While most people understand what circuit boards are, few comprehend how truly complex they are, and fewer are familiar with how they are created. Printed circuit boards have been around for a long time now, and their technology continues to grow as more and more is required of them. Simultaneously, the tools and methodologies used for their design have also evolved in response to the increased levels of circuit complexity now being designed. Yet, despite all of these advancements in PCB design and manufacturing technology, it still takes a PCB designer to lay out the circuit board.
The physical design of a printed circuit board, or “layout” as it is colloquially known, typically has a set pattern of steps that designers must use. And while some of these steps are narrowly focused on an individual layout, such as tool usage or each corporation’s design workflow, some basic global steps must be followed. Here we’ll detail those basic steps in an easy-to-follow PCB layout guide that can help those unfamiliar understand the circuit board design process better.
PCB Layout, What We Can’t Tell You
The first thing we will look at in this PCB layout guide is the design steps we can’t tell you. There isn’t any cause for concern, though, as we are not trying to keep secrets. It’s just that there are many aspects of PCB design that will vary depending on the board layout itself. For instance, consider the following:
- Requirements, specifications, design rules, and constraints: All of these will change from design to design. Power circuits will require one trace width while signal routing will require another, and those widths have a lot of latitude in them depending on the circuit board type.
- Specific layout processes: These processes will change depending on the specifications of the design and your company’s requirements. For instance, some companies may panelize their designs, while others will not.
- PCB CAD tooltips: Each CAD system has features and functionality differentiating it from other systems. How you draw a trace in one system will be completely different in another. Also, CAD systems differ in their base architecture from each other. For instance, one CAD system may require separate padstack elements in the database, while another may use embedded padstack attributes instead.
As you can see, specific details of the PCB layout have to be covered for each design. But we can show you the basic steps in this guide which we will do next.
PCB Layout Guide Tips and Recommendations for Efficient Circuit Board Design
In this list, you can see the basic steps that are important in a PCB layout:
Gather your design data
There is a lot of information that a layout needs before it can be worked on and completed. Designers who start without all the necessary data often risk delays or redesigns as any new data can change the completed work. This information includes the board layer count, stackup configuration, materials, controlled impedance requirements, manufacturing requirements, trace and space widths, via sizes and types, and much more.
Prepare your database
- Ensure the schematic is ready to go with its design rule checks completed and library parts updated to easily synchronize with the layout database.
- Install or create the library elements you will need for the PCB layout. These can include PCB component footprints, mechanical outlines, the board layer stackup, and documentation elements such as drawing formats and title blocks.
- Synchronize the schematic and layout databases to transfer component and net information to the layout.
- Fully configure your design rules and constraints for the technology type of the board being designed.
Floorplan your design
- Make sure to include height restrictions and other critical component placement zones in your rules or on the board outline.
- Organize the components by their function using the schematic, and group them in the layout in blocks of circuitry.
- Position these blocks of circuitry around the board’s perimeter where you can easily access their components for placement.
Component placement
- Start with fixed-location components such as connectors, switches, and other mechanical parts and place them according to specified positions.
- Locate critical components such as processors and memory chips centrally to the board for proximity to their connected components and disperse their heat throughout the board. Make sure to leave enough room for all associated circuitry around them.
- Locate power supply circuitry close enough to the parts they service with their assigned voltages but leave enough space between them for thermal management. It is essential to maintain separation between digital, analog, and power circuitry areas while placing these parts for the best signal integrity.
- Place decoupling capacitors as close as possible to their assigned pins on the critical components.
- Place high-speed components according to their signal paths as designated on the schematic.
- Finish up the placement with the remainder of the parts on the board.
- Remember to leave room between components for escape and bus routing.
Trace routing
- Route decoupling capacitors as tightly as possible to their assigned pins.
- Route power supply nets as short and direct as possible with wide traces to lower impedance and reduce noise in the power circuitry.
- Plan your escape routing and vias from large pin-count devices to open up routing channels on internal board layers.
- Be careful not to block off the clear signal return paths on the ground reference planes with large groups of vias, split planes, or board cutouts.
- Avoid using split ground planes whenever possible, and do not route high-speed signals over plane splits if you use them.
- Plan your bus routing to flow naturally between devices with as few vias as possible.
- Allow room for trace tuning on high-speed traces to match line lengths
- Follow the signal path connectivity in the schematic for high-speed circuitry, and keep those traces as short and direct as possible.
- Make sure that you haven’t left dangling traces or trace loops, as those can behave like antennas and ruin your signal integrity.
Finalize the design
- Conduct a thorough design review to find possible errors before completing the layout.
- Clean up the silkscreen to ensure the readability of its reference designators and other markings.
- Finalize any adjustments required for the solder mask and solder paste layers.
- Add the appropriate documentation text according to your company’s requirements.
- Create the manufacturing drawings.
- Prepare the final manufacturing files, including artwork, drawings, XY location, bill of materials, and any other instructions or readme files.
These basic steps should go a long way in helping to clarify the basic steps when laying out a printed circuit board. However, many other questions and steps will need to be taken care of, and we have a suggestion on where to find the help you need.
Who to Turn to for Additional PCB Design Help
Before you lay out a circuit board, it is always good to meet first with the PCB contract manufacturer building it. This step will give you a chance to plan the layout before you start so you know what design goals you need to accomplish. Here are some of the questions that you should be working on with your PCB CM:
- You need to determine the PCB technology type, board materials, layers, and manufacturing processes so you can input any associated information into your layout. Trying to change the board’s layer configuration in the middle of the design often creates more headaches than it’s worth for the layout designer.
- Plan out the controlled impedance requirements for the routing layers to work with the correct trace widths, copper weight, and spacing values from the start. Many designs have gone through costly re-layouts if this information changes partway through.
- Decide on the most cost-effective via strategy. While larger vias are usually better for both cost and manufacturability, there are many cases where using smaller thru-hole vias or microvias is preferable for space savings and improved signal performance.
At VSE, we have been helping our customers work through questions like this for over 30 years. We understand the challenges you face in PCB layout, and our engineering, procurement, and manufacturing teams are here to give you the help you need.
