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November 14, 2016
We’ve pulled together a few tips and best practices for PCB design that should be of use to novices and experienced designers alike. If you follow this advice you should cut your design time, reduce spins, and make life a lot easier on yourself.
1. Study Fabrication Methods and Chemistry
These days it isn’t uncommon for engineers to lack practical knowledge about what goes into creating PCBs from the files that they design. This frequently leads to PCB designs that are more complex than necessary. One common problem is orthogonal trace bends on tight grids. Not every PCB fabricator has the ability to produce that type of design with acceptable results.
Before you order your prototype, ask yourself if the design needed to be as complex as it is. You’ll also be able to cut costs and improve reliability if you choose a larger grid. Small via sizes, blind, and buried vias are valuable options, but only under the right circumstances.
2. Don’t Skip the PCB Design Schematic
It can be tempting to skip the schematic if your board is relatively simple, but it’s a bad idea. Building your layout from a complete schematic gives you a reference and helps make sure that all of your layout connections will be complete.
The schematic is your visual representation of your circuit. It lets you arrange components near their functional counterparts, regardless of where they will be physically placed. When each pin on each component is depicted in the schematic symbols, a disconnect is easy to recognize. Schematics help you determine if the basic circuit design rules have been followed by visualizing the design.
3. Don’t Over-Rely on the Autorouters
If you are using a professional-grade PCB CAD tool, it likely has an autorouter. It is great as a first pass or sanity check, but it is important that you also know how to route traces without it. Autorouter parameters require precise setup which must be changed from job to job and sometimes even between modules within a single PCB design. There are no “set-it-and-forget-it” options.
4. Consider Current Flow and Board Geometry
When you get a kink in your garden hose, water stops flowing. In the same way, electrons can be impacted by bottlenecks and choke points in your PCB design. PCB designers, especially newer ones unintentionally create electrical roadblocks. For example, using a 90-degree bend when two 45 degrees could serve the same purpose; bends bigger than 90 degrees, and switchback shapes. These approaches can have the effect of slowing down signal propagation or worse, they might melt at the point of resistance, sending you back to the drawing board.
5. Avoid Potential Slivers in your PCB Design
Slivers are narrow wedges of copper or solder mask produced during the PCB manufacturing process. They can be a big problem during fabrication. Slivers are created during the etching process and can happen in a couple of ways. Slivers can occur when an extremely long, thin feature of the copper or solder mask is etched away. Sometimes it detaches before it fully dissolves. In that case, it can float around in the chemical bath, and possibly land on another board, adding an unintended connection.
Slivers are also produced when a section of the PCB design is too narrow or too deep. Even if they are intended to stay attached to the board, if an etched section is narrow enough or the etching is deep enough, a sliver of material can completely or partially detach, either producing a floating sliver or a peeled-back sliver. Both of these options can have serious negative consequences on the circuit board’s function.
Slivers can be avoided by designing sections with minimum widths, minimizing the chances of producing slivers. A manufacturer will usually spot potential slivers with a DFM check.
6. Use the PCB Design Rule Checker
It is a good idea for design teams to craft a set of design rules that standardize the bare board build costs and maximize yields. They also make assembly, inspection, and test more consistent. These rules can also help create more consistency in the purchasing department as well. Pricing for the manufacture of the board is consistent, and purchasing can often reduce the number of specialized PCB manufacturing agreements that need maintaining.
To support this effort, many PCB design software platforms have a Design Rule Checker (DRC), or constraint manager built-in. This tool will call out design rule outliers right while you are editing. They usually err in the direction of reporting an error, but then let you decide if it is really a violation. We’re not going to lie, sorting through dozens of potential problems can be tedious, but it is worth doing anyway. It may very well save you from a failed prototype. It is also smart to consider that if your design is generating a bunch of possible problems, perhaps your trace placement should be revisited.
7. Know Who Will Fabricate Your PCBs
In order to set up the DRC correctly, you’ll need to know which fabricator you are going to use. It will also help with some additional pre-fab tasks. If you’ve chosen a good partner, they will offer help and advice on how to craft your design to keep down the number of iterations, make debugging easier, and improve the quality of outputs overall.
Each manufacturer has its own specifications, such as minimum PCB trace width, spacing, number of layers, and so forth. You need to understand your requirements and find a PCB manufacturer that can meet them. These requirements should include the grade of the materials. Grades range from FR-1 (paper-phenolic mixture) to FR-5 (glass cloth and epoxy). The most commonly used grade in prototyping is FR-4, but FR-2 is used frequently in consumer applications. The choice of materials will impact the board’s strength, moisture absorption, flame resistance, and durability.
If you have a good understanding of the PCB manufacturing process, you will be better positioned to make smart design decisions. You should know exactly which processes and methods will be used by the manufacturing partner that you select. It is a best practice to visit your PCB manufacturing partner and have a firsthand look at the process. You’ll be surprised by what you can learn when you do.
If you keep these things in mind as you work on your design, you are setting yourself up to get reliable PCBs for your project in short order.
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