Fabrication of printed circuit boards often entails a chain of events that starts with the design and finishes with mass production. A key factor that every designer needs to consider and specify entails the PCB material type. It has to get enclosed in their Gerber file when submitting their design for fabrication. The PCB material used defines the type of PCB product. As such, you get to hear FR4 PCB, Teflon PCB, etc., based on the laminate material used.
The most common PCB laminate is the FR4 comprising of the FR4 PCB substrate. But is it the best for your application? This article aims to delve into the diverse types of PCB laminates available in the market. It should help with the decision-making process regarding the substrate to use in your design and whether FR4 PCB should be your go-to option for your application.
PCB Material Types
A printed circuit board often contains four layers laminated together by heat to form one layer. Each layer needs a distinctive material due to its specific roles in the proper functioning of a circuit board. A circuit board designer should understand these materials and their respective characteristics that make them ideal for their respective applications.
The diverse printed circuit board materials include the substrate, copper, soldermask, and silkscreen arranged from bottom to top, respectively. The substrate usually comes as a composition of fiberglass. It is inferred as FR4 material because of its fire-retardant properties and provides the circuit board with its solid base. However, the thickness of the substrate layer differs from one PCB design to the other based on the intended application. The FR4 material, however, is not the only available PCB laminate. Cheaper alternatives exist, and these use other PCB substrate materials composed of epoxies and phenolic. More expensive options include PTFE.
Copper comes as the subsequent layer. It gets laminated to the substrate using an adhesive and some heat. The copper layer, while thin, often gets arranged based on the type and application of the circuit board. For instance, it can be one or two layers, though a single copper layer in PCBs is for cheaper electronic gadgets or devices.
The typical CCL or copper-clad laminate used in most PCBs gets classified into diverse categories based on the diverse classification standards. For instance, epoxy resin, PF resin, and polyester resin belong to the paper base class.
The silkscreen layer adds numerical and letter indicators that allow tech programmers to read the PCB while the soldermask always comes beneath the silkscreen layer. If you need advice on the right PCB material for your design, then RayMing PCB and Assembly will always be at your service.
FR4 PCB Material
It is a PCB base material that comes as a fiberglass laminate sheet reinforced with epoxy resin. It is a common laminate substrate in printed circuit boards because of its sturdy nature and flame-retardant properties. As such, it is vital to provide circuit boards with the required insulation. FR4 boards have largely replaced the G-10 boards as much as the G-10 boards still find applications where thermal destruction proves key.
An FR4 material complies with the UL94V-0 standards when it comes to plastic in-flammability. The standards ensure that it cannot propagate fire and extinguishes it immediately after the material starts to burn.
Fr4 materials use bromine. The halogen chemical element is resistant to fire and thus valuable in providing the fire-retardant properties to FR-4.
It also cannot absorb water, maintains the high mechanical strength, besides the incredible insulating capacity, especially in humid or dry environments.
FR4 material also has a high glass transition temperature of between 115 and 200 degrees Celsius. However, this depends on the methods of manufacturing and the resin types used. In most instances, the FR4 PCB will contain an FR4 sandwiched between two copper layer laminates.
The FR4 material can come as the standard FR4, the high TG FR-4, the high CTI FR-4, and FR4 without any laminated copper. A standard FR4 has a heat resistance of 140-a50 degrees Celsius, while a high TG FR4 possesses a glass transition value of 180 degrees Celsius. On the other hand, the high CTI FR4’s index exceeds 600V, and the FR4 devoid of laminated copper proves ideal for board support and insulating plates.
The thickness of the FR4 material plays a crucial role in diverse PCB applications. Therefore, picking the correct thickness should take into account the following.
- Compatibility of the FR4 thickness with components
- The need to save space on the board
- Flexibility and design
- The type of connections required
The benefit of an RF-4 Material
It is affordable compared to other PCB substrates and thus becomes standard in fabricating small PCB series. However, it is vital to remember that it is not suitable for high-frequency circuits or in instances where the components prove ideal for flexible PCBs.
Flex PCB Material
Unlike standard printed circuit boards, a flexible PCB uses a flexible polymer instead of the typical FR4 material. The flexible polymer commonly deployed entails the polyimide film substrate or a polyester substrate. It is a strong material that cannot soften under heat and stays flexible upon thermosetting. Plenty of thermosetting resins such as the PI stay rigid after applying heat, thus making it ideal for flex PCBs.
Another key flex PCB material commonly used alongside the PI includes an adhesive material. It helps in the attachment of the layers. However, modern trends and the need to limit the unreliability that results from the adhesive have resulted in the development of adhesive-less PI. It permits thinner designs but is devoid of risking via breakages. The PI also develops the coverlay film to protect the flex circuit.
Additionally, it is important to consider mixing both rigid and flex materials when handling rigid-flex PCBs. The rigid parts can deploy fiberglass as substrate materials, while the flexible parts deploy the adhesive-less PI. However, it is prudent to note that signals cannot get transferred between these flex and rigid parts in such instances. Further, multilayer rigid-flex circuit boards will include Prepreg fiberglass in the middle layers (substrate).
Besides the substrate, aspects such as the copper layer material, the soldermask, and silkscreen in flex circuits remain the same. Therefore, before you delve into the design of flex or flex-rigid PCBs, checking such flex PCB materials becomes crucial.
Benefits of Polyimide
- Greater temperature adaptability
- High mechanical performance
- Outstanding compressive and tensile strength.
- Exceptional chemical resistance.
- Radiation resistance
- Transparency in plenty of microwave applications
- Superior wear and bearing properties
Copper Clad Board Material
It also gets inferred as a copper-clad laminate (CCL). The CCL is a PCB raw material reinforced with wood pulp paper or glass fiber. It is an important aspect of the PCB and a product with copper laminations clad on either side of the reinforcing material, especially after getting drenched in resin.
Copper-clad laminate has diverse classification standards. The classification can hinge on the following.
- CLL’s mechanical rigidity, including Flex CLL and rigid CLL (Cem-1, FR-4). Rigid PCBs rely on rigid CLL, whereas flex PCBs rely on flex CCLs.
- Classification based on the insulation structure and material
- It encompasses organic resin CLL comprising of CEM and FR-4.
- CLL’s thickness. It can either be the standard thickness CLL or the thin CLL. The thin CLL can go as low as 0.5mm. However, the copper foil size (thickness) gets excluded from the thickness of the CLL.
- The type of reinforcing material. It includes a glass fiber cloth base CLL (it includes FR-5 and FR-4CEM), a compound CLL (CEM-3, CEM-1), and a paper-based CLL (XPC).
- Applied insulation resin. It predominantly comes as a phenolic CCL (composed of XPC and FR-1)
Some of the important attributes that play a significant role for such copper clad board materials include appearance, size, electric performance, chemical performance, physical performance, and environmental performance. Additionally, a CLL has Prepreg material composed of glass fiber, epoxy resin, 2MI, DMF, acetone, etc.
The CLL or copper-clad board materials offer low dielectric constant and coefficient of thermal expansion. It also provides a high heat resistance besides giving the board chemical resistance. Such features ensure excellent environmental performance when deployed.
Ceramic PCB Substrate
It is a valuable base material for circuit boards with application in high-speed and high-frequency tough environmental conditions. Ceramic materials entail a group of materials that have similar physical properties and chemical structures. It, therefore, includes substances such as beryllium oxide, aluminum nitride, and aluminum oxide. Similar substrates based on performance can fall under the ceramic materials category include boron nitride and silicon carbide.
The ceramic PCB substrate possesses a higher thermal conductivity compared to its counterparts, such as the FR-4. For instance, aluminum oxide has 20x TC to that of FR-4. Similarly, silicon carbide and aluminum nitride have 100x, with beryllium oxide having a higher thermal conductivity. Consequently, the ceramic substrates nullify the need for PCBs (when used as substrates) to have elements such as metal planes and vias unless circumstances demand them.
Apart from the fairly decent thermal conductivity, ceramic PCB substrates also have high thermal expansion coefficients. It is closer to conductor structures and thus reduces the stress that accumulates in thermal cycling. All this arises from the higher thermal conductivity that ensures a uniform and high thermal expansion.
Ceramics also have a desirable mechanical strength capable of withstanding high mechanical loads. It can include strong shocks and vibrations, making them ideal in PCBs for military and aerospace applications. The mechanical strength emanates from the low Young’s modulus that implies a lesser tendency to deform upon application of force.
- High resistance to corrosion and wear
- Dimensional stability also applies at high temperatures.
- High mechanical strength
- Chemical resistance
- It provides electrical insulation
- Can withstand high-temperatures
- It also possesses ferroelectric and dielectric properties.
It encompasses Prepreg and laminate materials. Such products get produced through an exceptional high-performance and multifunctional epoxy resin with a high glass transition temperature of 180 degrees Celsius. Such a system often applies to multilayer PWB applications requiring extreme thermal reliability and performance while retaining processability.
The 370hr Prepreg and laminate often consist of high-quality e-glass fabric with a superior CAF (conductive anodic filament) resistance. It thus provides superior thermal performance while ensuring a low CTE. The 370hr material also has moisture, chemical, and mechanical resistance properties equal to traditional FR4 substrates.
The 370hr materials also prove UV blocking and laser fluorescing for the ultimate compatibility with AOI (automated optical inspection) systems, solder mask imaging, and optical positioning systems. It is one of the best PCB materials for sequential lamination designs.
The glass fabric can come either as standard e-glass, spread glass fabric, or square weave glass fabric. Additionally, it can have different copper foil types, including reverse treat foil or RTF and the standard HTE grade 3. Copper weights vary, and the options come in 0.5, 1, and 2 ounces, though thinner and heavier copper are also accessible.
It has moisture-proof and chemical-proof properties that exceed traditional FR4 materials, providing more longevity and a longer product life cycle.
It also provides a better thermal performance based on its low CTE that improves the general performance of the PCB.
Teflon PCB Material
Teflon PCB material, also called polytetrafluoroethylene (PTFE), is the go-to substrate for high-speed and high-frequency PCBs. It thus has a wide application in satellite systems and mobile communication electronic products. PTFE has diverse and distinctive chemical and mechanical attributes that make it suitable for its application.
PTFE can withstand up to 260 degrees Celsius, with cryogenic temperatures of a similar standard. It has a high molecular weight compared to other laminate substrates besides having a low coefficient of friction. Other crucial attributes include decent dielectric properties, flame resistance, high electronegativity, temperature stability, and moisture resistance. It also has thermal and electrical insulation, UV and weather resistance, besides resistance to grease and oil.
PTFE as a material, however, requires special processes and equipment when it comes to PCB fabrication. As a result, not all PCB manufacturers prefer working with Teflon as a laminate. Therefore, it is important to ascertain that the PCB fabricator has the expertise, experience, and capacity to work with the material before enlisting their services.
- Teflon is non-stick and thus can self-clean.
- PTFE can prove effective in diverse temperature ranges. It can handle cold or hot temperatures, which makes it ideal for numerous applications.
- Resistance to environmental elements such as UV, oxidation, embrittlement, and discoloration makes it durable.
- It demonstrates flame and high-temperature resistance because of the high auto-ignition temperature and melting point.
- PTFE has a low coefficient of friction resulting in less wear and tear, less heat, and reduced fire risk.
- Teflon is resistant to corrosion, implying that most chemical compounds do not affect it. It, therefore, becomes the go-to substrate for making gaskets in PCBs where aggressive chemicals get deployed.
- Teflon also has excellent electrical properties that provide high dielectric strength and electrical resistance.
The laminate material comes as a default alternative for most PCB fabricators globally. It is instrumental for high-performance PCB applications but with mid-glass transitional temperature requirements. It, therefore, means that Shengyi S1141 is not suitable for high-frequency applications. Additionally, it cannot be ideal for electrical and electronic equipment that attain very high operating temperatures.
The Shengyi S1141 has a glass transition temperature of 140 degrees Celsius, which falls under the mid-range Tg group. It also comes compatible with AOI, besides having UV-blocking properties. The incredible mechanical processability of Shengyi S1141 makes it a terrific low-cost PCB material option for diverse circuit board applications.
It functions primarily as a PCB stiffener in flex printed circuit boards. While possessing decent properties such as surface and volume resistivity, flexural strength, dissipation factor, and thermal stress, it often ranks lower on these aspects with its closest equivalent- FR406. However, it has diverse application areas. It includes instrumentation, computers, communication equipment, VCR, electronic gaming machines, etc.
- A low-cost option for high-performance printed circuit board applications
While crucial in the various application areas, Shengyi 1141 cannot get deployed in anti-CAF applications. It is also not ideal for applications that feature heavy copper, HDI board, and high layer counts that equal or surpasses 12 layers.
Polyimide PCB Material
It is a PCB core substrate that consists of imide monomers. The polymer group can entail both synthetic and natural materials like wool and silk. But for PCB production, synthetic polyimides get deployed after manufacturing. The production process of synthetic polyamides often yields divergent results based on the additives and chemicals used. Some of the polyimides that result from the process include pure polyimides, 3rd gen polyimides, filled polyimides, and low-flow polyimides.
Pure polyimides, also inferred as 2nd generation polyimides are produced without additive and brominated flame retardants. It results in extra thermal stability besides the temperature resistance of the material compared to existing alternatives.
3rd generation polyimides incorporate additives during production, and this enhances flammability resistance. Such a trait helps eliminate the possibility of electrical fires. While the 3rd gen polyimides have less thermal stability, they still provide enhanced production times, requiring less time and lower temperatures to cure.
Filled polyimides, on the other hand, have fillers and polyimide. The filler functions to diminish resin shrinkage, reducing the development of cracks when drilling and curing. The polyimide, on the other hand, is instrumental in giving the needed flexibility and thermal resistance.
Lastly, the low-flow polyimides have numerous flow and resin restrictors that diminish the material’s flexibility. However, it is expected that as things develop, more polyimide variations will come to the fore.
It provides excellent tensile strength, flexibility, thermal stability, durability, and resistance to chemicals. Consequently, it can be deployed in diverse and tough application areas like automotive electronics, computer electronics, consumer electronics, military, and the medical industry.
Isola PCB Materials
Isola printed circuit board material comes as a high-performance laminate. It integrates resin constituents engineered to satisfy performance requirements that prove demanding. As a result, it applies to electronic equipment for aerospace and high-grade consumer electronic sectors.
Isola materials deploy no-flow Prepreg that is polyimide-based for high-temperature demanding PCB applications. It primarily gets composed of a polyimide blend besides a thermoplastic resin. The two come fully cured and devoid of using methylenedianiline (MDA). As a result, the materials possess a high glass transition temperature that lacks low bond strength and brittleness.
The Isola PCB Prepreg material has diverse attributes that make it distinctive. It includes the ability to sustain the bond strength at high temperatures. It also has a higher thermal performance besides possessing halogen-free alternatives. Isola also has a reduced delamination requirement due to machining. Other vital attributes include a long-lasting resin system and improved processing from reduced brittleness.
Isola PCB laminates include Astra MT77 that has excellent electrical features for microwave/RF PCB designs. The DE104 offers an incredible thermal resistance resulting from a low coefficient of thermal expansion. It also has a unique resin system in the Z-axis. On the other hand, the ED13OUV allows for PCB construction using UV-blocking materials. FR402 uses tetrafunctional epoxy resin and the FR406 acts as the industry benchmark for high-performance materials. Other Isola materials include the FR408 and the FR408HR for high-performance applications.
Wonderful electrical attributes, thermal resistance, and a low CTE
Isola PCB material also blocks UV and offers high-performance applications. Such attributes are instrumental in the diverse application areas of Isola PCB materials.
Considerations for Picking a PCB Material
Plenty of PCB materials exist out there, with each having different properties ideal for specific applications. As a designer, selecting the right material for your printed circuit board fabrication becomes crucial. For instance, having the thermal properties of the laminate material aligned closely with the components forms a major objective when selecting PCB materials.
Material characteristics that should always figure needs to include the coefficient of thermal expansion (CTE). It also features glass transition temperature (Tg), the decomposition temperature (Td), and time to delamination (260-280 degrees Celsius)
Plenty of PCB materials exist to choose from when designing your PCB for fabrication. However, from the discussion in each type of PCB material, the FR4 comes out strongly as a favorite for multiple PCB application areas. It has attributes domiciled within the standard range for most applications and proves relatively cheap compared to other substrate types. However, only pick an FR4 PCB material if your application area proves suitable.