If you have ever had to deal with electronics in nearly any shape or form, the chances are that you have encountered the acronyms “PCB” and “PCBA” on multiple occasions. While people often use the terms interchangeably in everyday speech, they are not the same thing.
PCB stands for “printed circuit board.” A PCB is a sheet of insulating material covered by one or more layers of laminated conductive material, usually copper foil. The copper coating is divided into tracks, connecting pads, and other features that allow for the mounting of electrical components to form a circuit.
PCBs are ubiquitous in today’s day and age. Compared to rival technologies such as point-to-point construction and wire wrap, PCBs are easier to design and lend themselves to mass production. As a result, they are now a common feature of most electronic devices and some electrical products.
PCBA stands for “printed circuit board assembly.” The term describes the process of populating a bare PCB with electrical components to create a fully functioning circuit assembly. In that sense, PCBA is also used to refer to the finished product.
To learn more about the components of PCBs, as well as how they turn into PCBAs, keep reading.
What Are the Main Components of a PCB?
A PCB is a board that consists of an insulating substrate that serves as the foundation for mounting and connecting various electronic components.
While the dimensions of a PCB board largely depend on the industry and type of device, standard thickness options include:
- 031 inches (0.78 mm)
- 062 inches (1.57 mm)
- 093 inches (2.36 mm)
Copper Foil Coating
The board is coated with one or more layers of laminated copper foil. Typically, PCBs have four, six, or eight layers of foil, with boards with four and six layers being the most common. A two-layered PCB will have copper on both sides, whereas a multi-layered PCB will have several copper layers stacked one on top of the other and separated by layers of insulating material.
If a board has more than two layers, the first two layers may feature mostly solid copper. That way, they can function as ground planes and provide shielding and power supply, while the rest contain the wiring between the various board components.
Once applied, the copper foil needs to be imprinted with a predefined conductive pattern, also known as the artwork. This pattern divides the copper layer into various elements. These include lines called “circuit traces” or “tracks,” connection pads, and electrical connections known as “vias.”
Together, these elements create a conductive pattern that will connect the various electrical components that will be fitted onto the bare PCB during the PCBA process. The tracks, for example, function as wires. The board substrate material and the surrounding air provide insulation to these “wires.”
The conductive pattern can be imprinted in one of two ways.
Traditionally, PCBs would go through a manual process known as chemical etching, which essentially removes parts of the foil to form the conductive pattern.
This etching process typically uses a light-sensitive material, or photoresist, that dissolves upon contact with light. The PCB is first coated with photoresist. Then, the board is covered with a patterned template or “mask” in the desired pattern. The exposure to light causes the photoresist to dissolve in the uncovered areas. Finally, the board gets a thorough cleaning to remove any chemical residue.
Most modern manufacturers use special layout software to design their PCBs. The process involves the following steps:
- First, the designer creates a schematic capture using an electronic design automation (EDA) tool.
- Then, the board dimensions and the pattern template are chosen based on the PCB type and the specific needs of the devices for which it is intended.
- Next, the manufacturer determines the heat sinks and the positions where the electrical components will later go.
- The ground and power planes and the layer stack for the PCB are then decided. A PCB can have anywhere from one to tens of layers.
- Next comes the line impedance, which depends on factors such as the dielectric layer thickness, routing copper thickness, trace width, and trace separation.
- Finally, the system places the components automatically and generates Gerber files for manufacturing.
However, you do not need to have a manufacturing plant to start designing and creating PCB boards. For a quick tutorial on how to make your first steps in PCB design, check out this video:
PCB Design: Getting Started & Design Rules
For further details, make sure to watch this tutorial on PCB design for beginners as well:
Step-By-Step Printed Circuit Board Design for Beginners
Additionally, PCBs may have a protective coating to prevent corrosion, oxidation, unwanted contact with stray bare wires, and solder shorts between traces. This coating is known as “solder resist” or “solder mask.”
What Are the Properties of the PCB Circuit?
Each trace consists of a flat and narrow section of the copper foil that has survived the etching process intact. The thickness, length, and width of a trace determine its resistance, which needs to be sufficiently low to be able to carry the current. Signal traces must be narrower than both power and ground traces.
In the case of microwave circuits, the transmission lines may be laid out in a planar pattern, such as a microstrip or a stripline. These have precisely controlled dimensions that ensure consistent impedance.
In fast-switching and radio-frequency circuits, the capacitance and inductance of the PCB conductors are typically undesired significant circuit elements. However, in antennae, fuses, and distributed-element filters, PCB conductors may be a deliberate feature of the circuit design. The reason for this is that they dispense with the need for any extra discrete components.
What Does a PCB Do?
PCBs are essential components that support and provide electrical connections between the electronic components of various appliances.
What Do We Use PCBs For?
Most modern electronic devices rely heavily on PCB boards. Some of these appliances include:
- Mobile phones
- Graphic cards
- Computer motherboards
In addition, PCBs play a big role in the production of medical and lighting equipment and industrial machinery, as well as in the automotive industry. They are also used to manufacture certain electrical products, such as passive switch boxes.
Why Are PCBs So Popular?
Industry observers expect that, by 2024, the global market for bare PCBs will reach a whopping $79 billion.
The soaring popularity of PCBs is due to their unique features and advantages compared to alternative technologies like wire wrap or point-to-point construction. In fact, PCBs are so much better than these rival technologies that manufacturers now rarely, if ever, use these once-popular methods.
The main selling point of PCBs is that once designed, the printing of a single pattern can be automated — and the bulk of the work is done by electronic computer-aided design software. What’s more, the conductive pattern lends itself to mass reproduction in much the same way in which photographs mass-duplicate film negatives with a photographic printer.
As a result, manufacturers can mount and wire the components in a single operation. In addition, PCBs are lightweight and small in size yet pack an impressively high-density wiring.
All that makes the manufacturing of PCBs both cheaper and faster than other wiring technologies.
What Types of PCBs Are There?
By Number of Layers
As mentioned above, PCBs come in a variety of layers.
Also known as single-sided PCBs, these are easy to design and manufacture, making them the simplest and most widespread PCBs in electronics. These boards feature a single copper layer covered by a solder mask.
You can find these types of PCBs in low-cost, mass-market appliances such as radios, printers, and calculators.
Double-layer or double-sided PCBs have a copper coating on the top as well as the bottom side of the board. While they are more costly than single-layer PCBs, they offer greater flexibility and come in smaller sizes, which allows for more compact circuits.
Common applications of double-sided PCBs include:
- UPS systems
- Industrial controls
- HVAC applications
- Power monitoring units
These PCB models have more than two layers of copper foil stacked one on top of the other. To secure the board and to prevent overheating, manufacturers alternate layers of copper with layers of substrate material using glue.
Each copper layer is individually etched, and any inner vias that will not reach both outer sides of the board are plated through. Finally, all layers are laminated together. Only the outer layers are additionally coated with a solder mask; the inner copper foil layers are protected by the substrate.
Multi-layer PCBs are commonly found in complex applications such as:
- File servers
- Satellite systems
- GPS technology
- Medical equipment
- Data storage equipment
By Range of Flexibility
Flexible PCBs are also known as flex boards, flex circuits, flexible circuit boards, flexible printed circuit boards, and flexible electronics. They can bend, snap, twist, and fold into virtually countless configurations.
Flex boards are used in most high-performance and/or ultra-compact devices, including smartphones, tablets, cameras, GPS units, and wearables. Not only do these PCBs provide greater sophistication, but they can also be easier to install from a technical perspective.
Rigid PCBs, on the other hand, are unable to twist and bend. Nevertheless, these boards have an incredibly wide range of applications, including:
- Audio keyboards
- Solid-state drives (SSDs)
- Laptop and desktop computers
- Flat-screen TVs and monitors
- Children’s toys
In addition, rigid PCBs are usually less expensive than flex circuits.
As their name suggests, flex-rigid PCBs bring the best of both worlds. These hybrid circuit boards feature elements from both rigid and flexible circuits. They are flexible in some areas on the board and rigid in others. As a result, flex-rigid circuits can be bent as needed while maintaining the integrity of any areas that may need extra support.
Flex-rigid PCBs are usually multi-layered, and consist of flexible circuit substrates joined by rigid boards. The flexible layers are built in and fully penetrate the rigid sections.
What’s more, flex-rigid PCBs are quite slim: the standard circuit is between 0.001 to 0.002 inches thick. That makes it ideally suited for ultra-light and ultra-thin manufacturing needs.
Other Terms for PCB Boards
When a board has no components yet, it is sometimes referred to as a “printed wiring board” (PWB) or an “etched wiring board.” However, the former term has fallen out of use in recent years.
What Is PCBA?
Printed circuit board assembly, or PCBA, is the assembly process of fitting the electronic components onto the bare PCB board to create a printed circuit assembly (PCA).
The process involves passing a bare PCB board through procedures known as Surface-Mounted Technology (SMT) and Plated Through Hole (PTH).
During these processes, the electronic components are mounted and soldered onto the PCB board via:
- Solder paste printing
- Component placement
- Hand, wave, or reflow soldering
To learn a basic soldering technique on how to solder a through-hole connection on a PCB board, check out this short video:
Basic PTH Soldering Technique
A single PCB board may use both PTH and SBM technology for mounting components. In fact, PCBs with through-hole mounted components only are now fairly uncommon.
Surface mounting is used for diodes, IC chips, transistors, resistors, and capacitors. Larger components, including electrolytic capacitors and connectors, often employ through-hole mounting.
Plated Through Hole
The very first PCBs employed through-hole technology, and many still do to this day.
PTH involves inserting electronic components through plated holes using leads or pins. The leads enter through one side of the board and are then soldered onto traces in the copper layer on the opposite side.
Typically, through-hole parts are installed horizontally using a couple of two axial leads, such as diodes, capacitors, or resistors. The two leads are bent in the same direction at a ninety-degree angle and are then inserted into the PCB board. On the back of the board, the leads may be bent in opposite directions, as that improves the mechanical strength of the unit.
The leads are then soldered either manually or by using a wave soldering machine. Finally, they are trimmed off at the ends.
All this results in the creation of electrical connections between the components and the substrate.
PCBA boards may be either single-sided, where only one side has plated components, or double-sided, with components soldered on the two sides.
The main disadvantage of PTH manufacturing is that it requires the precise and accurate drilling of many holes, which adds to the production cost. In addition, PTH limits the routing area for signal traces on multi-layer boards.
That is why, with the advent of SMT technology, through-hole mounting is usually only used for components that are too large for surface mounting due to either mechanical limitations or power requirements. PTH is also used in areas where there is mechanical stress which might damage the board or its components.
SMT technology was first introduced in the 1960s. The method gradually gained popularity in the 1980s and eventually became ubiquitous by the mid-1990s.
In contrast to PHT, SMT involves installing electrical elements by mounting their leads to traces directly on the substrate on one side of the board rather than through holes. Components were intentionally redesigned so as to have tiny end caps or metal tabs that lend themselves to soldering directly onto the PCB. That dispensed with the need to have wire leads pass through holes.
The components themselves became much smaller, which made it possible to place them on both sides of the PCBA board. The new technology also allowed for much smaller PCBA units that had much higher circuit densities.
What’s more, the SMT method allows for a high degree of automation, which significantly lowers production costs and increases output rates compared to PTH technology. Furthermore, because surface-mount components can be anywhere from one-quarter to one-tenth of the weight and size of through-hole elements, they are much cheaper.
The finished PCBA board then goes through several rounds of quality control inspection to ensure that they are fully functional.
There are several ways to test a PCBA board:
- With the power off, the board may go through a visual inspection or an inspection using an optical inspection tool. It can also undergo an analog signature analysis, or “power-off testing.”
- With the power on, it is possible to carry out in-circuit testing as well as a full functional test to establish whether the PCBA is working properly.
A finished PCBA can have a silkscreen with a legend printed on both sides. The silkscreen is not essential to the functionality of the PCBA, but it is still useful as it displays key information such as the switch settings, component designators, and test points. The legend may also contain other details regarding the assembly, testing, servicing, and use of the PCBA board.
Originally, manufacturers used actual silkscreen printing. However, nowadays, they use faster and more accurate printing methods that allow for finer and more detailed printing. These include liquid photo imaging and inkjet printing. Inkjet printing, in particular, can print data that is unique to each unit, such as a bar code or a serial number.
Protection, Packaging, and Safety
PCBA boards, especially those for use in extreme environments, often have a conformal coating. It consists of a thin polymer film that conforms to the contours of the board and protects its components from corrosion, shorting due to condensation, and leakage currents. The main disadvantage of conformal coating, however, is that it can make servicing of the PCBA unit extremely difficult.
The conformal coating is usually applied either by dipping or spraying the PCBA board after all components have been soldered. Alternatively, some manufacturers sputter plastic onto the PCBA board in a vacuum chamber.
The first conformal coats used wax. However, modern coats are usually dilute solutions of acrylic, epoxy, polyurethane, or silicone rubber.
Many PCBA boards are sensitive to static. As a result, they must be stowed in special-purpose antistatic bags during transportation.
It is vital that users handling PCBA boards are grounded or earthed. Otherwise, they are at risk of transmitting accumulated static through the PCBA unit, which may result in damaging or even destroying some or all of the components.
Static damage may not immediately affect the functionality of the PCBA. Later on, however, it might cause failures and intermittent operating faults. It is also likely to narrow down the electrical and environmental conditions under which the PCBA can function properly.
What’s more, even bare PCB boards can sometimes be static sensitive. The traces on the conductive patterns have now become so fine that it is possible for static discharge to blow a trace or alter its characteristics. That is especially true with regards to MCMs, microwave PCBs, and other non-traditional PCB boards.
A Note on the Terminology
A PCB with mounted electronic components may be called a PCBA, as well as “printed circuit assembly” (PCA) or “printed circuit board assembly.” However, the IPC prefers the term “circuit card assembly” (CCA).
Things are made even more confusing by the fact that in informal speech, people often refer to PCBA as a “printed circuit board” for short. You can also often hear the term “card” is another informal term for PCBA.
PCB and PCBA: Conclusion
Hopefully, this guide has helped you differentiate between PCBs and PCBAs. While they make for a rather tech-heavy subject, these devices are part and parcel of our modern world.
To recap your knowledge of PCB and PCBA boards, have a look at this video:
What Are PCBs and PCBAs? PCB Basics.