One of the most important aspects of any successful printed circuit board (PCB) project is an efficient and accurate PCB design. Unfortunately, about 75% of the orders we see at the quoting stage contain design errors, and upon further discussion, we often discover that many potential customers do not understand how to successfully design and build a board that is manufacturable. We also find that some orders don’t follow IPC guidelines.
Why is all of this so important? Two words—time and money. A flawed PCB design will burn up precious engineering time and accrue additional costs since it will need to be revised, which can also cause project timeline delays. So let’s review some of the most common PCB design mistakes—and how to avoid them.
Failure to understand the electrical parameters of a design: Before starting a PCB project, it’s important to understand the electrical parameters of the system. This includes:
- Current maximums
- Voltages
- Signal types
- Capacitance limitations
- Impedance characteristics
- Shielding considerations
- Type and location of certain circuit components and connectors
- Detailed net wire listing and schematic
Different types of PCBs have different electrical requirements. For example, RF and microwave PCBs operate on signals in the megahertz to gigahertz frequency ranges. High-speed PCBs have specific factors that will impact the board’s design, such as whether there are controlled impedance circuits.
Not fully understanding the environment in which the product must function: Certain types of printed circuit boards are ideal for specific applications and industries. For example, aluminum-backed PCBs are often used in RF communications since adding aluminum to the PCB with a conductive adhesive draws the heat out and channels it into a chassis, which means a more efficient board. High-density interconnect (HDI) boards feature thin lines, closer spaces, and more dense writing, all of which create a faster connection. HDI PCBs are used in everything from telecommunications, military/aerospace, and automotive electronics systems, as well as smartphones, laptops, and Apple watches.
Drilled hole location too close to the copper feature: If the hole is closer than .010″ mils (.25mm) to a trace or another hole in a multilayer board, it must be fixed.
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Gerber file and the schematic drawing contain conflicting information: One of the first steps of a PCB design is creating a schematic. Once this is complete, the design is converted into something called a Gerber file, which is used to fabricate the PCB. Both the schematic and the Gerber file should contain the exact same information. We run into problems when, for example, the drawing says the minimum trace is .005” and the minimum space is .005”, but the Gerber file has 3/3 as opposed to 5/5.
Poor communication between the PCB provider and customer: Good communication is important in any circumstance, and the design of a PCB is no exception. It’s essential for PCB engineers and customers to communicate regularly during the initial stages of any PCB design project to discuss everything that is needed. Open, consistent communication also ensures everyone is on the same page, which can cut down on project delays and costly revisions.
Stubs in the design: A stub is a trace that has no plated through-hole at the end and that does not have a connection. Sometimes stubs are intentional in RF designs, but in most cases—when they’re an accident—they result in a delay in manufacturing and require new files to be generated, all of which means valuable time is lost.
Incorrect trace and space vs. copper weight: As a general rule, the thicker the copper, the more spacing is required between the copper features on the board. For example, 2oz of copper needs a minimum trace of .008″ mils (.2mm) and .008″ mils of space—but the customer or designer may specify .005″ mils (.11mm) for each, which isn’t enough.
PCB designer who doesn’t have enough experience in how a circuit board is manufactured: It is important to engage with your PCB shop to learn how PCBs are manufactured. It’s also crucial for PCB designers to be knowledgeable in IPC guidelines. For current or aspiring PCB designers, consider broadening your skillset to become proficient in a number of different software programs and design tools.
At the end of the day, it’s important to note that it’s rare for a PCB to be 100% perfectly designed on the first try. But by taking steps to mitigate the potential problems outlined here, customers and designers can avoid costly delays and multiple, extensive revisions.
RESOURCES
Cirexx International | www.cirexx.com
PUBLISHED IN CIRCUIT CELLAR MAGAZINE • JUNE 2023 #395 – Get a PDF of the issue
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Don Kaufman, a senior member of the Cirexx International team, has more than 33 years of technical experience in the printed circuit board industry. He possesses extensive knowledge of integrated circuits, materials, specialty laminates, chemicals, and equipment. He also prides himself on staying up to date with the latest techniques and procedures in PCB layout and manufacturing.
