PCB Design Basics for New Designers

Let’s set the scene: you’ve just installed a new PCB design program and are ready to start your first PCB layout. Where should you begin? Most engineers are comfortable with handling schematics and selecting core components, but PCB layout newcomers might find the process overwhelming. PCB design software comes with a plethora of tools and follows specific workflows, but beginners can complete the design process by understanding some fundamental PCB design principles.

In this article, I will introduce some essential design principles that every new designer should follow to ensure a successful design process. Some of these points may challenge traditional views on how circuit boards are constructed, but they are aimed at helping balance low-noise signals, manufacturability, and ease of layout.

 

Basics of Starting a New PCB Design

The PCB design process follows a standard workflow, and it is crucial for any new designer to understand how this workflow unfolds. The standard workflow for PCB design focuses on performing the engineering design, completing the physical PCB layout, and preparing the final design for manufacturing.

Understanding the PCB Design Workflow

Before moving on to the next steps, it is beneficial to take some time to briefly familiarize yourself with the standard workflow, so please read the link above. The design workflow proceeds according to the following list of steps:

Select Core Components: Choose core components that are in stock and available from major distributors.

Create and Review Schematic: Create the schematic according to engineering requirements and review it.

Create Blank PCB: Create a blank PCB, build the stack-up, and establish design rules.

Import Component Data: Import the component data from the schematic into the new PCB layout.

Place Components: Place the components and review their placement to ensure they meet engineering requirements.

Route Traces: Route traces and connections between components.

Clean Up Layout: Clean up the PCB layout and perform a final design review.

Prepare Design Output: Prepare the design output and send it for manufacturing.

After gaining a good understanding of the design process, read the link below to learn how the PCB design workflow unfolds and the standard set of tasks each designer must perform to complete a new circuit board.

 

Schematic and Layout Must Be Synchronized

If you are just beginning your journey as a PCB designer, you are likely creating schematics for your device. Whenever changes are made to the design, you must synchronize these changes between the schematic and the layout. Specifically, this includes any of the following design changes:

Adding, removing, or swapping components

Adding, removing, or changing nets

Grouping nets into net class objects

Changing component parameter information (part numbers, supplier information, etc.)

Any other electrical rules or directives applied to objects in the schematic

After making any changes, they must be applied to the PCB layout. This can be done using the import functionality in PCB design software. This ensures that everything in the design is synchronized, and the design rule engine in the software will correctly read your design data.

Each Object in the Schematic Must Be Linked to an Object in the PCB Layout

 

To maintain this synchronization, ensure that component edits are made in the schematic and then imported into the PCB layout, not the other way around.

 

Understanding the PCB Manufacturing Process

Any design you intend to produce as a physical product should be scalable for mass manufacturing. ECAD software allows you to implement any conceivable design feature in your PCB layout. However, your design decisions need to be manufacturable within standard processes, which means designers need to be familiar with the manufacturing constraints of PCBs.

This means every PCB designer should take the time to learn the basics of PCB manufacturing to ensure their designs are fully manufacturable. This involves understanding an important set of practices that should be included in the design process, known as Design for Manufacturability (DFM). Take some time to learn about some common design mistakes that could derail manufacturing and cause your boards to be sent back for redesign before production.

 

Design Rules Are Key to Success

Understanding the manufacturing process and some of the standard capabilities of PCB manufacturers will make it easier to comprehend the restrictions imposed by design rules. Most DFM issues are related to the dimensions of copper features or the gaps between different features in a PCB. The default design rules programmed in PCB design software are generally somewhat conservative, and you should be aware of the limits when violating these design rules.

For example, consider the following package. The pad-to-pad spacing of this package is approximately 9 mils. However, when the component is transferred to the PCB layout, it triggers a design rule error (shown in green). You can easily violate this design rule and set the limit lower, as most manufacturers require a minimum spacing of around 5 mils. Before disregarding design rules, be sure to check the gap requirements on the manufacturer's capability sheet.

 

The default minimum gap might be too large, and importing components might violate the design rules. Ensure that the gap is set to the correct value in the design rules.

 

Using Ground Planes

Every PCB needs a clear ground connection to ensure that all components in the design can form a complete circuit and be powered throughout the design. The ground can be connected to a battery on the board or an external power source. Regardless of how the connection is made, it needs to be provided to all other components in the PCB. By far, the simplest method is to use a ground plane, where one layer in the PCB is dedicated to ground and is covered with a large copper plane.

I am always surprised to see many new designers hesitant to place ground planes in their PCB stackup. Many fundamental noise problems affecting digital and analog signals can be traced back to a lack of ground planes in the device. In modern PCBs, the standard guideline is to use solid ground planes throughout the device. In some exceptions, split planes or star grounding on a PCB are appropriate, but these methods are not suitable for the vast majority of digital and analog designs.

Complete Placement Before Routing

After transferring the schematic data to the new PCB layout, you must place the components around the board. At this point in the design, your goal is to place components to ensure design solvability, meaning it can be easily routed. Therefore, it is best to avoid any routing before placing and approving the placement of all components. If you route before placing all components, you will inevitably have to change the routing. Try to minimize the number of net crossings in your layout before doing any routing.

 

In this example design, the goal of component placement is to eliminate crossover lines.

Routing the PCB

Once all the placement work is done and finalized, you can begin routing the PCB. If you have followed the advice above and reviewed the design rules before starting the PCB layout, you are more likely to route the design successfully. When we use the term "success" to describe routing, we typically refer to the following aspects:

Minimize the Need for New Signal Layers: Aim to route all signals without adding extra signal layers.

Reduce Inter-Layer Transitions: Minimize the use of vias, as they can add impedance and potentially create signal integrity issues.

Minimize Noise: Achieve this by using ground planes in the PCB stackup to provide a return path for signals and reduce electromagnetic interference (EMI).

Keep Traces Short and Direct: Wherever possible, ensure traces are as short and direct as they can be to reduce resistance, inductance, and potential signal degradation.

This list of guidelines is not exhaustive, but they apply to most of the signals you will route throughout the PCB. High-speed, low-speed, low-frequency analog, and RF designs all benefit from these PCB routing techniques, so get accustomed to implementing these same practices in your PCB layout.

 

Don't forget: Your goal is production!

Once all components in the layout are routed and finalized, your work isn't finished yet. As a designer, you need to create manufacturing files based on the PCB layout. PCB design software includes tools to automatically generate these output files. Even after preparing the outputs, you should still review these files to ensure no errors occurred in applying settings from the output file exporter. Take some time to review everything before sending it to the manufacturer.

 

Gerber files are the standard format used to create manufacturing tools for producing new PCBs.

 

Next steps

Congratulations! You've completed the design phase, and now you'll move into the manufacturing phase. Your manufacturer will review the design to ensure it meets their manufacturing capabilities. If everything passes the final Design for Manufacturability (DFM) review, your circuit board will proceed to the production and assembly stage.