Printed Circuit Board: An Introduction and the Basics of Printed Circuit Boards

September 27, 2019 Cadence PCB Solutions

 

Picture of a standard printed circuit board

 

All around us are electronic devices. Some are these devices are subtle, while others attract a lot of attention. But whether the device is something that silently monitors your health, or is the smartphone that annoys you with constant interruptions, they all will have some sort of printed circuit board at their core.

Printed circuit boards have been around since World War II when they were developed for military applications. Once this technology was released for commercial use, electronic manufacturers quickly adopted it as it provided a much more cost effective solution than the traditional point to point construction of electronics. 

Over the years circuit board manufacturing has continued to grow in order to keep up with the increasing demands of newer, faster, and more complex electronic circuitry. How a PCB is created for it to do what is required is a subject that could fill a library, but we will give you the basics of what it takes to design a printed circuit board here.

What is a Printed Circuit Board?

A printed circuit board is a rigid structure that contains electrical circuitry made up of embedded metal wires called traces, and larger areas of metal called planes. Electronic components are soldered to the top, bottom, or both layers of the board onto metal pads. These pads are connected to the board circuitry allowing the components to be interconnected together. The board may be composed of either a single layer of circuitry, circuitry on the top and bottom, or of multiple layers of circuitry stacked together. 

Circuit boards are built with a dielectric core material with poor electrical conducting properties to make the circuitry transmission as pure as possible, and then interspaced with additional layers of metal and dielectric as needed. The standard dielectric material used for circuit boards is a flame resistant composite of woven fiberglass cloth and epoxy resin known as FR-4, while the metal traces and planes for the circuitry are usually composed of copper.

Printed circuit boards are used for a variety of purposes. You can find simple circuit boards in toys or controllers, while advanced circuit boards are used in computers and telecommunications. 

Some boards are made with flexible materials thereby allowing them to be used in unique cases and enclosures that require them to bend around other features of the device. Some boards are built with specialized materials due to the high frequencies that they operate at, while other boards have heavy layers of copper in them for high powered circuits used in industrial control panels and other similar applications. 

There are boards designed for extreme environments such as underground sensors or the engine compartment of your car. Specialized circuit boards are used for aviation, space, and military applications that impose strict tolerances on their manufacturing and performance.

Although there are many different applications for printed circuit boards, they are usually designed following a common process. This is what we will examine next.

 

Image of engineers working in a lab doing electronic or printed circuit board design

An engineering team at work designing a printed circuit board

 

The Process of Printed Circuit Board Design

To build the physical circuit board you must first design it, which is accomplished using PCB design CAD tools. PCB design is broken into two main categories: schematic capture to create the circuitry connectivity in a diagram, and then PCB layout to design the actual physical circuit board.

The first step is to develop the library CAD parts that you will need for the design. This will include schematic symbols, simulation models, footprints for PCB layout, and step models for 3D printed circuit board display. Once the libraries are ready the next step is to create the logical representation of the circuitry on a schematic. CAD tools are used to place the symbols on a schematic sheet, and then connect them together to form the circuitry. 

At the same time circuit simulation is run to verify that the design will work electrically the way it is intended to. Once these tasks are completed, the schematic tools will send their connectivity data over to the layout tools.

On the layout side of PCB design, the schematic connectivity is received and processed as nets that connect two or more component pins together. With an outline of the intended board shape on the screen, the layout designer will place the component footprints into their correct locations. Once these components are optimally organized, the next step is to connect the nets to the pins by drawing the traces and planes between the pins. The CAD tools will have design rules built into them that prevent the traces of one net from touching another net, as well as governing many other widths and spaces needed for a complete design. Once the routing is complete, the design tools are used again to create manufacturing drawings and the output files that the manufacturer will use to build the board.

The design and manufacturing of a circuit board is a step by step process: schematic creation and simulation, setting up PCB design grids and DRCs, component placement, PCB routing, power planes, and finally assembling the BOM and building the board. Next we will look more in depth at these steps.

 

Picture of a printed circuit board and the schematic that generated it

A circuit board and its schematic

 

How to Make a Printed Circuit Board Step by Step

Although the design and manufacturing of a printed circuit board assembly can be generalized as schematic capture, PCB layout, and circuit board fabrication and assembly, the details of each step are very involved. We’ll take a look here at some of the more specific aspects of each of these steps.

Schematic Creation Using Schematic Diagram Symbols to Identify Electronic Components

Before you can start the design of the board within the CAD tools, you have to make sure that you have the library parts to work with first. For the schematic this means creating logic symbols for the parts that you will be working with; resistors, capacitors, inductors, connectors, and integrated circuits (IC’s). With these parts ready for use, you can then begin to organize them on the schematic sheets within the CAD tools. Once the parts are roughly placed, you can then draw in the lines of connectivity between the pins of the schematic symbols. These lines are known as nets and they can represent single nets, or groups of nets for memory or data circuits. During the schematic capture process you can move the parts and nets as needed to optimally create the circuitry that is required.

 

If you’d like to, read this to learn more about schematic symbols.

Please look here for more information on the schematic capture process..

Using SPICE with Demo Circuits and Netlists to Simulate the Circuitry

With the parts and nets organized on the schematic, the next step is to verify that the circuit as drawn will work the way you intend for it to. To verify this you will simulate the circuits in a Simulation Program with Integrated Circuit Emphasis tool otherwise known as SPICE. These tools allow PCB engineers to test the circuits that they are designing before building the actual hardware. As such, they can save time and money making these tools an essential part of the PCB design process. 

 

Read this to find out more about analyzing a design using SPICE..

PCB Design Grids, DRCs for Routing Verification Procedures, and Other CAD Tool Setups

The design tools that PCB designers use have many different capabilities. Among those are the ability to set up design rules and constraints that will keep you from touching nets that shouldn’t be connected, and maintaining the correct amount of distance to different objects. You also have many other aids available to you such as design grids that can help you to place components and route traces in a neat and orderly manner. 

 

To find out more about setting up a PCB design database, you can read about design rules and setting up design grids

 

Screenshot of a printed circuit board schematic created with OrCAD Capture

An example of a schematic created using OrCAD Capture

 

Precise Component Placement Layout for Maximum Performance

With your design database set up correctly, and the net connectivity information has been imported from the schematic, you are ready to start the physical layout of the circuit board. The first step here is to place the component footprints within the board outline in the CAD system. Each footprint will have the net connections displayed as a “rubber-band” image to show you which parts that they connect to. You will want to place these parts for their best performance being mindful of what they connect to, areas of excessive heat and electrical noise, and other physical obstructions such as connectors, cables, and mounting hardware. You also need to place the components so that they can be best assembled by the manufacturer.

 

Please look here to learn more about printed circuit board layout component placement.

The Proper PCB Routing Angles Will Connect Your Circuitry Together

With the components placed (although they can be moved as needed), it is time to connect the nets together. This is done by converting the rubber-band net connections into drawn traces and planes. Your CAD tools will have many features that will enable you to do this including some automated routing features that can save you a lot of time. Great care must be taken however when routing to make sure that the nets are the correct length for the signals they are conducting, as well as making sure that they do not cross areas of excessive noise. This can result in cross-talk or other signal integrity problems that may degrade the performance of the built board.

 

Read this for additional information on PCB routing.

Power and Ground Planes, The Importance of a Good PCB Return Current Path

Usually each active component on the board (IC’s and other related components) will need to be connected to power and ground nets. This is easily accomplished by flooding areas or layers with solid planes that these components can tap into. But designing power and ground planes isn’t as simple as it seems. These planes also do the important job of returning the signals that are routed with traces. If the planes have too many holes, cut-outs, or splits in them, it can cause these return paths to create a lot of noise and degrade the performance of the PCB.

 

Read this to learn more about working with power and ground planes in a PCB layout..

Using the Tools for Manufacturing Cost Estimation, and Fabrication and Assembly Drawing Creation

With the component placement, trace routing, and power and ground planes finished, your PCB design is nearly complete. The next step is to run a final rules check on it, and set up the different text and markings that will be silk screened on the exterior layers. This will help others to find components, and label the board with names, dates and copyright information. At the same time you will create drawings that will be used during manufacturing to both fabricate and assemble the final boards. PCB designers also will use their tools to come up with cost estimates on building the board so that there aren’t any surprises later.

 

For more information on preparing your PCB design for manufacturing, you can read here about manufacturing cost estimations and the steps of generating manufacturing drawings.

 

Screenshot of 3D printed circuit board layout using OrCAD PCB Designer

Here is an example of a printed circuit board created with OrCAD PCB Designer

 

Building the Board: Multilayer PCB Fabrication Process and the PCB Soldering Process

At this point, the board is ready to be built, and the first step is to send the output data files to a facility for fabrication. This process includes etching all of the traces and planes onto the different metal layers, and then compressing them all together producing a bare board that is ready for assembly. At the assembly facility, the board is loaded with the components it needs, and then run through different soldering processes depending on the type of components being used. After this the board is inspected and tested, and the final product is ready to be shipped back to you.

 

For more information on the printed circuit board fabrication process, please read here.

Learn more about printed circuit board assembly..

It Takes the Highest Quality PCB Design Tools to Create a High Quality PCB Design

The processes of printed circuit board fabrication and assembly are exact and demanding. To build the board so that its circuitry delivers the required performance means that the manufacturers need precise design data to work from. Not only do the PCB design tools that you use need this level of sophistication and precision, they also must have the features and functionality that you need for creating complex designs. This includes specialized features that help you to route intricate traces for high speed circuits, and design rules that you can easily set up for specific area clearances. It also includes having the best simulation tools available to streamline your schematic capture process as well as providing a wealth of library parts to work with.

The good news is that there are PCB design tools already available to you on the market today that can handle this level of design expertise that we’ve been talking about. An example of a PCB design system that has all of the features and capabilities that we’ve talked about here is from Cadence. OrCAD PCB Designer has the tools and functionality to expertly take your design from concept to final manufacturing files. With OrCAD you have access to libraries, schematic capture and SPICE tools, as well as all of the PCB layout features that you need.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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