As an electronics enthusiast, you cannot fail to understand the importance of printed circuit boards to electronics by now. It ensures the proper functioning of electronic devices and equipment for all application areas in the global market. However, a PCB becomes complete due to the production process that begins from design, fabrication, assembly, to packaging. While most of these aspects prove instrumental, one of the essential considerations in your design entails the PCB material. It not only ensures the functional integrity of the printed circuit board but defines its application as well.
We will, through this article, canvass the diverse types of PCB materials available in the market, their attributes, and application areas to facilitate your understanding.
Overview of PCB Materials
All printed circuit boards can get segmented into four components: the non-conductive layer, conductive layer, solder mask layer, and silkscreen layer. You must consider these PCB components when designing your PCB, regardless of the type and application area of your PCB. However, each component layer needs a distinct PCB material that varies with the intended application of the PCB. You, therefore, have to consider the PCB board material for each PCB component before starting with the actual fabrication.
A crucial aspect you have to consider as a serious electronics designer for a PCB material entails its performance. Because of this, it becomes prudent to acquaint yourself with essential material properties like temperature resistance, tensile strength, adhesion, flexibility, dielectric constant, dielectric strength, electrical, thermal, and other physical factors that become integral. The integration level and performance of the printed circuit board solely depend on the material deployed to fabricate it.
You will always find different printed circuit board materials in the market. Such materials possess distinct properties, with continuous enhancement as technology evolves. But before getting ahead of ourselves, let us look at the different circuit board material types utilized in the PCB fabrication process.
PCB Material: Important Types and Corresponding Attributes
You can use diverse materials as your PCB substrate or components in fabricating your printed circuit boards. Material selection, however, depends on your application needs as different materials have different attributes and thus will provide the PCB with varying qualities. Because of this, the PCB performance will get impacted in a distinct set of circumstances. Sometimes as the designer, you can pick materials based on electrical performance, especially for thermal or mechanical survivability or high-speed application. It also becomes critical to adhere to specific regulatory requirements like the RoHS, especially in selecting a PCB material.
RoHS comes as a directive that prohibits the utilization of hazardous materials (materials having restricted metals or chemicals). Another popular consideration for materials entails the UL (underwriters’ laboratories) material quality. You can always tell whether an electronic circuit developed with the material can self-extinguish in the event of a fire. It is, therefore, becomes critical as consideration for consumer electronics.
Laminates comprise cloth fabric and resins that provide distinct insulation abilities. It can include Teflon, FR4 epoxy, Polyimide, and other PCB substrate or laminate types utilizing a mix of resin coatings and glass. Many distinctive electrical and thermal factors determine the type of laminate that will prove suitable for your specific PCB design. So before we get into the different material types, let us first understand the material attributes of any PCB substrate.
PCB Material Attributes to Consider for Your PCB Design
A PCB laminate can influence the performance or functionality of your PCB in different ways. Part of coming up with an excellent printed circuit board design entails understanding the diverse material properties and their potential influence on your PCB’s functionality. Below, some of the pivotal ones.
Every important electrical property you need to consider for your modern PCB substrate can be accounted for under the dielectric constant. The dielectric constant constitutes the most important factor for consideration during the design of your stack-up for high frequency or high speed printed circuit board. It comes as a complicated quantity and a frequency function that offers two dispersion forms in printed circuit board substrates.
- Velocity dispersion. The dielectric constant comes as a frequency function, and thus different frequencies result in divergent travel speeds and loss levels.
- Loss dispersion. Signal attenuation comes as a frequency function. However, simple dispersion models detail that the loss will increase with an increase in frequency. But the statement cannot get regarded as entirely correct because some laminates have a complicated loss versus frequency spectrum.
Velocity and loss dispersion contributes to distortion levels of a signal during propagation. If you are operating an analog signal at a narrow bandwidth, dispersion becomes a non-factor. But when dealing with digital signals, dispersion becomes a key element you need to consider. It proves a significant challenge for modeling or dealing with the Interconnect designs for high-speed or high-frequency digital signals.
Your printed circuit board structure, along with the substrate, influences the electrical and thermal properties of the circuit board. Such properties get personified in two approaches: the glass weave and copper conductors’ roughness styles.
Glass Weave Style. In this style, the substrate has spaces or gaps which correlate with the board’s resin content. A combination of the glass and resin proportions defines the substrate’s volume-average DK (dielectric constant). The gaps created within the glass weave style bring about the fiber weave effect – where the different dielectric constants of other substrates along the Interconnect develop skew, losses, and resonance. Such effects become prominent above 50 GHz, affecting multi-gigabit Ethernet, radar signals, etc.
Copper Roughness. It infers to the structural properties of PCC (printed copper conductors) and contributes to the electrical impedance of the Interconnect. The conductor’s surface roughness enhances the skin-effect resistance, especially at higher frequencies. As a result, inductive losses occur because of induced eddy currents as the signal gets propagated. Some of the common contributors of surface roughness include the method of copper deposition, copper etching, and the Prepreg surface.
You must consider two sets of thermal properties when using laminates during and after the PCB lamination process. It becomes pivotal to consider these properties (both PCB substrate and laminates) before substrate material selection.
Specific Heat and Thermal Conductivity. The total heat you need for raising the circuit board’s temperature by a degree unit is quantified as the substrate’s specific heat. On the other hand, the total heat transferred or transported via a substrate for every unit gets quantified as thermal conductivity. Both elements combine to provide the circuit board material properties insofar as thermal equilibrium is concerned. For instance, utilizing a high thermal conductivity substrate becomes prudent if you intend to deploy your board in a setting that needs rapid heat dissipation.
Thermal Expansion and Glass Transitional Temperature. CTE and TG properties of materials high correlate. Every PCB material type possesses a CTE (coefficient of thermal expansion) value, and this typically comes as an anisotropic quantity. Therefore, immediately the boards’ temperature surpasses the glass-transitional temperature, the value of the CTE increases. However, you should always ensure a low CTE value (preferably within the specified range) while the TG value needs to come high. A cheap FR4 substrate can have a glass transition temperature value of 130°C, though most manufacturers, including RayMing PCB and Assembly, will provide a laminate and core of 170 °C TG.
Here, you can consider aspects related to dimensional stability, flame retardant properties, mitigated loss factors, mechanical strength, etc.
Dimensional stability entails the extent to which the material can move besides the consistency of such movements during thermal cycles, manufacturing, and exposure to moisture or humidity.
Mechanical strength includes the tensile, shear, and other instrumental mechanical properties that can prove necessary when the PCB gets deployed.
Mitigated loss factor becomes crucial for high-speed applications, in which the speed of signaling proves key.
Flame retardant properties entail the ability of the circuit board material to stifle any spread of fire on the circuit board. You can always gauge the material’s ability by checking its UL qualification.
Popular PCB Material Types Used in the Production of PCBs
For the Non-Conductive Layer
FR4 Epoxy Laminate and Prepreg
One of the most popular and common laminates or substrates for a printed circuit board’s non-conductive layer entails the FR4. FR4 describes a material class that meets a specific set of requirements as laid out by NEMA LI 1-1998 standards. It possesses decent electrical, mechanical, and thermal characteristics besides an auspicious strength-to-weight ratio. Because of this, the FR4 material proves suitable for almost any type of electronic application.
The FR4 Prepreg and laminates comprise epoxy resin and glass cloth. It costs the least in the market and can become your new favorite for low-layer count printed circuit board projects (less than fourteen layers). Further, you can blend the base epoxy resin with additives to enhance its UL flame rating, electrical and thermal performance. Such enhancements can help you escalate their use in higher thermal stress and layer count applications. It also ends up reducing the cost of high-speed circuits with incredible electrical performance.
FR4 Prepreg and laminates prove adaptable and versatile when it comes to some popular manufacturing techniques with predictable outcomes.
Classes of RF4 Material
The circuit board material can get categorized into four, which mainly depend on its application and properties.
- Standard RF4. It has a TG or glass transition temperature of 130°C with or without UV blocking. It is the most typical and widely utilized, and besides, the most affordable as well.
- Higher TG FR4. It has a glass transitional temperature range of 170°C – 180°C and comes compatible with lead-free reflow tech.
- Halogen-free and attuned with the lead-free reflow tech.
- FR4 with normalized CTI index of ≥ 400, ≥ 600
Benefits of RF4
- It provides versatility besides also coming at low costs.
- Its composition makes it easy to drill and metalize (the fiberglass material impregnated with epoxy resin)
- It offers good performance, especially at microwave/RF frequencies.
Drawbacks of RF4
- It possesses a high dielectric loss at microwave frequencies owing to its dissipation factor.
- Unsuitable for high-speed or high-frequency digital and analog applications, respectively above some GHz.
Polyimide Prepreg and Laminates
The thin polyimide laminates provide advanced temperature performance compared to FR4 materials. Additionally, its electrical performance also proves an improvement to that of an FR4. However, you will need to pay more for a polyimide material, as much as you will get to enjoy its survivability in high-temperature and harsh environments. It also comes with better stability during thermal cycling as it possesses fewer expansion attributes. You, therefore, can use it for high-layer count fabrication.
Composite Epoxy Material (CEM)
It comes as low-cost PCB material and denotes a NEMA classification. It comprises paper and a double layer of phenol and woven glass epoxy compounds. The family contains CEM-1, CEM-2, CEM-3, CEM-4, and CEM-5. CEM-1 comes at a lower price than FR4, and you can use it to manufacture single-sided PCBs. The laminate proves incompatible with the metallization process, especially in the holes.
Further, its dielectric properties look like those of FR4. However, it has poor mechanical endurance. CEM-3, on the other hand, comes with similar properties to FR4 and has four more subclasses.
Benefits of CEM
Almost all the CEM subclasses are cost-effective, with a practically similar dielectric property set to FR4.
It mostly has a poor mechanical endurance
It is a PCB material that proves suitable for PCB stack-ups. It has a low-temperature co-efficient when it comes to dielectric constant (DK). The feature becomes instrumental in minimizing the phase variations besides the z-axis CTE.
Teflon Bonding Plies and Laminates
Besides the bonding plies, Teflon laminates provide superb electrical properties, making them suitable for high-speed circuit applications. If you want to use Teflon for your PCB fabrication needs, you must contend with the higher material-associated costs. It is expensive than other materials like Polyimide and FR4 but gives you the high-speed abilities you need. You can coat glass fabric with Teflon material. However, you can also get it manufactured as a standalone film or as composites of special additives and fillers to enhance the mechanical properties.
You, however, have to rely on an exceptionally skilled workforce and a piece of dedicated equipment, besides processing approaches. Further, you also need to anticipate diminished manufacturing yields.
A flexible laminate comes thin and offers you the ability to bend or fold it within the electronic design but without losing any electrical continuity. Flexible laminates lack the glass fabric necessary for support, though they get used as a plastic film for support. You will also find it equally effective when folded into the applicable device. Therefore, a one-time flex will function with the same efficiency as a dynamic flex requiring consistent folding upon deployment. In most instances, you will get flexible laminates from high-temperature substrates or materials like LCP and Polyimide. Additionally, you can opt for low-cost PEN and polyester materials in making the flexible laminate.
A lot of other boding and laminate materials exist in the market. It includes ceramic, cyanate ester, blended systems (combining resins to offer the ideal mechanical and electrical performance), and BT. However, the volume for such material-oriented PCBs proves lower, and thus the manufacturing process becomes difficult. Further, such distinctive materials also prove expensive as a PCB design alternative.
Hybrid Printed Circuit Boards
It is not uncommon to find certain manufacturers combining laminate materials to form hybrid systems. In fact, a common variant of this system encompasses the rigid-flex PCBs that combine rigid and flexible circuit materials to offer the best of both worlds in a single circuit package. You will find some of the circuit segments flexible, allowing them to get bent as desired. Alternatively, you can flex it multiple times (in thousands) while maintaining electrical continuity.
Some of the sections will come rigid, and these sections offer a higher density of electrical routing needed for contemporary electronics. As a result, rigid-flex circuit boards offer the perfect package method that current electronic designers can enjoy.
Another hybrid variant will see you include a layer of the Teflon material within the traditional Polyimide or RF4 circuit board. Such a layer offers you as the designer an optimized layer for high-speed signals in a PCB package that can easily get manufactured.
No- and Low-Flow Prepreg
You may not know about this type of material, though it proves crucial in fabricating rigid-flex circuit boards. The no- and low-flow Prepreg get produced in a similar way to that of the traditional Prepreg. However, the resin gets advanced curing, which ensures the moderate flow of resin that leads to the formation of a Prepreg sheet. Immediately the resin attains a specific temperature, it thermosets and toughens.
The low and no flow Prepreg becomes essential for the manufacturing of rigid-flex boards, especially allowing the flow of resin to the edge of the board’s rigid section. It prevents any extra flow or spillage to the flexible segment of the circuit board. As a rigid-flex manufacturer, you can avoid issues like the one stipulated (spillage) when you decide to use the traditional Prepreg. Trust me; you do not want to have the flexible parts of the circuit board turn rigid due to resin spillage. Further, low and low flow Prepreg can become useful when bonding the materials to the printed circuit board. Materials such as stiffeners and heatsinks play a pivotal role when attached to the flexible PCB.
Please note that the availability of no- and low-flow Prepreg proves limited. You, therefore, need to pick a laminate approach or system that comes equipped with a corresponding no- and low-flow Prepreg. It becomes risky to otherwise rely on the traditional Prepreg for your rigid-flex PCB manufacturing process. Further, the no- and low-flow Prepreg PCB material has limitations, especially when heavier copper gets deployed as the resin fails to flow sufficiently and cover the circuitry. It also needs a specialized fabrication consideration that you must accommodate to succeed in using them.
For the Conductive Layer
The most common PCB material for the conductive layer you will encounter as an electronic designer or enthusiast involves copper. However, it is not the only conducting material used in printed circuit boards. You can always use silver and gold, to mention a few. Gold and silver have better electrical and thermal conductivity properties, though they often prove more expensive than copper. Further, PCB material types like aluminum and iron also get utilized for printed circuit boards. Such materials permit the use of SMT (surface-mount technology) when it comes to integrating electronic components. You can also bank on their ability to give mechanical durability. As a result, the product lifecycle of metal-based PCBs proves longer.
When to Deploy the Various PCB Material Types
You can always use flexible materials when space becomes an issue or proves limited. It also applies when the materials need to bend during the application of the printed circuit board. If you also desire the board weight to come light, using flexible materials can also become key. Finally, the need to have your printed circuit board operate in extreme conditions without any issues can also demand that you use flexible PCB materials.
Rigid materials become instrumental in developing printed circuit boards for specific applications. However, if you don’t know when to use these materials, consider them under the following circumstances. If strength ranks high on your PCB needs, then rigid materials can work a treat. Further, the need for easy maintenance and repair necessitates rigid materials as it has marked electronic components. Rigid materials also feature well-organized signal paths besides costing less when ordered in large volumes (when you eliminate connectors).
The rigid-flex materials become instrumental when you plan to use your PCB for automotive sensors and radar systems. Additionally, rigid-flex materials can work a treat in making RF identification tags besides deployment in power amplifiers and antennas located in cellular base stations.
You can use the RF4 PCB material when you have a limited budget, prototyping, or your targeted application entails a low-speed digital circuit. Further, if your design proves complicated and needs several layers, an RF4 can also come in handy.
A lot of PCB materials exist in the electronic markets. However, your PCB design, intended application, and budget should act as proper beacons when selecting the best PCB material fit for your PCB. But understanding the various circuit board material comes as the starting point, something we hope to have ultimately achieved.