Printed circuit boards are widely used for the functionality of electronic devices. It is an essential item for prototyping. It has many layers of different materials. There are many varieties of printed circuit boards in terms of the materials used in their manufacture. Depending on the material, the choice of material determines the properties of any circuit board. Among the various options, high temperature FR4 is one of the high-end materials used in high temperature PCBs.

What is FR-4?

FR-4 is the grade name for flame retardant glass fiber reinforced epoxy material. Therefore, the heat resistance of FR-4 PCBs is much higher than that of ordinary PCBs. FR-4 circuit boards are divided into four categories, defined by the number of copper trace layers contained in the material:

Single-sided PCB/Single-layer PCB
Double-sided PCB/Dual-layer PCB
Multi-layer PCBs may have four or more than ten layers
PCBs with high transient gradient (TG)
Tg is the temperature at which a polymer changes from a hard glass-like substance to a softer rubber-like substance and is one of the basic properties of epoxy resins. Flame retardant circuit board materials are required; it cannot burn and can only soften at a specific temperature. In scientific terms, the glass transition temperature is called the point (Tg).

The higher the Tg point, the higher the lamination temperature, and the board will become hard and brittle, affecting the electrical quality of holes and devices. At high temperatures, the mechanical and electrical properties of general PCB materials will be greatly weakened, resulting in performance loss.

In general, the Tg of FR4 is between 130 and 140 degrees Celsius, the medium Tg is between 150 and 160 degrees Celsius, and the high Tg is above 170 degrees Celsius. The larger the Tg, the better the PCB’s heat resistance, moisture resistance, chemical resistance, stability and other properties.

The code FR4 refers to the grade of epoxy glass used in the construction industry. (The temperature at which glass begins to break.)

High-temperature PCBs will change from glass to liquid after reaching the melting point (i.e., the temperature above its Tg value), affecting its performance. In addition, the PCB size is also related to this value.

High-Tg printed circuit boards are usually produced using Tg 170°C PCB materials. Due to the rapid development of the electronics industry, high Tg materials are increasingly used in computers, communication equipment, precision equipment and instruments, etc.

High TG PCB Advanced Circuit Specifications
The glass transition temperature (abbreviated as “Tg”) of a PCB indicates the point at which the PCB material begins to change. Suppose the operating temperature exceeds the authorized Tg value. In this case, the circuit board will change from solid to liquid, which may have a negative impact on its working ability.

Standard PCBs are manufactured with materials with a TG value of 140°C and can survive operating temperatures of 110°C. At the same time, this may not be suitable for extreme temperature procedures commonly found in applications such as automotive, industrial or high-temperature electronics. PCBs made of FR-4 materials may generally be the most acceptable choice for high-temperature PCBs.

How is FR4 manufactured?

High-temperature FR4 is made of prepreg sheets, which are made of glass fiber mats impregnated with epoxy resin. A basic 1.6 mm circuit board consists of 8 layers of prepreg and an outer copper layer. The prepreg and outer copper foil layers are stacked together and pressed into a fairly large press that joins all the layers into the final laminate.

The copper foil is manufactured on a slowly rotating stainless steel drum that is immersed in a copper plating solution. As the drum rotates, copper is electroplated onto the drum and subsequently scraped off as the plated areas leave the solution.

Fiberglass fabric is woven from fine fiberglass strands. The yarn is in turn woven from fine fiberglass strands. The hardness of the glass in the fabric provides strength to the laminate and enables it to cut through the edge of a steel drill bit in seconds.

Epoxy resin is typically made from vinyl chlorohydrin [epichlorohydrin] and bisphenol A. Once the PCB is made, the resin is virtually unbreakable unless attacked by high temperatures or solid oxidizing solutions.

Alternative PCB Materials

There are many alternative board materials to FR4. These include:

FR2
Canadian Board of Education 1
Continuing Education 3
These are all paper-based boards. However, the robustness of FR4, especially for multilayer boards, is a key factor that distinguishes it from other alternatives and has become the industry standard. Today, there are other materials used in single-sided, double-sided, non-plated through-hole (NPTH), and multilayer PCBs besides FR4.

FR4 vs. Other Board Materials

Comparing FR4 to ceramic boards, or any other board, is all about heat conduction. Ceramic PCBs like aluminum nitride (AlN) and aluminum oxide (Al203) are thermally conductive. FR4 PCB materials are not. Let’s say you’re using the board in an application like LED lighting that really needs to dissipate heat. In that case, you might want to move from a standard FR4 board to a ceramic panel or other metal core PCB.

Metal core boards can more easily carry excess heat away from hot spots that could eventually shorten the life of the semiconductor junctions and damage the board.

In addition to aluminum and beryllium, other metal core PCB materials may include copper and steel alloys. Steel alloys have rigidity that copper and aluminum don’t have, but they are poor thermal conductors.

For printed circuit boards, copper has the greatest ability to conduct and dissipate heat. However, because copper is a bit expensive, many companies use aluminum as a cheaper, but still very effective, alternative to high-temperature FR4 boards.

In what situations should you avoid using FR4 as a board material?

As mentioned above, in most cases, FR4 is an excellent choice for your board material. If your board has any of the following issues, then FR4 may not be your best choice:

If you need to use lead-free soldering
If your board will be sold in Europe and must comply with the Restriction of Hazardous Substances Directive (RoHS) or your customers require the use of lead-free soldering, consider using a different material. For lead-free PCBAs, the reflow temperature may rise to 250°C, which greatly exceeds the Tg of several FR4 versions.

Due to high-frequency transmission
The impedance of FR4 boards fluctuates greatly at high frequencies, causing the signal to be attenuated by reflections. This is caused by too high a DK value.

When the board needs to withstand extremely high temperatures,
It is also unwise to use FR4 if the PCB is operating in a high-temperature environment. The engine compartment of an aircraft is a good example.

FAQ
Is FR4 an effective insulator?
FR4 is a flame retardant material used in numerous components for electronics and industrial or maritime applications. Flame retardant 4 is known for its excellent dielectric properties as an electrical insulator.

What does FR4 stand for?
The glass-reinforced epoxy laminate standard FR4 was developed by NEMA (National Electrical Manufacturers Association). This indicates that the material meets the UL94V-0 standard for flammability of plastics.

Is FR4 a water-absorbent material?
FR4 material absorbs only 0.10% of moisture after being immersed in water for 24 hours.

In antennas, why do we use FR4 as a substrate?
FR4 offers a good balance between electrical performance, cost, and availability. Unlike FR-4, better results can be obtained using other materials. Since FR-4 is abundant enough and affordable, it is widely used for frequencies < 1GHz. The dielectric constant of the substrate is almost constant at these frequencies, which is why.

Is FR4 a brittle material?
Similar to FR4, polyimide is not as brittle as polymers. Polyimide can have a specific gravity as high as 1.6 g/cm3. Its dielectric constant is 4.2 at 1 gigahertz (GHz).

What is FR4 made of?
Fiberglass mats soaked with epoxy resin are used to make prepreg sheets, which are then laminated together to form FR4. Fiberglass yarns are woven into fine strands to form glass fabric. The small glass strands are twisted together to form yarn.

What is FR4 made of?
Rigid printed circuit board substrates are mostly organic substrates, such as FR-4 epoxy fiberglass cloth, which uses epoxy resin as a binder and electronic grade fiberglass cloth as a reinforcement material.

Each type of PCB material has its own impact and cost. If you want to use a metal core printed circuit board, you can use a traditional FR4 board. Metal core PCBs made of ceramic are an option.

In PCB manufacturing, FR4 is the most common material used for high temperature PCBs. High temperature FR4 boards must be strong, waterproof, and provide adequate insulation between copper layers. However, FR4 is not the best material to use for circuit boards in high frequencies or environments.