I’ll admit that when I first started laying out printed circuit boards, my primary goal was to get to the cool stuff like component placement and trace routing. I didn’t spend enough time ensuring that the design was ready, which undoubtedly caused many avoidable headaches. I should have put more effort into configuring the board layer stackup, confirming the outline with mechanical design, and setting up my PCB footprint library. Fortunately, the more experienced and wiser designers I worked with quickly taught me the importance of a thorough approach to PCB layout before I could cause too many problems.
However, there is another aspect of circuit board design that the same lack of thoroughness can plague: ensuring the integrity of the schematic before you go into layout. In some cases, this is due to different departments handling schematic capture and PCB layout, and the resulting lack of good communication creates a poor exchange of design data. In other situations, designers may simply not be aware of the potential problems by not following through with all of the necessary steps for layout. It is crucial to take basic steps to ensure design integrity before converting the schematic to a PCB layout.
Verify the Design Libraries Before You Convert the Schematic to PCB Layout
Before going into the PCB layout with a schematic, you need to verify the library parts you are using in the design. The schematic will be populated with logical symbols that drive the layout’s PCB footprints. Each of these parts must be correct to ensure that the completed circuit board doesn’t have any electrical or physical errors in it.
Many corporations use a library structure where approved parts and those components still in development are separated. Due to tight design schedules, these “dev” parts are often used as placeholders for fully approved parts. It is essential that all of the parts in your schematic are fully approved or that the project management has approved the use of non-approved parts before you complete the design.
The next step is to make sure that the parts you are using are available. Although a symbol may belong to a valid component, the actual part may not be available or too expensive to use due to shortages in supply chains. Here is where you need to check the parts you are using before committing to a course of action that may delay production or cause significant cost increases. Your PCB contract manufacturer can usually help by verifying your bill of materials to their suppliers to ensure that the parts you need are available and not cost-prohibitive.
You will also want to check each schematic symbol to verify it is associated with the correct PCB footprint. Although most design libraries couple their symbols and footprints together, so this isn’t a problem, some processes still rely on manual part assignments. Not only can this create errors in the part assignments, but in some cases, the pin-out of a part may not match a unique part being used. Examples include polarized capacitors, transistors, and diodes, which may have a pin-out different from the symbol in the schematic.
When all of your parts are verified, you can move on to validating the schematic itself by running it through the design rule checking.
Run the Schematic’s Design Rule Checking, and Clean Up Any Errors
PCB layout tools have design rule checks to find several problems, from short circuits to un-manufacturable circuit boards. These checks are critical in the board’s layout to ensure that it can be built as designed. A schematic capture tool also has design rule checks, which are just as important. Some of the problems that the DRCs are looking for are either electrical or physical errors on a page:
- Single-pin connections
- Missing pin connections
- Incorrectly connected nets, such as power & ground shorts
- Floating wires or unused nets
- Pin and text visibility
The DRCs will also look for problems scattered throughout the entire design across multiple pages:
- Offpage connections or issues with ports
- Global power and ground connections
- Duplicate net names
- Incorrect high-speed rule properties
- Overlapping parts or nets
To use your schematic DRCs effectively, you must understand what the rules are trying to accomplish. Designers sometimes ignore DRC errors and warnings because they don’t understand them and therefore don’t assign the importance to them they deserve. Often a DRC will report a problem that is perfectly acceptable, such as a non-connected pin, but it is up to you to look for those noted errors and decide if they are okay or not.
However, to check whether the circuitry will work as designed takes an additional CAD tool, and we will look at that next.
Use a Good Circuit Simulation Tool to Verify the Functionality of Your Design
Many designers will work up their circuits on a breadboard before schematic capture. While this can answer many questions about the circuitry you are designing, it isn’t the most efficient way to validate an entire circuit board due to the cost and time involved. A much better method is to use a circuit simulation tool like SPICE (Simulation Program with Integrated Circuit Emphasis). These tools model each part you are using in your design and simulate the operation of the circuitry virtually, saving time and money in building multiple prototypes to debug.
By following these recommendations in your design, you can save yourself a lot of time and frustration by avoiding errors when converting your schematic to a PCB layout. Another thing you can do is to work with your PCB contract manufacturer while your project is still in the design phase. PCB CMs, like VSE, have years of experience working with customers who are designing circuit boards for manufacturing. They can help you with BOM verifications, component choices, board layer stackup configurations, and much more.