While most people understand circuit boards, only some comprehend how complex they are, and even fewer are familiar with their manufacturing process. Printed circuit boards have been around for a long time, and their technology continues to grow as more and more is required. Simultaneously, the tools and methodologies used for their design have also evolved in response to the increased circuit complexity now being designed. Yet, despite 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. While some of these steps narrowly focus on an individual layout, such as tool usage or each corporation’s design workflow, some basic global steps are necessary. Here, we’ll detail those basic steps in an easy-to-follow PCB layout best practices that can help those unfamiliar understand the circuit board design process better.
|The closer a design adheres to technological precision and general manufacturability, the better the production in terms of time, cost, yield, and quality.
|Boards with poor signal integrity are more susceptible to electromagnetic interference (both to and from nearby devices); electromagnetic compliance is expensive, and manufacturers will generally not submit a board for testing with a chance of failure.
|Reliability often comes down to manufacturing tolerances/precision and manufacturing materials. However, designers can assist the process with a layout that supports passive thermal routing (thermal vias and copper pours for power circuits, wider traces for power nets, etc.)
Where PCB Layout Varies
The first thing we will look at in this PCB layout guide is the design steps a CM can’t answer universally. There isn’t any cause for concern, though, as many aspects of PCB design will vary depending on the board layout. For instance, consider the following:
- Requirements, specifications, design rules, and constraints will change from design to design. Power circuits will require one trace width, while signal routing will need another, and those widths have a lot of latitude depending on the circuit board type.
- Specific layout processes vary depending on the design specifications and your company’s requirements. For instance, some companies may panelize their designs, while others will not.
- PCB CAD tooltips have features and functionality that differentiate them from others. Drawing 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.
The design must account for specific PCB layout details, but the basic guidelines remain constant.
PCB Layout Best Practices: Start to Finish
Although the details may change, the general workflow of PCB development does not alter board-to-board. The following list highlights the steps designers should expect to encounter for every board:
Collating Design Data
There is a lot of information that a layout needs before placing a component, trace, or via. Designers who start without all the necessary data often risk delays or redesigns, as any new data requires altering the finished 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.
- Ensure the schematic is ready with its design rule checks completed and library parts updated to synchronize with the layout database easily.
- Install or create the library elements necessary for the PCB layout. These 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 the design rules and constraints for the board’s technology type.
- Include height restrictions and other critical component placement zones in the 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 for easy placement accessibility.
- 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. Maintaining separation between digital, analog, and power circuitry areas is essential while placing these parts for the best signal integrity.
- Place decoupling capacitors close to their assigned pins on the critical components. Prioritize the placement of the smaller decoupler capacitors over any larger farad capacitors.
- 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.
- 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 the 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 signals over plane splits.
- Plan 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.
- Ensure there are no dangling traces or trace loops, which can behave like antennas and ruin signal integrity.
Document Preparation and Output
- 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 company 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.
Your CM Can Optimize Your Board for DFM
PCB layout best practices should always begin by speaking with the PCB contract manufacturer building the project. This step lets the designer plan the layout to comply with manufacturing precision. Here are some of the questions you should be working on with your PCB CM:
- Determine the PCB technology type, board materials, layers, and manufacturing processes to input any associated information into the 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 revisions 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 through-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.