Estimated reading time: 6 minutes

Elementary, Mr. Watson: Ensuring Design Integrity

Back in February, many of us watched the "Big Game." It reminded me of the saying, “It's not how you start that is important, but rather how you finish." It is perfectly okay when you are talking about sports, you get off to a bad first half and need to recover in the second half. However, when it comes to PCB design, this is not a good practice. If things start badly, they usually don't recover. They continue down that same path, costing more money and losing design time.  

Before the age of consolidated ECAD suite software, it was not uncommon to have several programs doing various tasks in PCB design. We would have one package to capture the schematic and export the netlist, which was then pulled into another program to do the PCB layout. Another one is to create the output drawings for fabrication and assembly. I recall conducting some major "mental gymnastics" using third-party software packages and making the leap across the design.

Even for the most uncomplicated PCB design, it was a very difficult and broken-up process. So, it was easy to see why designing a PCB was viewed as an isolated process rather than a unified procedure.

However, with the introduction of the ECAD Design Suite, a new paradigm shift occurred that streamlined the design and centralized it around coherence and the unification of the data and the process. Maintaining integrity in the design is vital to accomplishing that. The maintenance and the correct transference data must be done cleanly. At each design stage, you are building on what was done before it, which explains why things can go downhill quickly sometimes.

What Is Integrity?
The PCB design is summed up in a single word: integrity. It’s defined as the state of being whole and undivided. I like that definition. There are four major parts of a PCB design: the library, the schematic capture, the PCB layout, and what I refer to as the file creation and outputs (documentation). Of course, there are a subset of minor steps in each, but generally, those are the four major stages.

Integrity in the context of a PCB design refers to four different principles of the design: consistency, accuracy, reliability, and robustness throughout its lifecycle. It encompasses several key aspects:

• Consistency: This refers to maintaining uniformity and coherence across various design aspects, including component placement, routing, documentation, and design practices. Consistency ensures that the design is easy to understand, troubleshoot, and maintain, regardless of the stage of the design process or the individuals involved.
• Accuracy: A PCB design must accurately reflect the intended circuit functionality, component specifications, and electrical connections. Errors or inaccuracies in component footprints, netlists, or routing can lead to functional issues, reliability problems, or even catastrophic failures in the final product.
• Reliability: A reliable PCB design performs consistently and meets the specified requirements under various operating conditions. Integrity in PCB design involves considering factors such as thermal management, signal integrity, power distribution, and manufacturability to ensure the reliability of the final product. Design choices should be made with reliability in mind, aiming to minimize the risk of failures or malfunctions.
• Robustness: A robust PCB design is resilient to external factors and variations, such as environmental conditions, manufacturing tolerances, and component variations. Designing for robustness involves incorporating features such as proper grounding, EMI/EMC considerations, mechanical stability, and adequate margins for tolerances to ensure that the PCB can perform reliably under real-world conditions.
• Documentation: Integrity in PCB design also extends to documentation. Comprehensive documentation, including schematics, PCB layouts, bill of materials (BOM), assembly drawings, and fabrication files is essential for maintaining the integrity of the design. Accurate and up-to-date documentation enables efficient collaboration, troubleshooting, and future revisions, ensuring the design remains consistent and well-documented throughout its lifecycle.

Overall, it's the PCB designer's responsibility to maintain a stringent level of integrity throughout the PCB design process. It requires attention to detail, adherence to best practices, and a commitment to quality to ensure that the final PCB design meets or exceeds the requirements and expectations of its intended application.

The rule is that all four stages of the PCB design should match and stay matched throughout the design process. The schematic should sync to the library, the schematic then links to the PCB, and the PCB to the source file for the outputs and documents. It's common to have changes occur to the design at some point. Keeping a unified design throughout is what's meant by the integrity of your PCB design.

Putting It Into Practice
What are some practical steps to ensure integrity from these basic definitions? We can now look at exactly how we maintain the integrity of a PCB design from the library through to the final document packages, which involves several crucial steps to ensure consistency, accuracy, and reliability. These essential principles would be on the level of requirements in your design.

Library
First, it's essential to establish a standardized and well-curated component library. This library should include verified component footprints, symbols, and associated data such as pin assignments and electrical characteristics. By using standardized libraries, designers can ensure consistency across different projects and avoid errors arising from incorrect or outdated component data.

Schematic
Careful component selection and placement are paramount when transitioning from the library to the schematic design phase. Designers should verify that the chosen components match the requirements of the circuit and accurately represent the intended functionality. Proper symbol placement on the Schematic is also crucial for clarity and comprehension. Components should be logically arranged and labeled to facilitate easy understanding and troubleshooting during later stages of the design process.

PCB Design
As the design progresses from the schematic to the PCB layout, maintaining the integrity of the design becomes even more critical. Designers must accurately transfer components from the schematic to the PCB layout while preserving their relative positions and orientations. Any discrepancies between the schematic and PCB layout could result in functional errors or connectivity issues. Additionally, designers must pay close attention to routing integrity, ensuring that traces are routed carefully to maintain signal integrity, minimize interference, and ensure proper electrical connectivity. Design rule checks (DRC) should be performed regularly to identify and rectify any violations related to trace width, clearance, and spacing, further ensuring the integrity of the design.

Documentation
Clear documentation is essential for maintaining integrity and facilitating collaboration among team members throughout the design process. Design documentation should include detailed schematics, PCB layouts, bill of materials (BOM), assembly drawings, and fabrication files. Comprehensive documentation provides a clear roadmap for understanding the design and allows for easy verification and validation of the design at various stages of development. Additionally, version control should be implemented for all design files to track changes and revisions accurately, ensuring the design remains consistent and up-to-date.

In summary, maintaining the integrity of a PCB design from the library to that final document package requires careful attention to detail, standardized processes, and precise documentation. By following these steps diligently, designers can ensure the final PCB design's accuracy, consistency, and reliability.

John Watson is a professor at Palomar College, San Marcos, California.