In this article, we present some of the golden rules of PCB design, which remain largely the same as they were 25 years ago when commercial printed circuit board design first emerged. These rules can generally be applied to any PCB project and will be a useful guide for both electrical engineering students and more experienced designers.
Below are ten useful elements that electronics designers should consider or practice when designing PCBs for commercial manufacturing using design software. They are not listed in chronological order nor assigned a specific level of importance, but together they make a significant difference to any design.
Rule 1: Use the right grid. Find a grid whose spacing accommodates the number of components you'll be using and make the most of the available space. While the idea of using multiple grids might seem appealing, a little extra thought in the early design phase can prevent space-related difficulties and allow you to maximize the board's potential. Take advantage of the fact that many devices are available in various sizes. Also, because the polygon shape is important when adding copper to the board, and boards with multiple grids often produce discrepancies in how the polygon is filled, using more than one grid can make things more difficult than necessary.
Rule 2: Traces should be as short and direct as possible. It sounds simple and obvious, but remember this rule at all times, even if it means you have to adjust the layout of part of the board to optimize trace length. This is especially true for high-speed analog and digital circuits where impedance and parasitic effects can limit system performance.
Rule 3: Whenever possible, use a ground plane to manage the distribution of lines and grounds. Filling the ground plane is a quick and easy option in most PCB design software. It applies sufficient copper to common connections and really helps ensure that power flows as effectively as possible with minimal impedance or voltage drop and that ground returns are adequate. If possible, route multiple power lines in the same area of the board, and remember that if the ground plane is routed over a very large section of a layer, it can positively impact feedback between lines routed above or on adjacent layers.
Rule 4: Group related components and test points together if needed. For example, place the discrete components required for an operational amplifier close to the device so that the capacitors and bypass resistors are located with it. This helps with the trace lengths required by Rule 2 and makes testing and troubleshooting easier.
Rule 5: Create panels of your PCB by replicating it multiple times on a larger board. Using the size that best suits the equipment used by your chosen manufacturer will improve prototyping and manufacturing costs. Start by laying out the board as a panel. Contact the manufacturer to obtain their preferred panel size specifications. Then, once you have corrected the design rules, do your best to repeat your design multiple times on the preferred panel size.
Rule 6: Consolidate component values. As a designer, you will have selected some discrete components of varying values that function in the same way. Consolidating these components into a standard range of values simplifies the bill of materials and is likely to be more economical. Furthermore, long-term stock decisions are made easier if you have a variety of boards based on the preferred device values.
Rule 7: Check the layout rules as often as possible. The DRC function for this purpose in PCB design software is quick and easy, and checking the rules frequently can save you hours on much more complex designs. It's an excellent habit to develop. Every board layout decision is important, and constantly checking the layout rules ensures you always keep the most important factors in mind.
Rule 8: Use silkscreen wisely. Silkscreen can be used to communicate useful information to the board builder, maintenance or testing engineers, and even the person installing or operating the device. Clear labels for functions and test points are obvious, but the orientation of components and connectors should also be considered as much as possible. Annotations are used even though they sometimes end up covered by components after assembly. Using silkscreen on both sides of the board can reduce the need for rework and simplify production.
Rule 9: Decoupling capacitors is not optional. Don't try to optimize your design by avoiding line decoupling and relying on the absolute limits in component datasheets. Capacitors are inexpensive and durable. Take the time to install them whenever possible, and remember Rule 6 about using a range of standard values to avoid inventory problems.
Rule 10: Generate your own PCB manufacturing data and verify it before sending the boards to production. Most board manufacturers do this directly from the download file, but if you upload your Gerber files first and check them using a free tool to ensure they look as you expect, you can avoid misunderstandings. You might even find errors unintentionally before they're permanently printed on fiberglass, resin, and copper.
As circuit designs become more openly shared and more design teams rely on reference designs, we believe it's crucial that these rules continue to be observed in printed circuit board (PCB) design. By maintaining a firm grasp of the fundamentals, designers also retain the flexibility to add value to their products and get the most out of every board they design. Finally, anyone just starting out in PCB design will learn much faster and gain greater confidence if they consistently keep these basic rules in mind.
Edwin Robledo @CadSoftTech is the General Manager of CadSoft USA and has been with the company for 20 years. Jorge Garcia is a member of the Technical Support team.
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By Edwin Robledo and Jorge Garcia of CadSoft
