PCB should use FR4 material if selected.

 A Guide to FR4: When Can You Use It and When Can You Not

Most electrical engineers and individuals involved with printed circuit boards are familiar with the material FR4. FR4 is the backbone material upon which most rigid circuit boards are built. However, many are unaware of what FR4 is, let alone why it is the most popular PCB base.

Read on to learn more about FR4 printed circuit boards, such as what they are, why they are so popular and how FR4 PCB specifications compare to other options within the industry.

What Is FR4 Material? 

FR4, also written as FR-4, is both a name and a rating. The name is applied to the fiberglass-reinforced epoxy-laminated sheets used in printed circuit board manufacturing. However, the name also functions as a grade used to rate epoxy laminate sheets. The designation essentially indicates the base quality of a laminate sheet, meaning a variety of sheet materials and designs fall under the FR4 rating. The “FR” in the name stands for flame retardant, while the 4 differentiates the material from others of the same class.

The material widely referred to as FR4 is a composite structure. The most basic layer of the material is fiberglass woven into a thin, cloth-like sheet. The fiberglass gives FR4 its necessary structural stability. This innermost fiberglass layer is then surrounded and bound by a flame-resistant epoxy resin. This resin is what gives the material rigidity, among its other physical properties.

FR4 sheets are widely popular among electrical engineers and designers as a PCB base material. The low cost and versatility of the material, as well as its wealth of beneficial physical properties, accounts for that popularity. FR4 sheets are electrical insulators with high dielectric strength. They also feature a high strength-to-weight ratio and are lightweight and resistant to moisture. Add this to their relative temperature resistance, and FR4 can perform well in most environmental conditions.

The FR4-PCB Connection

These qualities make FR4 an ideal default substrate for quality PCB manufacturing processes. When used appropriately, these properties can also form the foundations of high-quality and low-cost PCBs.

Within a PCB, FR4 forms the primary insulating backbone. This is the base upon which the manufacturing company builds the circuit. Once prepared, the FR4 board is laminated with one or more layers of copper foil using heat and adhesive. This copper forms the circuits in the finished product and may cover one or both sides, depending on the design of the board.

Complex PCBs may use more than one side or even layer the circuit board to produce more sophisticated circuits. From here, the circuits are drawn and etched out before being covered with a solder mask layer, preparing the board for the final silkscreen layer and the subsequent soldering process.

How to Select FR4 Thickness

When ordering a laminate board for a PCB project, the designer or electrical engineer must specify the FR4 PCB thickness. This is measured in inch-based units, such as the thousandth of an inch, or thou, or millimeters, depending on which is most appropriate for the setting. The thickness of a sheet of FR4 ranges widely depending on the needs of the project, but it tends to range from ten thou to three inches.

While board thickness may not seem like a significant factor in the design of a PCB, in reality, it is an essential feature. Board thickness affects several aspects of the board’s functionality, which is why several factors are considered in determining the thickness of a board for design. These include the following.

Space: Thinner May Be Better

If space concerns the designer, a thinner FR4 board tends to be preferable. This is a predominant factor for the manufacture of smaller devices, like USB connectors and many Bluetooth accessories. Even for larger projects, smaller FR4 PCBs tend to be favored to save space within the device.

Connections: The Wrong One Could Result in Damage

A two-sided PCB design requires an edge connector to join the two sides. This can be a major limiting factor for the final size of the PCB since PCB edge connectors only fit a particular set of PCB thicknesses. The mating portion of a connector has to fit snugly on the side of the PCB, or else risk slippage or damage to the PCB. This is one of the primary reasons why circuit design comes before choosing materials for the circuit.

Impedance Matching: Essential to Maintaining Board Function

Every multi-layered PCB acts as a capacitor on adjacent layers. This is why the thickness of this board is so important — the thickness of the PCB FR4 determines the thickness of the dielectric, which in turn affects the value of the capacitance.

This is an especially key factor for some high-frequency PCBs, such as RF and microwave designs. High-frequency designs focus on impedance matching as an essential component to maintain optimal board function, so getting the right capacitance for each layer is crucial.

Flexibility: Depends on the Application

Thinner boards can flex in some capacity. While an unusual trait, flexibility can be a positive or negative feature, depending on the application.

More flexible boards tend to be preferable in some applications where the product is regularly stressed or flexed. For example, those using boards for medical and automotive applications often prefer flexible boards due to the constant stress and flexing to which these PCBs are often subjected.

However, flexibility can be a detriment to the PCB manufacturing process, resulting in serious problems later in assembly. When handled by a machine, a more flexible board may flex when dealt with by a soldering machine, causing the component to be soldered at an angle. Additionally, this flexing has the potential to break freshly placed components and connections already on the board.

Design Requirements: Intended Use Impacts FR4 Thickness

Thin boards aren’t preferable in all cases, primarily due to the limitations thin boards put on PCB plans. Thin FR4 boards can’t feasibly feature grooves, and they can’t be too large without risking fracture. Thicker boards, however, can accomplish both. Always account for this when weighing options between FR4 thicknesses.

Component Compatibility: May Work With a Small Range

The thickness of a board can also affect the compatibility of components with the board. Just like edge connectors, many components work with a small range of board thicknesses. This is especially true for some through-hole components, for example.

Weight: Lighter Products May Be More Attractive

The thickness of an FR4 board will, logically, affect the weight of the final PCB. While weight is less of an issue in some applications, it is often a consideration in consumer electronics. Lighter PCBs make for lighter products, which makes them cheaper to ship and, in some cases, more attractive to consumers.

When to Use FR4

Many electronics applications choose to use the epoxy-based FR4 board. Their strength, reliability and relatively low cost make them a logical default choice for electronics engineers. However, FR4 isn’t ideal for all situations, especially high-frequency designs. For these designs, high-frequency laminates tend to be a more common choice.

Choosing between these materials may be difficult, which is why we list more detailed guidelines below.

When trying to decide whether to build a PCB with FR-4 or a high-frequency laminate, consider the following key features of each.

 Cost: FR4 Will Be Cheaper

FR4 is a very common PCB base, primarily for its relatively low cost. High-frequency laminates, on the other hand, are considerably higher in price, which is a significant drawback for cost-concerned designers and manufacturers. This is the primary limiting factor for companies when choosing between FR4 and high-frequency laminates.

Signal Loss: Lower Df Leads to Lower Signal Loss

Signal loss is, in many cases, a critical part of a PCB design, especially in high-frequency situations where signal loss is more of a problem. For these designs, FR4 is not an ideal choice — FR4 has a higher Df, or dissipation factor, than high-frequency laminates. This means FR4 circuits will suffer more signal loss than an identical circuit on a high-frequency laminate.

The Df of FR4 is around 0.020, while the Df of most high-frequency laminates is around 0.004, a fourth of FR4’s Df. The lower the Df, the less signal loss overall. The other issue is the Df of FR4 increases as a signal’s frequency increases, meaning as the frequency of the signal increases, more signal loss occurs. Since high-frequency laminates have more stable Df characteristics, they experience less loss at higher frequencies.

Impedance Stability: Dk Stability Matters

Stable impedance is another important factor for many designs, as it often means more predictable performance, especially for larger circuits or high-frequency designs. Again in this area, FR4 and high-frequency laminates offer different results. Maintaining stable impedance requires a material to maintain a stable Dk, or dielectric constant, across the entire material as the temperature of the material changes.

In this regard, FR4 is not a suitable material for maintaining stable impedance, as its Dk value varies widely within a single board and as the temperature of the board changes. High-frequency laminates don’t have Dk values that vary as widely, remaining fairly constant across the area of a board as temperature changes.

Temperature Management:

Temperature performance is another factor to consider when choosing between an FR4 board or high-frequency laminate for a PCB base. When comparing temperature performance, the thermal coefficient of dielectric constant provides a decent gauge. This number measures how much a material’s dielectric constant changes with temperature:

For FR4, this value rests around 200 parts per million per degree Celsius change in temperature

High-frequency laminates feature a thermal coefficient of dielectric constant of around 40 parts per million per degree Celsius.

Though both may seem like small numbers, they offer widely different results. The significant coefficient of FR4 means an FR4 board’s Dk may vary considerably across its area. In comparison, the lower coefficient for high-frequency laminates indicates less variation in temperature across the board’s area. This is particularly important to consider when designing boards intended for hotter environments.

Dielectric Constant: Choose the Right Dk Value

Sometimes a circuit board’s dielectric constant can be just as important as any other feature of a board. The dielectric constant of a circuit board impacts the size of a circuit’s transmission lines, especially in high-frequency circuits like RF or microwave designs. The smaller the transmission lines can be, the smaller the circuit itself.

Therefore, if a smaller circuit board size is desirable, a board material with a higher Dk value is the best choice. FR4 features a Dk of about 4.5, which is lower than high-frequency materials’ Dk, which usually lies around 6.15 to 11. Using these high-value Dk materials can have significant results, shaving 25 percent or more off the size of the final circuit board.

Operational Environment:

The operational environment for the circuit is yet another thing to keep in mind when choosing between an FR4 and a high-frequency laminate board. This includes the environmental conditions, like moisture and temperature. In both cases, high-frequency laminates provide more moisture and heat resistance than FR4, meaning the operational environment will have significantly less impact on the function of the circuit. This is a critical consideration if the PCB will operate in outdoor or extreme industrial environments.

In short, high-frequency laminates feature a wide range of physical characteristics, many of which are superior to FR4. The only drawback high-frequency laminates suffer over FR4 is a relatively higher cost, which is a significant factor for cost-focused designers. The big decision, then, is when the advantages outweigh the costs of high-frequency laminates.

Making the Choice: FR4 Versus High-Frequency Laminate

So, using all the information listed above, how can an electric engineer or PCB designer choose between FR4 and a high-frequency laminate? First, the designer must list and compare the electrical and mechanical requirements for the final PCB, using the most specific parameters as possible. The designer must then examine how well FR4 will meet these parameters as compared to a laminate designed to handle high-frequency signals. More precise data for each of the points above is readily available using industry data sheets.

In short, FR4 is a widely relevant material, popular mostly for its low cost and relative mechanical and electrical stability. While this material features extensive benefits and is available in a variety of thicknesses and sizes, it isn’t the best choice for every application, especially high-frequency applications like RF and microwave designs. In these latter cases, a high-frequency laminate is the better choice. Still, it’s easy to see why FR4 is still in such common use to this day.

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