply with insulating properties

– Guaranteed control of impedance characteristics

c. Warpage

– Meet PCB processing and component mounting, and assembly requirements

d. Tg

– Satisfy the physical heat resistance performance index. The maximum temperature to maintain the rigidity of the sheet determines the processing and use temperature of the sheet

e. Peel strength

– Meet the strength requirements. The peel strength will affect the reliability of the PCB. Too high, it is not conducive to etching. Too low, poor reliability, disconnection occurs.

Key Quality control points of FR-4 CCL

1. Resin formulation

Using resins and curing agents with longer molecular chains and better flexibility can reduce substrate warpage. For example, paper-based copper-clad laminates have effectively solved the problem of copper-clad laminate warpage since using tung oil-modified phenolic resin.

2. Quality control of glass fiber cloth

There are obvious differences in the quality of FR-4 CCL products provided by different manufacturers of glass fiber cloth. We found that the same glue solution and the same production conditions are produced using paper from different manufacturers for the same copper-clad laminate production. The warpage direction of single-sided copper-clad laminates will be different for copper-clad laminates. In addition, according to this phenomenon, in producing paper-based copper-clad laminates, we can mix paper with different degrees of warpage to glue, mix, and stack materials. We can also make copper-clad laminates with good flatness. From the above, the quality control of glass fiber cloth is very important in procurement and production.

3. Strictly control the technical parameters of each production link

In the production process of CCL, strictly control the technical parameters of each production link to ensure the consistency of resin content, fluidity, and gelation time of the prepreg, which is a necessary measure to improve the flatness of the CCL. Among them, determining the technical indicators of fluidity and gelation time is a very technical problem. Collecting data through many production practices is necessary to find a better control indicator and production process conditions. At present, a flow pressure meter is used to measure the prepreg. It is a popular way to control the technical parameters, and its control accuracy is relatively high, and it can be used for dynamic simulation of the lamination process test.

4. Tension Control

When gluing the substrate, the tension of the gluing machine should be small rather than large. The warp and weft of the laminated material must not cross, the cloths of different manufacturers should not be mixed, and the cloths of different specifications should not be mixed.

5 Temperature control

When the product is hot-pressed, it is best to heat it with heat-conducting oil, and the temperature difference between the hot-pressing plates is smaller than that of steam heating. The heating rate should be moderate, pay attention to the fluidity and gelation time of the prepreg, and make appropriate adjustments depending on the type and thickness of the laminated product to minimize the amount of glue flowing. Suppose the piping arrangement of the hot pressing plate and the position of the inlet and outlet of the heat source are unreasonable. In that case, it will also cause uneven temperature distribution of the hot pressing plate, which will increase the warpage of the product. At this time, the hot pressing plate should be reformed. In addition, the cooling rate of the outer layer board is higher than that of the inner layer board. The production process can be cooled by stage cooling. First, use warm water or warm oil to cool so that the cooling of the first stage of the product is relatively gentle, and then use cold water or cold water. Cool the oil.

6. Reasonable lamination process design

Good design of the preheating temperature and the heating and pressure increasing rate during the lamination process is the key point to pressing the board well and reducing warpage. In addition, the number and softness of the backing paper also have a certain influence, which needs to be adjusted in time. The thickness and hardness of the stainless steel plate will also affect the flatness. You can try to use a thicker and harder stainless steel plate.

7. Reduce molding pressure

Using a vacuum press for hot pressing, the higher the degree of vacuum, the easier the discharge of low-molecular substances. The lower the pressure, the higher the density of the product. The lower the pressure, the smaller the internal stress of the product. Low-pressure molding and reducing glue flow are important to improve the flatness of CCL and reduce white corners.

8. Packaging and StorageThe substrate should be stored in a moisture-proof sealed package. When the substrate is stored, it should be flat, not vertically. If stacked and stored, there should be a hardboard between the bags. The practice has proved that if the hardboard is not separated, the products below will be deformed.

9. PCB circuit graphic design balance

The PCB circuit pattern design needs to be more balanced. If there is a large-area conductive pattern, it should be gridded as much as possible to reduce stress.

10. Reasonable processing process

Before the substrate is put into use, it is best to bake the board (bake at the temperature near the substrate TG for several hours) to relax the stress of the substrate, which can reduce the warpage of the substrate during the PCB manufacturing process. In addition, try to use lower processing temperatures in the PCB manufacturing process to reduce thermal shock. Finally, the direction of the trademark characters on the CCL substrate indicates the direction of the force in the product processing process, commonly known as the longitudinal direction. In the PCB manufacturing process, the direction of the lines in the circuit pattern should be consistent with the longitudinal direction of the substrate as much as possible to reduce stress. When making a multi-layer board, attention should be paid to making the prepreg longitudinal direction consistent with each layer of the PCB.

Several leveling measures for warped copper-clad laminates

1. Roller leveling:

A roller-type leveler is used to pick out the board with a relatively large warpage first, and then it is leveled by a roller-type leveler and then transferred to the next process. For the final product, if there is still a product with excessive warpage, it is also sent to a small roller-type leveling machine to be leveled again. This method is effective for PCB boards with relatively thin thicknesses and less warpage deformation.

2. Press leveling method:

For the PCB board that has been completed and the warpage is obviously out of tolerance and cannot be leveled by a roller leveling machine, some PCB factories put it into a small press and press the warped PCB board for several hours to ten years. Cold-press leveling for several hours.

3. Bow dies leveling:

For the warpage of the completed PCB board, and the use of other leveling methods has no effect, it is recommended to use the bow die and its hot pressing leveling method. First, according to the area of ​​the PCB board to be leveled, make several pairs of very simple bow molds. You can clamp the warped PCB board into the bow mold and put it in the oven to bake and level it, or soften the PCB first and then clamp it into the bow mold. Press and level.

Engineers should pay attention to the below technical details: when the mold is made, the arc must be smooth; the temperature and heating time should be appropriate to avoid damage to the appearance of the laminate to be leveled.

For large-area laminates, the pressure should be suitable when a large-area laminate is thermally leveled with a large press. Large to avoid other deformation of the laminate.

Wrap Up

Eashub has many years of experience in FR4 material and FR-4 CLL PCB production. We continue to provide high-quality FR-4 CCL PCBs to our customers at a competitive price, and we prove to be a reliable partner to our customers on high-quality PCBs.