PCB Design Review: Catch Errors Before Production

Constructive feedback guides good decision-making, which is evident in all aspects of electronics manufacturing. The earlier feedback is received on a PCB layout, the easier for manufacturers to incorporate the information into the design. Inversely, design feedback received later on can create disruptive, albeit necessary, changes to the manufacturing process. Although portions of the developed printed circuit board can be verified only after manufacturing, such as EMC testing, manufacturers can adopt most feedback before a single panel enters fabrication.

The PCB design review is a multi-disciplinary analysis to ensure the layout is prepared and ready for production before manufacturing begins. Additionally, the review can significantly accelerate the development process by circumventing common design issues that often arise.

Sample of Layout Review Items
Placement	Routing
Bypass capacitors: Bypass capacitors will be placed as close to respective IC power pins as possible. The lowest capacitance capacitors are placed closest radially so fast charging/discharging cycles can support PDN responsiveness. Crystal oscillators: Oscillators are responsible for timing the circuits. A short, direct path to the IC clock pin is crucial and will ensure low impedance for clock signals to maintain the expected frequency. Terminating resistors: Resistors matching the real-valued component of a transmission line’s characteristic impedance are used to minimize the effect of reflections. They are placed to avoid creating reflections of their own through trace stubs. Fiducials: Fiducials are critically important targets providing references for computer vision systems during manufacturing. Fiducials should be placed at three adjacent corners of the design on the top and bottom layers.	Power/ground trace width: Power and ground traces will be sized to the diameter of the via hole (or the largest supportable trace width) to allow for sufficient current-carrying capacity. These trace widths will help prevent current crowding and its associated effects (reduced device performance, thermal hot spots). Via array: Larger copper features like polygons and pours will require multiple vias to connect between layers of the board. Using too few vias is akin to a narrow trace width on a power/ground via and can limit the current flux between layers. A good rule of thumb is a via for every 0.5 amps of current, while simulations will yield more precise via dispersal. Reference plane: Signals that run across split planes or gaps in planes significantly contribute to EMI as a signal return’s path becomes more circuitous, and the strength of the magnetic field grows according to Ampere’s Law, especially in the case of high-speed signals. Signals will reference continuous planes over their transmission path.

Placement and Routing: The Twin Threads of PCB Layout Review

After the PCB layout is finished, the product development teams will want to discuss the design from a production standpoint. With DFM, layout designers will already have established communication between the engineer and manufacturing manager to ensure the layout maximizes performance, design intent, and producibility. Yet precisely-followed design rules may still leave a board in an unoptimized state. Advanced layout practices, like providing clear signal return paths that aren’t blocked by planes or split planes, may not be checked by automated DRCs. Board layouts with an extensive revision history may have legacy anomalies that could be more conducive to manufacturing and require correction.

The prevailing mentality of “do it right, or do it twice (or more)” is intended to avoid costly PCB manufacturing problems. A board not focused on design for manufacturability guidelines will require additional layout revisions, increasing turnaround times and costs. VSE values PCB design reviews as the cornerstone of agile manufacturing for NPI, which minimizes the total time for our customers to receive completed boards.

Our PCB layout review will focus on a few key aspects of the board:

• Placement: Components should be placed not only for the best electrical performance but also for how their location will affect manufacturing.
• Are connectors easily accessible, or are they surrounded by tall profile components that may make plug insertion/removal difficult?
• Are solder shadows present that could impact automating soldering processes, requiring manual work on small or short components that can be less effective and more costly?
• Can test points be accessed by automated or manual probes, or are they obstructed?

Clearance issues like these demonstrate how often standard design rules only encompass some of the concerns of DFM.

• Routing: Routing issues tend to be observed less during manufacturing and more during testing and performance metrics. ECAD tools are robust enough to cover the various shapes, groups, and conditions of the boards’ nets. However, routing that doesn’t account for potential EMI/EMC interactions between traces and components may result in manufacturable boards that fail during runtime. Our engineers take note of high-speed signals and large inductive loads (namely inductors in switched-mode power supplies) that can influence nearby signals through induction. Conversely, sensitive lines will also receive extra consideration of how nearby signals may interact with them.

The Impact of Documentation on Producibility

A perfectly laid-out board may still suffer from poor documentation. Technicians on the manufacturing floor rely on unambiguous directions to ensure the initial design intent is expressed in the final product. Artwork must convey visual information accurately, and detail processes like surface finishes when a design contains specific requirements. As a turnkey PCBA solution, we ensure your documentation is concise and thorough for error-free manufacturing.

Documentation also extends to the board itself. Clear and legible silkscreen will greatly aid technicians in locating components and test points during test and debug. Unlike a CAD program that can zoom in and out, the finished PCB can only be viewed at its actual scale. Designers should view a board from a 1:1 scale on the screen to determine whether a silkscreen is easily readable without visual aid. Designers should also check the following silkscreen elements:

• Pin 1 indicator or polarity/orientation markers. Many components and connectors can be installed and soldered in two or more orientations. For components like electrolytic capacitors, this could damage the board and pose a safety hazard to technicians and operators. Have pin 1 indicators visible and unobscured by component bodies.
• Reference designators. Designators should be placed as close as possible to the component. Designers should utilize visual elements, such as lines, arrows, etc., on the silkscreen to associate components with their designators if there is ambiguity. Keep reference designators in one vertical and horizontal orientation for clarity. In densely populated assemblies, designators for components like resistors, non-polarized capacitors, etc., can be hidden in favor of polarized components, ICs, connectors, and test points.

From PCB Design Review to Debut: Your Contract Manufacturer Covers NPIs And Beyond

DFM can be a broad design methodology, and best practices may shift with the scope of the production. An NPI design may be focused on minimizing the turnaround time to get prototypes or finished products back as quickly as possible. On the other hand, high-volume production will want to focus on reducing per-board costs as much as possible. A longer layout time can minimize feature redundancies, such as lowering via count through in-depth analysis of trace routing, and improve board quality.

VSE is equipped to handle both ends of the manufacturing spectrum with our NPI-focused facility in San Jose and our high-volume Reno location. VSE also offers pre-production analysis of your product design beyond PCB layout reviews. Our engineering team will also review your design enclosures for form, fit, and function as part of our comprehensive box-build services. We ensure that every part of your electrical system meshes to maximize performance, even in the most complex designs.

At VSE, our engineers are committed to building electronics for our customers. Alongside our valued manufacturing partners, we aim to deliver nothing less than the highest quality in PCBA engineering, manufacturing, and testing for life-saving and life-changing applications.