A PCB Layout Review Checklist Gets You to Manufacturing Quickly

PCB designers can ensure their own successful manufacturing run when they keep a PCB layout review checklist. Once the final design checks and sign-offs are complete, your design is ready to hit the fabrication line. However, you can get through manufacturing quicker and prevent any redesigns if you know what your manufacturer will check in your design.

The right time to locate the important DFM requirements and create design rules for your PCB layout is before any of the components are placed. If you did some homework before and during design, you’ve probably implemented the best DFM practices and provided plenty of documentation for your manufacturer. Let’s look at some of the common points you should examine in a PCB layout review so that your manufacturer does not derail your build schedule with technical questions.

A Complete DFM and Requirements Analysis

Your goal in a PCB layout review is to go a step beyond a DFM review; it is to ensure a comprehensive review against product requirements as well as manufacturability constraints. Manufacturability does not guarantee you conform to all product requirements, and similarly, complying with your product’s requirements list does not automatically align your design with manufacturing capabilities. The PCB layout is where these two groups of constraints meet and, sometimes, interfere with each other.

In addition to basic DFM and product requirements, some PCBs have their own unique DFM requirements that must be checked as part of a PCB layout review. These are particular to the design type in question and do not apply to all PCB layouts. Ideally, these should be defined as design rules early in the process to ensure conformance with all manufacturing requirements, but some of these requirements cannot actually be defined as a design rule in ECAD software.

DFM Review Points for All Designs

First, let’s look at the most common review points that are relevant for any PCB layout and should be part of a PCB layout review. These are listed below.

• Trace and space: The most basic constraint is copper feature sizes that can be etched into a PCB layout. These will differ on inner and outer layers.
• Component spacing: While typically limited by copper pad size, some component bodies extend outside the copper pad boundaries and can end up being placed too close together, causing problems for automated pick-and-place machines.
• Drill bit sizes: It’s best to consolidate to fewer drill bit sizes where possible, and those drill bits must obey the aspect ratio limitations imposed by the fabrication house.
• Drill clearances: Plated and non-plated holes require certain clearances from copper and other drills to account for positional and diametrical tolerances.
• Missing or thin solder mask between pads: This can result in bridging during reflow soldering, which can short two pads or pins on a component. This is one common mistake that can be spotted by simply turning on your solder mask layer during a final review.
• Thermal reliefs: Missing or unnecessary thermal reliefs might be present on some pads because there is a setting override in the PCB layout, or they were defined in the associated PCB library.

The above list would best be considered a “basic” surface-level review of the PCB layout, and it is certainly applicable to all designs. Before looking at specific types of designs, such as HDI or rigid-flex, there are some more advanced checks that can be seen as applicable to every design but may still be rubber-stamped in simpler boards. Some of these deeper aspects of PCB layout review arise in high-speed designs, bottom-terminated parts, dense via/pin fields, or multi-board designs.

Many of these items cannot be evaluated independently because changing one fabrication parameter can change several layout constraints at once. Copper weight can change allowable trace spacing, while stackup changes can affect impedance, hole structures, and finished thickness. The best PCB layout review therefore compares the complete design against an approved fabrication capability set rather than checking individual rules in isolation.

HDI-Specific PCB Layout Review Items

HDI designs carry additional constraints primarily related to the use of blind and buried vias. These could be mechanically drilled vias with a stackup built using sub-laminations, or these designs can use laser-drilled microvias, or both. The layout review must then incorporate the additional via and copper etch constraints as they relate to:

• Dielectric layer thickness
• Stacking microvias vs. staggering microvias
• Annular rings on laser-drilled microvias
• How clearances/feature sizes are impacted by copper weight
• How clearances differ for mechanical vias vs. laser-drilled vias

The constraint definition for these items in an HDI PCB is entirely driven by the stackup design. The stackup design needs to be approved by a fabricator and must align with capabilities before design rules can be developed to account for capability constraints. Once the stackup is approved, these fabrication limits can be translated into PCB design rules and used as the baseline for the final HDI layout review.

Rigid-Flex-Specific PCB Layout Review Items

Rigid-flex designs are also often PCB stackup driven, but for different reasons. The flex layers can have bending constraints imposed on them based on the design of the stackup, where rigid and flex regions will interface or border each other, and any additional mechanical fixation that may be used in these designs.

Discrepancies Between Your BOM, Your Netlist, and Your Schematic

One of the most common errors seen during a PCB design review is a discrepancy between these three pieces of documentation. This is common when multiple designers collaborate on a new project, and frequent changes are made by multiple parties. Eventually, someone forgets to synchronize something or compile the project, an old BOM/netlist gets marked as the newest BOM/netlist, reference designators get changed without being synchronized to the BOM/netlist, or some other mistake occurs.

This is understandable, especially if your team is trying to get to the production line quickly. However, it is also preventable if you use the right design software. Sub-standard design tools force you to manually compile a BOM, or they force you to manually execute synchronization between your BOM, netlist, schematics, and PCB layout. Eventually, someone forgets to do this, and this can cause incorrect information to be sent to the fabricators.

The best design software for ensuring all your design, assembly, and component information remains synchronized, works in a unified design environment. In this type of environment, the underlying design engine automatically synchronizes your design data across your schematic, layout, BOM, Gerber files, and any other documentation required for production.

Non-DFM Requirements Definition and Review

Product-specific requirements often sit outside the normal DFM rule set. A PCB might need defined connector locations, restricted component heights, specific creepage distances, test-point access, thermal limits, isolation zones, or mechanical keepouts. These constraints can affect the PCB layout even though they do not originate from the fabrication process.

Requirements Portal provides a structured location for defining these requirements and linking them to the corresponding hardware project. Requirements can be organized, assigned, tracked, and connected to design information rather than remaining isolated in spreadsheets or specification documents.

Once the project is connected, requirements can be accessed from the Requirements panel in Altium Designer and placed as active requirement instances on schematic or PCB documents. This gives the designer an in-context reference while implementing the layout and creates traceability between the original requirement and the design.

During PCB layout review, each applicable requirement can then be checked against the completed design and moved through its verification process. This creates a second review path alongside DRC and DFM checks, helping ensure that a manufacturable PCB also satisfies the product-specific constraints that drove the design.

The Verdict Comes From Your Manufacturer

After you send your design data over to your manufacturer, their engineers will go through your design with a fine-toothed comb, looking for anything that doesn’t comply with their constraints or that may result in scrapping fabricated boards. In prototype runs, yield analysis is typically not a factor in a manufacturer’s DFM review, but larger orders or sustained volume production will get a review for any factors that can impact yield.

Want to Save Time? Do a PCB Design Review in the Cloud

Today's PCB design software helps expedite many design tasks needed to create high-quality layouts and manufacturing documentation, but many designers still rely on email chains and chat programs like Skype to complete a PCB design review. There's a faster way to complete a PCB design review: use Altium Agile Teams cloud platform. Having access to an Altium Workspace allows you to store and share your design projects in a managed cloud environment.

Best of all, Altium Agile Teams enables you, your manufacturer, and your design team to open, inspect, modify, and release projects for fabrication without using an external PCB design review application. Here are some of the important tasks that Altium Agile Teams makes easy:

• Share your in-progress and finished projects with collaborators and your manufacturer
• Compare Gerber file revisions within Altium Designer or in your web browser
• Migrate your component libraries, quickly import components into new projects, and create new components from old symbols and footprint
• Inspect all design documents in your web browser or by opening shared projects in Altium Designer
• Control user access to all projects through a secure platform.

With the layout and schematic design tools in Altium Agile Teams, you can easily address all the standard items on your manufacturer’s PCB design review checklist. Any important DFM requirements can be defined as design rules. These design rules are then checked automatically as you create your board, rather than being checked at the end of your design. You can quickly identify DFM violations and correct them early. Once you're ready, share your projects with collaborators and prepare for manufacturing.

Learn more about Altium Agile Teams →