Almost every circuit used in the modern era is now implemented using a Printed Circuit Board (PCB), and while other options do exist none are as robust, reliant, and convenient as the PCB. Each unique circuit one wishes to construct requires an equally unique PCB in order to implement it, and while the design of a PCB will significantly affect the end device's function and performance, the materials used are equally as important. In this article, we will provide an overview of the materials and construction of PCBs as well as the effects that one's choices of materials and construction will have on performance.

PCB Construction

The general construction, and description, of PCBs, is that of a laminate board with conductive layers (often called the cladding or simply copper in the case where one uses copper foil) sandwiched with insulating, dielectric layers (usually termed the substrate).

We will begin with the substrate, the most commonly used types of substrate materials are probably "Epoxy-glass" (FR-4 being the most common of this type) and Phenolic Paper (also called Synthetic Resin-Bonded Paper, SRBP) with Epoxy-glass substrates being by far the most commonly used material in modern PCBs due to their highly desirable properties and ease of manufacturing.

The conductor almost always chosen is high-quality, pure copper (often coated with gold) as it is cheap and has acceptable properties for most circuits. However, on rare occasions, other materials may be desired such as pure gold.

And whilst all of this provides a general description of the manufacture of the most common and basic type of PCB there are always exceptions and different options, which we will see in the remainder of this article.

PCB Types

PCBs obviously consist of much more than just a conductor and substrate sandwich, and they can be further classified by the way they are constructed and by any specialized materials or construction schemes used. Specifically, we can further segregate them by the number of layers they might have, how layers might be connected together, the types and placements of components used, and any other special manufacture required for specific applications.

Single-Sided PCB

A single-sided PCB is one where the conductor only exists on one side and usually, by extension, the components are only found on one side. One side contains all of the copper tracks, polygons, etc. that conduct current and connect the various components, the soldering of the various components is on this side as well. The other side contains all of the component bodies. These are only possible in the case of extremely simple designs and as such are not often found in modern fab houses, if it is offered by a manufacturer then it is very likely to be extremely cheap and fast to produce. If you have completed some kind of formal education in Electronics you may have produced one of these types of boards via the acid etching method or using a router of some kind in a laboratory or a practical work exercise.

Double-Sided PCB

‘Double-sided’ printed circuit boards have copper (or conductor) on both sides of the insulating material, components are also usually on both sides of the PCB but it may be beneficial to place only on one side to provide a solution for mounting, cooling, or other design specifications.

There are two more additional sub-categories referred to for double (also multi-layer etc.) boards, which are:

• Plated Through-Hole connection (PTH); and
• Non-Plated Through-Hole connection (Non-PTH).

PTH boards have circuitry on both sides connected together by drilling a hole through the board and then metalizing that hole, of course, the two sides of the PCB will have copper, tracks, etc. where the hole and metalizing are constructed so that a connection is made. PTHs (also called vias) are essential to allowing circuits of high complexity, and there are many subtypes that one may wish to use, but that shall be discussed in further detail later.

On the other hand, we have non-PTH which is the simplest form of double-sided board (basically an extension of single-sided PCBs) with no built-in connections between the two layers. Non-PTH PCBs are cheaper than PTH boards but connections between the layers (if they are desired) must be completed by soldering component leads on both sides with the appropriate copper connections or using "jumper" wires/connections which must be soldered into place. If a single-sided style component placement approach is taken when designing this type of PCB (i.e. if components are placed such that the bodies are only found on one side and lead ends on the other) it is generally a good idea to place most, if not all, the connections on the soldering side to make component removal easier.

Multi-layer PCB

As the name suggests this is a board with more than just two layers of conductor. Eventually, design complexity becomes such that 2 layers are simply not enough, and while PTH technology can help solve conductor cross-over and connection requirements, there is a limit as well as other problems that may need to be solved with a design.

Particularly when working with high-speed design and multi-pin ICs such as BGAs (Ball Grid Arrays) the packaging density causes a concentration of interconnections that can only be solved with a multi-layer approach. This complexity has other problems which can only realistically be solved using multi-layer techniques, these problems include cross-talk, noise, impedance matching requirements, stray capacitance, and voltage drops from parallel signal lines.

Multi-Layer boards can typically be manufactured between 2 and 32 Layers (but select manufacturers can offer up to 50 layers) with almost any desired substrate material and dimensions (thickness). They are made by bonding together a number of PCBs using a material known as "prepreg", which is usually just the insulating substrate (eg. glass epoxy) that hasn't been fully cured.

Usually one will need to consider the use of a multi-layer PCB in the following circumstances:

• When weight and volume savings per connection are a significant design requirement, e.g. military designs and aerospace applications;
• When the complexity of the design requires complicated and expensive connections;
• When frequency requirements are present necessitate precise control and uniformity of impedance along signal propagation
• When connections (board-to-board) or shielding of a considerable number of connections is required, high capacitance distributed between layers gives good decoupling of power allowing good high-speed signal propagation

Although sometimes expensive, there are a number of other advantages to multi-layer designs:

• There are more via (PTH) options available including blind vias, buried vias,
• All interconnections (or most) can be placed or wired internally. allowing for a very large "heat sink" of copper on the outer layers
• Power and ground planes can be established to provide power across the board in a simpler manner.

Rigid, Flex, and Rigid-Flex PCB

There are other types, or classifications, of PCBs now (somewhat) commonly used or found in devices, this is regarding whether the insulating material is rigid, flexible, or a mix of the two. The board types that have been discussed before now are basically all assumed to have been rigid. Rigid PCBs can be constructed using several insulating materials, have PTHs, and include other design features that will be explored later.

Flex PCBs on the other hand are usually constructed using polyester or polyamide as the substrate and, as the name suggests, are flexible.

In the case of a purely flex PCB (as opposed to, say, Rigid-Flex) the conductor is usually rather thin (in the range of 0.1mm) and the PCB will often only have a double-sided construction for simplicity (flex PCBs tend to be expensive enough to begin with and besides that can't permit much power). The holes for the placement of components are not normally drilled but punched.

Rigid-Flex PCBs are a combination of rigid and flexible techniques, these types of PCBs are particularly suited to realizing three-dimensional designs or structures that use flexible portions bonded together with rigid boards in order to connect together those rigid boards (which hold the components). These PCBs provide highly efficient use of space and can be made in single-, double-, and multi-layer configurations with PTH or non-PTH as desired.

High Density Interconnect (HDI) PCB

As manufacturers began to use fine pitch ball grid array (BGA) and other evolving technology factors, they needed to come up with new design and fabrication techniques in order to accommodate components with extremely tight pitches. In addition, extremely high clock speeds and bandwidths require board configurations that mitigate the negative effects of radio frequency (RF) and electromagnetic interference (EMI).

Microvia circuit interconnects can address these problems, these are usually considered to be vias of less than or equal to 150 microns in diameter. They are most often used to create blind and buried vias. Microvias allow for the construction of high-density assemblies and whenever a design requires them it is usually considered to be an HDI PCB.

Aside from this general "layman's" definition, the IPC chose High-Density Interconnection Structures (HDIS) as a term to refer to microvia designs, the components can be placed closer together in the board, making more room for signal tracks between them.

If the reader is interested in knowing more about the various IPC standards involved with HDI design, below is a list of some relevant texts:

• IPC-2226 - Section Design Standard for High Density Interconnect (HDI) Printed Boards
• IPC-JPCA-2315 - Design Guide for High Density Interconnects and Microvias
• IPC-6801 - IPC/JPCA Terms and Definitions, Test Methods, and Design Examples for Build-Up/High Density Interconnect (HDI) Printed Circuit Boards

When designing for HDI boards there are a number of considerations that must be made, but the most important of them is material selection. Glass-reinforced laminates and resin-coated copper foils are often the most popular and suitable HDI materials.

HDI boards have controlled impedance requirements much higher than conventional PCBs. When edge speeds are fast and traces are long in comparison, then impedance needs to be taken into account. In the case of HDI design usually, both low-voltage and high-speed signals are involved, thus the acceptable noise margin will be lower and more susceptible to impedance changes.

Another thing to note for high-density interconnection boards, assembling almost always has to be done using only automatic and CNC machines, as there are significantly smaller pad sizes, thinner materials, and finer circuitry features involved for which hand soldering may be possible but is not feasible, especially for quantities found in production runs.

PCBs for Light Emitting Diode (LED) Applications

This is a PCB manufactured especially for the high power and high-temperature application of LEDs, there are other times when you may wish to use similar technologies in a PCB, namely when you have high temperatures present and wish to mitigate against any effects on performance that high temperature may produce.

There are several names or specific technologies that can be used for this type of PCB. Still, in the industry, you will most commonly hear the terms aluminum PCB, aluminum clad PCB, Metal Clad PCB, Metal Core PCB (MCPCB), and Insulated Metal Substrate (IMS). The basic idea is the use of a metal (usually aluminum) in one way or another to help dissipate the heat being produced by the LEDs (or whatever other component is producing large amounts of heat).

The most basic form of Aluminum PCB is one-sided, consisting of a copper conductive layer on which components are mounted, a small dielectric substrate for insulation, and then the aluminum cladding which dissipates the heat. There are many other available technologies, techniques, and sub-types to these PCBs which will be explored more thoroughly in another article.

For now, just know that Aluminum PCBs are mostly for high-temperature applications and can be manufactured in a number of ways including Through Hole, flexible, multi-layer, high thermal conductivity, high-frequency, and hybrid (bonding a normally manufactured multi-layer PCB to an aluminum base at the end of the assembly procedure).

Blank or Bare PCB

Blank or bare PCB is the term used to refer to a PCB that has undergone all of the PCB manufacturing processes but has not had any components placed or soldered. This type of board will be manufactured in the fastest possible time, but you will have to place and solder all of the components yourself if you order one.

The Main Types of PCB Substrates

Here, we will only be dealing with rigid substrates/laminates, rigid-flex and flexible substrates (more accurately called films) will be covered more comprehensively in another article.

Substrate / Laminate - An Overview

To accurately differentiate between terminologies here, it is necessary to point out that the term "laminate" refers to the combination of the substrate material and chosen conductor once all fused together, this results in a panel of material that can be used to mount components and create interconnections between components.

Meanwhile, the substrate is a composite, insulating material which the conducting layers are placed upon, it is made of a filler or reinforcement material and a resin.

The filler material provides mechanical strength and rigidity to the substrate or laminate. There is a very large range of fillers that can or have been used, including different papers, cotton fabric, asbestos sheet, glass (in multiple forms such as fiber, cloth, and continuous filament mat), ceramic material, and molybdenum. By far, the most common filler material in use today is glass fiber.

The resin is used to impregnate and bind together the filler material (as well as the conducting material). The most commonly available resins are epoxy and polyimide, but there are others such as cyanate ester, polyester, and phenol. Again, the most commonly used of these is probably epoxy. The resin material has by far the largest effect on the overall substrate/laminate's various properties. Often, the chosen resin will have additives introduced to achieve even more desirable properties than the base material would normally have.

The properties of substrates vary depending on the grade chosen, these grades are based on specifications (with limits) provided at the national and international levels by, amongst others, the Institute for Interconnecting and Packaging Electronic Circuits (IPC), International Electrotechnical Commission (IEC), Department of Defense Design Standard or Military Standard (MIL), the American National Standard Institute (ANSI), and the National Electrical Manufacturers Association (NEMA).

We shall heavily focus on the NEMA AND IPC standards as they are generally the most commonly used in the industry.

Additionally, it is important to note that the properties of laminates are/can be greatly affected by environmental factors such as humidity, temperature, corrosive atmosphere, etc. But this shall be covered shortly.

Prepreg

Also called "B" stage, this is basically a substrate that has been impregnated with resin and semi-cured.

When manufacturing full laminate panels there is a stage in their production (called treating) when the materials reach the same state as prepreg, the liquified resin is combined with the filler material using immersion and rollers, carefully controlling the process so that the filler is thoroughly wet and the quantity of resin absorbed is exact, then the wet material is semi-cured using a drying oven that is either infra-red or air-convection heated. At this stage, the material is dry to the touch, but further processing is required to press on the copper and fully cure the substrate.

Substrate Properties - A Quick Explanation

Below are some laminate properties that affect electrical performance (directly or indirectly) and a short explanation:

• Dielectric constant; Also called the relative electric permittivity (ε). Permittivity is a basic measurement that describes a material's reaction to an electric field but it can be used to describe the amount of electrostatic energy that can be contained by a material in a given volume (directly related to its capacitance). The dielectric constant (or relative electric permittivity) is the ratio between the permittivity of free space/air/vacuum and the substrate in question. The non-relative type is measured in Maxwell's equations using SI units Farads/meter^3.
• Dielectric breakdown strength or Dielectric strength; is a measure of the substrate materials' ability to resist the flow of current. In the industry, this is basically tested by subjecting a laminate (without its copper) to an increasingly high voltage and measuring the current. Usually measured in Kilovolts.
• Dissipation factor; Also termed the loss tangent or tanδ. This is a frequency-dependent measurement of the ratio between power dissipated due to resistance in the substrate material and stored power (usually only by electrical/capacitive means but it technically also includes magnetic/inductive means). This is a dimensionless unit (notice that it makes use of the tangent function) that is as small as possible to avoid losses in the PCB. Note this is also temperature and moisture-dependent.
• Arc resistance; Similar to dielectric strength but the measurement is looking for the voltage required to produce an arc across free space to the material.
• Absorption of water; This is the measure of how much water the substrate material will absorb, measured by submerging a given volume of material at a given temperature and pressure for a given amount of time. It is highly important that this value be as low as possible to prevent an increase in losses in the material, changes in impedance, and dimensional warping of the PCB.
• Insulation Resistance (Volumetric or Surface-based); Simply, this is the resistance (the ratio between voltage and current) measured using a galvanometer either across a given surface area or a given volume.

More mechanically (or manufacturing) relevant properties, note that under the right conditions these will affect electrical performance:

• Tensile strength;
• Laminating difficulty;
• Peel Strength
• Copper adhesion;
• Bond Strength
• Flammability characteristics;
• Self-extinguishing characteristics;
• Compression;
• Shear;
• Flexural strength;
• Heat resistance;
• Impact strength;
• Environmental resistance;
• Machinability;
• Fungus resistance; and,
• Dimensional stability.

FR4 epoxy laminate and prepreg

FR4 (which stands for Flame Retardent 4) is a glass cloth substrate and one of the most commonly used in the industry. I boast a Tg value of 130~150°C, a Dielectric Constant (Dk) of ~4.4 @10GHz, and a loss tangent (Df) of 0.022.

FR4 is low cost, has good mechanical strength and machinability, low moisture absorption, and is fairly stable thermally, in addition, as the name suggests it is fire-resistant.

It is fairly suitable for single-, double, and most multi-layer applications, and can even be made to be suitable for RF and some high-speed applications. But as always there are some limitations and times when other materials are much better suited, namely; Boards with a very high layer count, when there are very high temperatures involved in operation or manufacture, and/or when there are very high-speed electronics involved, with low cross-talk, low propagation delay, etc. requirements involved.

There are also some special versions of FR4 available for specific applications, for example, high-performance FR4 (lower dielectric value, even lower water absorption, higher Tg, etc.) or high-Tg FR4.

Polyamide laminates and prepreg

Polyamide is most commonly used in flex PCBs as a film, but it can be used with glass, quartz, or aramid fibers as a substrate/prepreg material.

Polyamide has fairly good electrical properties for most applications, very good heat resistance and thermal properties (Tg ~220°C), decent machinability, and is very flexible in film form. It is very commonly used in aerospace and military applications, and (as mentioned) flexible or rigid-flex circuits.

However, if very tight impedance control, high-speed electronics are in use, or if high efficiency is required polyamide is not the most ideal candidate as it can have relatively high Df (4.3@10GHz) and Dk (0.020) values.

A very commonly available form of film-type polyamide is Kapton from Du Pont, who has also produced specialized versions for extra high-temperature applications.

Teflon laminates and bonding plies

Teflon (with glass fiber) is the ideal substrate for RF and high-speed applications that do not involve very high temperatures. It is not very commonly used in industry due to its thermal properties but does find some uses. It can also be used in its pure form as a release material in certain manufacturing processes.

Flexible laminates

We have mentioned one flexible substrate (polyimide film) but there are a few others and some specific applications for each.

There are two types of plastic used generally thermoset plastics and thermoplastics (confusing I know, but stay with me).

Thermoset plastics are those which will not become malleable again upon the application of heat, i.e. once formed they cannot be reformed polyamide is an example of this, as well as polyacrylate (which is seldom used in the industry). Polyamide is by far one of the most ideal materials for flex PCBs (as mentioned) due to its thermal properties but there are some situations when other materials may be more beneficial.

Thermoplastics are those that will soften again once heated, even after being initially formed, the best examples being polyester and fluorinated hydrocarbon, with polyester being another very likely candidate for use in flexible PCBs with commercial applications due to its low cost, good electrical, and mechanical properties

But if very tight impedance control or high-speed communications are required fluorinated hydrocarbon (or fluorocarbon) is the best choice as it has better dielectric properties.

Materials - Notable Mentions

Of course, not every single substrate material and combination has been mentioned here, and so some of the lesser-known materials have been left for discussion here.

PTFE (Polytetrafluoroethylene) is a thermoplastic that has specific applications for microwave and RF designs. Usually, it is mixed with glass (but can be mixed with ceramics) and offers exceptional dielectric properties across a wide frequency band, temperature range, and general environmental conditions (good moisture characteristics). However, it can be difficult to work with and machine and can also be expensive.

There is also polyester, silicone, and melamine, all of which have specific uses mostly due to cost, the difficulty of manufacture, or poor mechanical properties.

There is another type of general laminate process or strategy sometimes referred to as mixed dielectric laminates which are when a number of different substrate materials are used in different layers depending on which circuitry is on a given layer, this often allows for a great balance between cost and performance.

Key Considerations for PCB Materials and Common Industry Applications

The properties that you will have to prioritize and carefully select will, of course, depend on the application or requirements of the design that you wish to realize.

High Glass Transition (Tg) PCB Materials

The glass transition temperature of a substrate is the temperature at which the resin in the substrate begins to soften and the mechanical properties will begin to change. This is important for the manufacturing process of multi-layer PCBs, as well as the soldering process as it gives a good indication of the expansion one can expect from a material when soldering, as well as how it will stand up to high ambient temperatures.

Generally, high Tg values are desirable when the manufacturing process (usually post-assembly, as part of a larger device) involves very high temperatures, i.e. above the Tg of a standard material like FR-4. Or if the ambient operating temperatures will reach very high levels (again think higher than that of standard materials. One last reason could be due to reworking, if the PCB is a prototype and could require some rework (or if there is another reason rework will be done), a high Tg material should be chosen to ensure that the board will function well even after being repeatedly heated close to Tg.

When temperatures are very extreme the material of choice is usually polyamide (Tg 260-300°C) not only due to its high Tg value but because it will continue to maintain its bond to the foil quite well during high temperatures or rework. Another option is high-Tg FR-4 (Tg ~180°C), which is a version of FR-4 which has been modified to better withstand high temperatures.

Dielectric Constant (Dk) and Dissipation Factor (Df) values for PCB Materials

These have been previously mentioned above in the summary of relevant electrical values for substrates, but they are possibly two of the most important properties to be controlled for PCB manufacture, especially in any higher speed (more than 1GHz) applications

The dissipation factor (Df), as mentioned above, is also termed the loss tangent or tanδ. It is frequency-dependent and can be described as a measurement of the ratio between power dissipated due to resistance in the substrate material and stored power (usually only by electrical/capacitive means but it technically also includes magnetic/inductive means). This is a dimensionless unit (notice that it makes use of the tangent function) that is as small as possible to avoid losses in the PCB. Note this is also temperature and moisture-dependent.

It is hugely important for PCB efficiency and also when using high-speed electronics, i.e. circuits with higher clock rates, lower cross-talk tolerance, low propagation delay, better signal integrity, and lower attenuation. The higher Df the more resistance that will be inherent in the PCB, this has a knock-on effect, increasing the rise times of signals, the amount of power dissipated generally, changing the impedance of any given track, etc. This is especially important in modern portable electronics when efficiency is important and high frequency is becoming more and more commonplace.

The dissipation factor will also change with frequency and environmental conditions, an increase in temperature or moisture will usually result in an increase of Df, and an increase in frequency will result in a decrease of Df. So ensure you keep this in mind when designing your PCB.

The dielectric constant (Dk), again mentioned above, is also called the relative electric permittivity (ε). Permittivity is a basic measurement that describes a material's reaction to an electric field but it can be used to describe the amount of electrostatic energy that can be contained by a material in a given volume (directly related to its capacitance). The dielectric constant (or relative electric permittivity) is the ratio between the permittivity of free space/air/vacuum and the substrate in question. The non-relative type is measured in Maxwell's equations using SI units Farads/meter^3. Just a side note here, I always preferred the term loss tangent to dielectric constant, as this property is clearly not constant, it changes with frequency, temperature, thickness, and humidity.

This value inherently affects the impedance and capacitance of any track (along with dimensions) on a PCB and as such it is essential to control it. An increase in the dielectric constant causes an increase in capacitance which increases the propagation delay, rise time, and fall time of a signal.

Can PCBs Be Recycled?

The general answer here is: yes... Mostly.

However, there is a more nuanced answer, there are two general types of recycling used for PCBs: conventional techniques and novel techniques. Depending on the country you live in or the companies or individuals involved any of the following techniques may be used. Ideally, the entirety of a PCB would be recycled or at least disposed of in a way that is sustainable but that is unfortunately not always the case.

Conventional techniques (which include Incineration, Mechanical separation, and Gravity separation) have a problem in that they are not particularly environmentally friendly and often a low amount of the material is recovered in the process. These methods are significantly cheaper which accounts for their appeal to some countries and companies.

The more novel industry techniques for the recycling of PCBs (Pyrometallurgy, Hydrometallurgy, Biohydrometallurgy, Electrostatic separation, and High-intensity magnetic separators) can recover roughly 70~90% of the metals that constitute modern PCBs with a huge amount of variation on the percentage of non-metals that can be retrieved or properly disposed of. Some of these methods are brand-new and not fully adopted yet and some can be rather costly accounting for their lesser use.

Many factors drive the selection of one technique or another, should the reader wish to familiarize themselves further with this topic and try to ensure that their manufactured PCBs get correctly recycled, please consider reading: Electronic Waste and Printed Circuit Board Recycling Technologies by Muammer Kaya.

Solder Resist - What is the Green Coating found on PCBs?

Solder resist, or solder mask is the coating placed on PCBs to protect the copper from damage (chemical, abrasive, or manufacturing/rework related). Additionally, as the name suggests it stops solder from flowing onto the wrong or undesired locations on the PCB during the re-flow process of PCB manufacture. Solder resist prevents some manufacturing defects that can take place during production, such as "bridging", additionally silkscreen (the white stuff found on PCBs) is printed onto solder resist designating components and other information on the PCB.

Solder resist comes in several colors other than just green, and the whole rainbow is now generally available if one desires.

Conclusion

Remember, choosing the materials and other properties of a PCB for your design will always depend on the application, the material must match the application and be carefully selected to operate in all expected conditions.

Although hopefully after reading this article you now have a good overview of PCB materials and you feel more confident to tackle and make decisions regarding PCB design and manufacture.