PCB Design Basics for Engineers
Good design practices will ensure that your design can be produced at high volume and works at high speeds, regardless of whether you are designing a printed circuit board or moving at high speeds. This guide contains essential PCB design guidelines for modern circuit boards. While some specialty designs might need to adhere to additional layout guidelines, the PCB design guidelines presented here are an excellent place to start.
These guidelines are designed to help you with routing and manufacturability and fundamental signal integrity and assembly.
- Definition of design rules to ensure fabrication and assembly yield
- Component placement is where the goal of component placement is to ensure solvability as well as ease of routing
- To avoid routing all over the board, group components by type.
- The location of ground and power in the PCB stackup, including some points for mixed-signal PCB layouts.
- Respecting mechanical constraints such as enclosure limits and connector locations
#1 - Before you layout your PCB design rules, determine the following:
It can be easy to forget the design rules that will guide your printed circuit board project when starting a new design. It is possible to eliminate component shifting and rerouting by establishing simple clearances early in your design. Where can you find this information?
Talk to your PCB manufacturing house first. Good fabricators will often post their capabilities online or provide this information in a document. Send them an email to inquire about their capabilities if it isn't in a prominent location on their site. This is a good idea before you begin placing components. While you are at it, submit your stackup for review. Or look up their standard stackup data to use. 911EDA can support PCB manufacturing.
After you have compiled their capabilities list, you can compare them to the industry reliability standards that you will use (Class 2 vs. Class 3 or a certain standard). These can be encoded into your design rules. After you have determined these points, you will need to choose the most conservative design layout limits required for manufacturability or reliability.
Your design rules will guide you through the layout process and help eliminate any design mistakes that could lead to assembly or fabrication problems. Once you have established the design rules, it is time to start the placement process.
#2 - Fine-Tuning Your Component Placement
Component placement is a critical stage in your PCB design process. It requires a strategic assessment of all the available real estate on your board. Component placement is about creating a board that is easy to route, with a few layers transitions as possible. The design must adhere to the design rules and meet the requirements for component placements. Although these points can be hard to balance, a simple process can help designers place components that satisfy these requirements.
- Use Must-have components should be placed first. Sometimes components must be located in particular locations due to mechanical enclosure restrictions or their size. These components should be placed first and then locked in place before moving on to the next part of the layout.
- Use large processors and integrated circuits. Connect high-pin count ICs to other components in your design. This makes it easier to trace the layout of PCBs by centrally locating these components.
- Avoid crossing nets. Unrouted nets can be seen when components are placed in the PCB Layout. PCB designers should minimize crossing nets. Each intersection of nets will require a layer transition via vias. It will be easier for you to create the best routing guidelines for your PCB layout if you can eliminate net cross-sections through creative component placement.
- The SMDPCB design rules. It is recommended that all surface-mount devices (SMDs) be placed on the same side. This is because each board side will need its own SMD soldering line. Therefore, it's best to place all SMDs on the same side.
- Play with orientation. Rotate components to eliminate intersections. This can simplify routing by orienting connected pads to face one another.
It's easier to arrange the rest of your board if you follow points 1 and 2. Your board will also have a modern appearance and feel. A central processor provides data to all components around the board's perimeter.
#3 - Using Your Power, Ground, & Signal Traces
Now that all the components are in place, it is time to route power, ground, and signal traces. This will ensure that signals follow a clear and trouble-free path. These are some tips to help you navigate this stage of the layout process.
Where to place power and ground planes
It is common for power and ground to be placed on two layers within an internal layer. This might not work for 2-layer boards. You would place a large ground plane on one of the layers and then route signals or power traces to the second layer. Ground planes are better than trying to route ground traces. If components require direct power connections, it is recommended that you use common rails. Common rails are fine for components with a minimum of 100 mils.
Some guidelines state that plane layer placement should be symmetrical. However, this is not a requirement for manufacturing. This might be necessary for large boards to minimize warping. However, it is not required for smaller boards. First, focus on power and ground. Next, ensure all traces have a strong return path coupling with the nearest ground plane. Then, worry about perfect symmetry within the PCB stackup.
Routing Guidelines for PCB Layouts
Next, connect your signal traces to match your schematic. PCB layout best practices suggest that components should be placed as often as possible between each other. However, this may not always work on larger boards. If component placement requires horizontal trace routing, route the traces vertically to the other side. This is just one of the many important PCB design rules.
As the stackup grows in number, printed circuit board layout guidelines and rules for designing printed circuit boards become more complicated. Many PCB best practices commonly used in complex boards do not apply to specialized applications. You will need to design your PCB according to your specific application. If you do not separate the signal layers with a reference plane, your routing strategy will require you to use alternating horizontal and vertical lines in alternating layers.
Determining Trace Widths
Layouts of PCBs require traces to connect the components. But how wide should these trace widths be? Three factors influence the width of trace needed for different nets:
- Manufacturing. Traces can't be too thin, or they won't be able to be manufactured reliably. In most cases, you will need to work with trace widths greater than what your fabricator can produce.
- Current. This determines the minimum width required to keep the trace from heating up. The trace must be larger if the current is greater.
- Impedance. High-speed digital signals or radio waves will need a specified trace width to achieve the desired impedance value. This does not apply to all signals and nets. Therefore, you don't have to set impedance controls on every net in your design guidelines.
A trace width of 10 mils is sufficient for most low-current analog and digital signals. It may be necessary to make printed circuit board traces more than 0.3 A wider. You can check this by using the IPC-2152 nomograph. This will determine the PCB trace width required for current and temperature rise limits.
Connectors to planes for thermal relief of through-hole components
The ground plane acts as a heat sink and heats evenly across the board. If a via is connected to a grounded plane, the PCB designer will remove the thermal relief pads from that via to allow heat to flow to the ground plane. This will keep heat from escaping to the surface. If you use wave soldering to attach through-hole components to the board, this can be a problem. It would help if you had heat to stay near the surface.
The thermal relief feature of a PCB layout is required to ensure that a board can be manufactured in a wave soldering process. This refers to through-hole components connecting directly to planes. It can be challenging for process temperatures to be maintained when a through-hole via is connected directly to a plan. Therefore, thermal reliefs are recommended. Thermal relief slows down heat dissipation into the plane during soldering to prevent cold joints.
Designers will often tell you to use thermal relief patterns for any via or hole connected to an internal power or ground plane. This advice is often too general. It is essential to ask your fabricator for guidance before placing your board in production.
#4 - Keep Things Separate
You can find routing guidelines for PCB that will help you group and separate components and trace so that your routing is easy and prevents electrical interference. These guidelines are also helpful for thermal management, as you may need to separate high-power parts.
The PCB layout is a process of designing and laying out circuitry to make it easy to connect them with traces. It is best to group components in one place in the PCB layout. Because they could be part of a circuit and may only connect to one another, it is unnecessary to place them on different sides or areas.
Many layouts will have both analog and digital components. You should ensure that the digital components don't interfere with the analog components. This was the way it was done in decades past. However, modern design does not allow for this. This is an outdated design choice that can lead to EMI.
Instead, place a complete ground plan below your components. Please do not break it up into pieces. You can keep the analog components together with other analog components that operate at the same frequency. Keep the digital components together with other digital parts. This could be viewed as each component taking up a different area in the PCB layout. However, the ground plane should remain the same in most designs.
Separation of High Power Components
Separating components that produce a lot of heat on the board is also a good idea—separating high-power components on a PCB design can even out the temperature and avoid creating hotspots where high-temperature components are clustered together. You can do this by first looking at the "thermal resist" ratings for your component's datasheet and then calculating the temperature increase from the calculated heat dissipation. Engineers can use heatsinks or cooling fans to keep components' temperatures down. It can be challenging to balance the placement of these parts with keeping trace lengths small when designing a routing strategy.
#5 - Enhancing Your PCB Design
As you try to put together your final pieces for manufacturing, it's easy to become overwhelmed. It can make all the difference in whether your manufacturing project is a success or failure by double- and triple-checking.
It's a good idea to check your Electrical Rules (ERC) and your Design Rules (DRC) to ensure you have met all your constraints. These two systems allow you to easily define gaps, trace widths, and standard manufacturing requirements. This will enable you to automate PCB design review guidelines and validate your layout.
Many design processes require that you run design rule checks at every stage of the design process. This is to ensure that the manufacturing process goes smoothly. Using the right software to design, you can perform checks during the entire design process. This allows you to quickly identify potential design problems and fix them before they become serious. After your final ERC or DRC has produced no errors, you can check each signal's routing and verify that nothing is missing. You can also run through the schematic one wire at a time to confirm your findings.
Our top PCB design tips can be applied to all circuit board designs. Although short in number, these guidelines can be a great starting point for designing a functional and manufacturable circuit board. These PCB design guidelines are only a tiny part of the overall picture. However, they provide a solid foundation to build upon and continue improving your design processes.