What is a PCB?
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What is PCB? A printed circuit board (PCB) is an electronic assembly that uses copper conductors to create electrical connections between components. Printed circuit boards provide mechanical support for electronic components so that a device can be mounted in an enclosure. A printed circuit board design must include a specific set of steps that aligns with the manufacturing process, integrated circuit packaging, and the structure of the bare circuit board.
Conductive features on printed circuit boards include copper traces, pads, and conductive planes. The mechanical structure is made up of an insulating material laminated between layers of conductors. The overall structure is plated and covered with a non-conductive solder mask, and a silk screen material is printed on top of the solder mask to provide a legend for electronic components. After these fabrication steps are completed, the bare board is sent into printed circuit board assembly, where components are soldered to the board and the PCBA can be tested.
The printed circuit board design has grown into its own vertical within the electronics industry. PCBs play an important role in that they provide electrical interconnections between components, rigid support to hold components, and a compact package that can be integrated into an end product. Even the most basic circuit board has to be carefully designed using specialized software packages, and the best software can help take a design from concept all the way through manufacturing. In this article, we'll look deeper at what is a PCB meaning, its design description, and some of the important points to understand when designing circuit boards.
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All printed circuit boards are built from alternating layers of conductive copper with layers of electrically insulating material. During manufacturing, the inner copper layers are etched, leaving the intended traces of copper for connecting components in the circuit board. Multiple etched layers are laminated in succession until the printed circuit board stack-up is complete. This is the overall process used in PCB design fabrication, where the bare board is formed before passing through a printed circuit board's assembly process.
Before we can explain PCB board design, it is best to understand where PCBs have come from. In the past, electronics were designed and assembled from small integrated circuits and discrete components, which were connected together using wires. Today, standard designs can have high pin count components with many integrated circuits and very small passive components, making it impossible to manually connect components together with soldered wires. Instead, copper connections are deposited directly on insulating substrates to form electrical connections, and the manufacturing processes for circuit boards have evolved alongside the structural requirements for electronics packages and interconnects. Many of today's devices are advanced HDI designs with thousands of connections and multiple electrical interfaces, powering everything from smartphones to heart rate monitors to rockets.
Before printed circuit boards, components were packaged by attaching individual wires to components, and by mounting components to a rigid substrate. This original substrate was initially a material called bakelite, which was used to replace the top ply on a sheet of plywood. Conductive paths were formed by soldering metal components to wires, and larger circuits could contain many electronic components with many wires. The number of wires was so great that they could get tangled or inhabit a large space within a design. Debugging was difficult and reliability suffered. Manufacturing was also slow, where multiple components and their wired connections were manually soldered.
In the previous section, I focused on typical PCBs that are assembled on rigid substrates as these are the most common. However, there are other types of circuit boards that are constructed on a range of possible materials. The common types of these are:
- Single-sided - This board only has components mounted on one surface. The back surface is typically fully copper (ground) and coated with a solder mask.
- Double-sided - This type of circuit board has components mounted on both surfaces. Each surface is defined as a signal layer in the PCB stack-up, so the surfaces will contain traces that carry signals between components.
- Multi-layer PCBs - These boards have conductors on internal layers that carry electrical signals between components, or the internal layers could be conductive plane layers. Multi-layer PCBs may be single-sided or double-sided.
- Rigid PCBs - These boards are fabricated and assembled on rigid laminate material, such as FR4-grade epoxy resin-impregnated fiberglass laminate materials. Other types of rigid laminate materials are available as well, which provide different material properties for use in some specialized applications.
- Rigid-flex PCBs - Rigid-flex PCBs use a flexible polyimide ribbon that connects two or more rigid sections in a printed circuit board assembly. A rigid-flex board might be used when the design must have some movable element, such as a folding or bending enclosure.
- Flex PCBs - Fully flexible PCBs do not use any rigid materials and are made entirely of flexible polyimide ribbons. These boards can have components mounted and soldered on the, just like rigid and rigid-flex printed circuit boards.
- Metal-core PCBs - These boards use a metal slab in the core layer (normally aluminum) in order to provide much greater rigidity and heat dissipation than in typical rigid printed circuit boards. The metal-core PCB design manufacturing process is quite different from the standard rigid PCB design manufacturing process, and there are a few design points to consider to ensure solvability. These boards are common in high-power lighting and some industrial applications.
- Ceramic PCBs - These boards are less common and are used in applications that require very high thermal conductivity such that the board can dissipate large amounts of heat away from components.
The fabrication and assembly processes for these types of PCBs vary, but today's ECAD software can help designers create any of these boards as long as the right PCB design rules are enforced in the software.
In the above image, we have an older circuit board that largely uses through-hole components to provide the required functionality. Modern printed circuit boards have switched to largely surface-mount (SMD) components as they are more useful in high-density, double-layer designs. SMD components are now the standard component type used in most applications that require a small form factor, low power, and low cost. However, some applications still make use of through-hole components as they are more reliable and easier to assemble, including with hand assembly. The image below shows an example of a modern PCB with high-density SMD components.
Many of the important performance characteristics of a PCB are defined in the stack-up or the arrangement of layers in the PCB. The layer stack-up is built with alternating layers of conductive and insulating material, and with alternating layers of core and prepreg (two types of dielectrics used in the layer stack-up). The dielectric and mechanical properties of the core and prepreg will determine reliability and signal/power integrity in the design, and they should be selected carefully when designing for high-reliability applications. For example, military and medical applications need highly reliable designs that might be deployed in harsh environments, and a PCB for a telecom system might require a low-loss PTFE laminate in a small package.
An example of a PCB stack-up is shown below. In this example, the stack-up implements a 4-layer structure with two internal plane layers (L02_GND for ground, and L03_PWR for power). This type of stack-up is appropriate for IoT devices, lightweight embedded systems, and many other designs that use high-speed protocols. The internal plane arrangement helps ensure power integrity while also providing some shielding against external EMI. The internal plane layers also provide a consistent reference for controlled impedance signals. This type of stack-up is typical for many designs and is often a starting point for many modern printed circuit boards.
Before moving forward, it is important we define what a PCB layout is, which is a term you will run into frequently. It is a CAD drawing showing the location of all elements that will appear on assembled printed circuit boards. This includes all components and copper that will appear on both sides of a PCB. We'll walk through the creation of a printed circuit board layout in the next section step-by-step.
This is not to be confused with printed wiring board design, which is similar to a PCB layout but is a bare-bones version designed without the electrical components added.
When it's time to start a new design, printed circuit boards pass through several stages. Production-grade circuit boards are designed using ECAD software, or a CAD application that includes many utilities that are specialized for circuit board design and layout. ECAD software is built to help designers walk through a specific process for circuit board design, beginning with basic electrical drawings and ending with manufacturing file preparation. Circuit board design follows a basic process:
- Front-end engineering - In this stage, the major components are selected and some basic circuit diagrams are typically created so that the functionality in the board can be designed.
- Schematic capture - This is the stage where ECAD software is used to translate simple circuit diagrams into electronic drawings that define electrical connections between components. Schematic symbols are used to denote components in the design.
- Material selection and PCB stack-up design - In this stage, laminate materials are selected and the stack-up is designed to accommodate the need for plane layers, signal layers, dedicated routing channels, and specific material properties.
- Component placement - After the board shape is set and components are imported into a new PCB layout, components are arranged in the layout to comply with the design's mechanical requirements.
- Routing - Once the component placement is approved, it's time to route traces between components. Routing tools in ECAD software are used to set Trace geometry may be determined in this stage with the goal of ensuring impedance control (for high-speed signals).
- Design review and verification - Once routing is completed, it's always a good idea to inspect and evaluate the design to ensure there are no mistakes or unresolved problems. This can be done with manual inspection or using post-layout simulation tools.
- Preparing for Manufacturing - Once the design is completed, it's time to prepare for production by generating standard manufacturing files. These files are used in automated fabrication and assembly equipment.
If you want to easily get through all of these stages in the PCB electronics board design process, you need to use the best design software with an intuitive user interface and a complete set of PCB design features.
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