Navigating PCB Layout: Best Practices for Engineers

The layout of a PCB is more than just its appearance. A successful PCB layout arranges its circuits physically to achieve optimal electronic performance while also being fully manufacturable. This requires careful management of component libraries, CAD settings and parameters, component placement, routing, and the design of the power distribution network (PDN). Additionally, layout designers must ensure that their work is fully documented and that the final product is ready to be included in its intended electronic system.

There is a lot of work to be done, especially for engineers unfamiliar with the PCB layout process. To help with this workflow, it’s advisable to have a comprehensive set of PCB layout guidelines for reference. Industry and company standards will dictate the details of the design, but layout guidelines are essential for helping engineers navigate the PCB development process from start to finish. Here are some basic PCB design layout guidelines that can be used to formulate your own PCB development guide.

Before Starting PCB Layout

Several tasks need to be addressed before the layout process begins to ensure a successful design. One of the first tasks is to establish the PCB footprint library to be used.

When building your PCB layout library, it is crucial to use industry standards (such as IPC) or manufacturer specifications for package sizes and dimensions. However, personal, company, or technical requirements may also dictate variations for certain parts. For example, packages in RF designs may require smaller pad sizes than standard digital designs. Here are some additional guidelines for building your own PCB component footprint library:

• Ensure that any library components you build have acceptable pad pattern sizes with spacing that complies with the component’s standards.
• PCB footprints should include all necessary elements, such as part outlines, silkscreen markings, and reference designators.
• A good rule of thumb is to ensure that your manufacturer can build the parts you are designing before submitting them to the final design.

Another option is to use PCB footprints from external CAD library providers. Part manufacturers often build their own components for your design system in advance, and some tools have browsers for conveniently downloading these parts.

PCB Outline and Layer Stackup

Before beginning the PCB layout, you need to work with mechanical designers to obtain a good outline shape. While design factors can be changed later, any changes may force a significant redesign of the circuit to accommodate the new shape. Additionally, most CAD tools will accept data imported from mechanical design systems, making your work easier. However, even with imported data, you still need to ensure that the PCB outline is correct and contains all necessary CAD elements required for your design, such as keep-out zones.

The layer stackup should also be completed before the layout begins. Again, these can be changed later, but the potential impact on existing circuits could disrupt your design schedule and budget. The layer stackup should also be fine-tuned for your specific design to ensure the correct layer configuration is provided for impedance-controlled routing and other signal integrity requirements. Choosing the PCB material at this stage is also critical so that appropriate trace widths and other design calculations can be made based on the physical properties of the material. These properties include dielectric constant, insulation quality, moisture absorption rate, and dissipation factor.

CAD Parameters and Settings

It is not uncommon to find designers working with their CAD systems’ default settings. However, most CAD systems provide users with extensive control over colors, fill patterns, shading, as well as font sizes and widths. You can also change the display of certain objects, place one design element above another, set grids, and specify layout and routing preferences. These settings are designed to enhance your work efficiency, and you can save time by optimizing the settings in advance.

Component Placement Guidelines

Once the CAD libraries, PCB outline, and other setup tasks are completed, the design can begin. The first step in this process is placing the PCB component footprints on the board. Placing components on the board must meet three main requirements: circuit performance, manufacturability, and accessibility.

Circuit Performance

High-speed circuits need their components to be placed as close together as possible to achieve short, direct signal paths, but they are not the only components with this requirement. Analog circuits and power components also need to be placed so that their sensitive or high-current paths are as short as possible. This helps reduce inductance and improves signal and power integrity. However, in some cases, these components may need to be separated to accommodate bus routing or thermal isolation.

Manufacturability

To minimize production costs as much as possible, it is important to place components in a way that is as easy to manufacture as possible. For example, components that are too close to each other may not be able to be assembled automatically or may be difficult to solder with automated soldering processes. Higher chip components will produce a shadow effect when wave soldering is done in smaller parts in front of them, resulting in poor solder connections. Unbalanced copper between two solder pads of small chip components will produce uneven heating, causing one solder pad to melt before the other, pulling the other side up.

Accessibility

Circuit boards typically need to be manually tested and reworked, which requires access to the components that need to be addressed. If larger components obscure these components, it may make their work more time-consuming or cause incidental damage to adjacent components. Similarly, connectors, switches, and other human-machine interfaces that are inaccessible can also slow down the production of the circuit board.

A key guiding principle is that the layout should start with a basic plan view of the components on the development board. This will allow you to devise strategies for how to divide the different circuit areas on the board to avoid overlap of analog and digital signals.

PCB Layout Guidelines: Effective component placement leads to routing

PCB Routing Guidelines

For circuit board designers, laying out their circuit boards to create optimal signal and power integrity is crucial. Components should be placed in optimal positions to achieve short, direct routing. At the same time, the PCB must be laid out so that all networks can be fully routed. Balancing these requirements can be quite a challenge in high-density designs. The first PCB design layout guideline is to set up design rules and constraints for routing.

Design Rules and Constraints

Technically, configuring design rules and constraints should be included in parameters and settings. However, since most rules directly apply to track routing, we include them here. Rules and constraints are used to control track widths and spacings and can be set for individual nets, groups of nets called net classes, or as default settings for all non-specified nets. Design rules are also used to control which vias are selected for different nets, routing lengths, and matching lengths, and which layers are allowed for routing specific nets and routing topologies. Additionally, design rules are used to control component spacing, silkscreen rules, mechanical clearances, and many other constraints.

Signal and Power Integrity

To achieve optimal performance and signal integrity, PCB layout designers need to follow specific requirements for routing their circuits. Here, design rules and constraints will help—allowing designers to input physical routing parameters into CAD systems for routing. While exact values will vary based on the needs of the PCB, designers typically set rules to ensure the following guidelines are followed:

• Routes for high-speed transmissions should be short and direct.
• Track widths, spacings, and allowed layers for controlled impedance routing.
• Specified track lengths and length tolerances for matched length routing.
• Widths and spacings for differential pairs.
• Widths and spacings for sensitive signals like clocks and controls.
• Via types for different nets.
• Track widths and spacings for analog circuits.
• Track widths and copper weights for high-current power circuits.

Another important guideline to remember is to avoid crossing areas between analog and digital routes when routing in mixed-signal designs.

Effective Power and Ground Plane Guidelines

For modern high-speed designs, the optimal grounding strategy is often to use one or more continuous ground layers on inner layers. This provides excellent EMI protection and ensures clear signal paths, which will improve overall signal integrity. For areas where ground plane discontinuities occur due to unique PCB outlines or features, routing should be avoided in any ground gaps on the board. If there are no continuous and adjacent ground planes to serve as clear return paths for signals, your design may generate a lot of unwanted noise. Here are some power and ground plane guidelines to keep in mind:

• Ground layers need to be adjacent to signal layers in a PCB stackup with high-speed routing. This will help shield high-speed routing from interference and provide a good reference plane for signal return paths.
• Use thermal pads and carefully manage power and ground connections to the plane. The pad radius of the cushion strip must be wide enough to carry high current while eliminating the opportunity for these connections to act as radiators.
• Plan power connections and split power planes carefully to ensure that power is delivered sufficiently to all connected components throughout the PCB.

Avoid routing analog and digital circuits simultaneously in mixed-signal designs

Silkscreen and PCB Testing Guidelines

Once the PCB design is complete, it’s time to focus on finalizing the layout by cleaning up the silkscreen layer and adding test points. Reference marks, part numbers, and other company information are marked on the PCB with ink through the silkscreen process. Designers typically use the “silkscreen print” layer in their CAD system to design these marks.

To ensure that silkscreen layer marks are legible, designers follow these guidelines:

• Line widths should not be less than 4 mils.
• Font sizes should not be less than 24 mils.

Component reference designators should be renamed based on the company’s grid pattern to help locate specific parts on the circuit board. Move and rotate reference marks to make them easy to read. Include polarity and pin one markings where necessary.

Test points are crucial for circuit boards intended for mass production for automated assembly verification. Each network in the design should have a test point, whether it is an existing through-hole pin, via, or added surface mount test pad. Test points should be at least 50 mils away from other objects on the PCB, such as components or pads, and at least 100 mils away from the edge of the PCB. However, these values may vary by supplier, so be sure to check what the manufacturer’s test point requirements are.