There are a lot of undesirable things that can happen if you allow yourself to be distracted from your goal. Many years ago, I took my sons and a group of their friends out for some water skiing on the main river in our area. We usually went skiing on a smaller, slower river, and, therefore, I wasn’t familiar with the pull of this river’s current. While idling the boat to pull in a skier, I neglected to notice how swiftly we were drifting toward some yachts that were anchored nearby. Fortunately, we didn’t hit them, but we did disrupt their peaceful day of fishing, and they weren’t shy about voicing their annoyance.
When you are laying out a PCB design, it is also easy to become distracted by one or two aspects of your board—whether it be electrical performance needs that affect component placement and routing, thermal power and delivery concerns, or your design’s manufacturability—that you unknowingly make a layout mistake.
Here, we’d like to break each of these aspects down to help you avoid some of the most common PCB layout mistakes, as well as offer insight into how your PCB contract manufacturer should help along the way.
Keeping best practices in mind in all phases of the design process can help avoid PCB layout mistakes.
Component Problems That Can Lead to PCB Layout Mistakes
Before a component is ever placed or routed in a design, there are details that must be considered upfront to avoid problems later on in the layout. The first of these is to examine your bill of materials (BOM) for problems such as:
- Parts that are expensive or in short supply: These should be replaced or designed out to avoid schedule and budget problems.
- Parts that are not recommended for use: There are various reasons why a component may not be recommended for new designs. It may be discontinued or replaced in the near future, for example. These components should be replaced on your BOM, as well.
- Parts that are end-of-life (EOL): Sometimes, you might use components that are familiar to you, without realizing that the part is no longer available. These will also need to be replaced.
In addition, the following should also be checked:
- Your design’s library of components: This must also be verified as being correct. If the footprint models are out of date or incorrect, it could result in a layout that has to be corrected or redesigned before manufacturing.
- Mechanical features and dimensions of the board: Last-minute changes to the mechanical design of the system enclosure aren’t always updated to the layout. If they aren’t caught, it could result in another redesign.
- Board layer stackup configuration and materials: These must be confirmed by the CM as being manufacturable.
- Mechanical part placement: It’s advisable to look at the designated placement locations of the mechanical parts of the board. Connectors, switches, and other mechanical components can create problems if they aren’t placed conveniently for users to access after the board is built. Since the location of these parts usually drives the rest of the component placement on the board, it is vital that they be correct.
Many of these points are important to know as you’re creating your design and prepping it for manufacturing. Mitigating these issues early helps set you up for proper component placement and routing.
Component Placement and Routing Problems to Avoid
Component placement may be the most critical part of the PCB layout. Everything from how the board will perform to the ease and cost of its manufacturing is all based on how well the components are placed. Here are some ideas to keep in mind while you are placing parts:
- Component spacing: Parts that are too close to the edge can cause problems for automated pick and place equipment. Parts that are too close to each other can cause assembly and rework problems, as well as be susceptible to solder bridging.
- Component alignments and rotation: Parts that are not aligned correctly for production soldering processes can end up with poor solder joints.
- Component location for signal paths: For the best signal performance, parts need to be placed close together, according to their signal path, as laid out in the schematic.
- Component thermal issues: Some parts run hotter than others, requiring that they are placed in the best location for airflow and away from other hot components.
- Trace routing: Your parts need to be spaced out enough to ensure that you have the room you need for escape routing out of large dense parts and for regular bus routing throughout the board.
With a good component placement, the layout will lend itself to good trace routing. There are still some preferred routing techniques that should be followed, however, to avoid potential problems:
- Signal integrity: High-speed transmission lines that are not routed with adequate spacing to other traces can generate impedance.
- Signal path: Traces that connect high-speed signal paths should be as short as possible and not wander.
- Differential pairs: These pairs need to be routed together, even through congested areas of the board.
- Antennas: Care needs to be taken that trace stubs or partially deleted lines are removed so that they don’t inadvertently create antennas.
- Power planes: Not only do all parts need adequate power and ground, but good signal integrity requires an adequate return path through careful placement of the ground planes.
Post-Design Problems That Can Hinder the Layout
As a PCB designer, you may think your job is done once the placement and routing are completed, but there’s still work to do before the board can be manufactured:
- Design for test (DFT): Ensuring that your board is testable is actually something that should happen from the beginning. Your placement and routing must be planned out for the correct amount of test points and clearances for test fixtures.
- Silkscreen: Reference designators, pin numbers, and other important identifiers need to be clear of pins and positioned so that they can be read and understood.
- Markings: Board name, part numbers, company names, and other markings will also need to be included.
- Drawings: Fabrication and assembly drawings with all the necessary information for manufacturing need to be created.
- Output files: All image files, drawings, reports, and any other files need to be correctly packaged up to be sent to your manufacturer.
Clearly, there is a lot to think about to ensure your design is ready for manufacturing. And, as mentioned, it’s easy for a designer to get distracted with one aspect of the layout and perhaps not account for all the others that will make the layout successful. That’s why it’s critical to partner with a CM that can be your ally in helping you understand all of the requirements for your PCB layout.
Enlist the Help of Your CM to Avoid PCB Layout Mistakes
Your CM wants you to be successful in laying out your board so that they can be successful in building it. To that end, they should be happy to answer your questions and make some valuable resources available to you for your help:
- Board materials and layer stackup recommendations: When you take your design specifications to your CM, they should advise you on the best materials and layer stackup configurations needed for the job.
- BOM review: Your CM should look over your BOM and identify any components that should be replaced for EOL or other issues.
- Full design for manufacturing check: Your CM should also review your board’s layout for manufacturability, as well as provide suggestions for any needed changes.
- Engineering expertise: Whether it requires design changes or creating text fixtures, your CM should have the engineering staff on hand to optimize your board’s layout for completion.
At VSE, we have all of these resources and more standing by and ready to help. Our engineering and manufacturing teams will partner with you to ensure any PCB layout issues are resolved so that we can get your successfully built board back in your hands as quickly as possible.
For a CM with the engineering expertise to ensure your layout results in a PCB assembly that is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.
