Most medical prescriptions that are written for you today are done electronically, or at least that’s the way my doctor does it. It wasn’t too long ago though that these messages for the pharmacy were written out by hand, and sometimes it was unreadable. I’ve had many occasions over the years when the pharmacist had to call the doctor to confirm the medication and dosage in the prescription. Yes, the handwriting was really that bad.
Although the illegible handwriting of doctors has been a recurring theme for as long as I can remember, the problem of unreadable documentation is fairly universal. I’ve seen some electronic schematics laid out where I couldn’t tell what the design intent actually was. Having an unorganized schematic doesn’t just affect its readability however, it can also create problems for how the circuit board is designed and manufactured. To avoid these problems and create a clean and readable schematic, here are some tips on PCB schematic design capture to help.
The PCB Schematic Design, It’s More than Just a Document
Since we are used to reading schematics to determine their circuitry functions or to find specific design details, it is normal to consider them as a document. Although it is true that a circuit board schematic is a document, the reality is that an electronic schematic is also much more than that. Contained within the schematic database is a lot of precise information that the PCB design CAD system needs in order to do its job. If you were able to peel back the cover of an electronic schematic, here is some of what you would find buried inside:
- Component data: Each symbol on a schematic is the logical representation of an actual physical component that will eventually be assembled to the circuit board. As such, these symbols, or components, need to contain part numbers, values, tolerances, and other important data that will be reported to the bill of materials. Some of the CAD components will be composed of either multiple symbols, or multiple instances of the same symbol, and those associations have to be managed. For like pins and gates in a component that can be swapped electrically, that information also has to be managed as well. In addition, schematic components will need their associated PCB footprint and SPICE model information for layout and circuit simulation.
- Connectivity data: Most of the component pins will be connected to other pins in the design, and that information makes up the connectivity portion of the database. Some pins will be connected to global power and ground nets, while most of the pins will be connected as individual nets. These nets can be grouped together in busses to clarify their intent for the circuit board layout designers, which is helpful when dealing with large groups of memory of data connections. For those pins that aren’t connected, they will sometimes be flagged as a “non-connected pin” in order to satisfy the connection requirements of the schematic design rules.
- Design rules: Non-connected pins are only one of the design rules that can be specified in the schematic database. Electrical requirements can be added to nets to ensure that they are designed with the proper length or routing topologies in the PCB layout. In many tools, the actual physical constraints such as trace width and spacing can be specified as well in the schematic. This gives the PCB designer more control over how the circuitry will be physically laid out. There are also many other schematic constraints that can be added such as connectivity rules to determine if the nets are connected to the correct pins.
Of course, the schematic is still a business document, and as such it will also need to contain corporate and legal information. This includes part numbers and names as well as copyright and contact information. The electronic schematic is intended to drive the PCB layout database, and if the component, connectivity, and design rule information are not correct, it can result in multiple manufacturing problems later on.
How an Unorganized Schematic Can Hurt PCB Manufacturing
An incomplete or unorganized schematic can cause a lot of problems for PCB layout, and eventually circuit board manufacturing. For instance, incorrect component information can halt manufacturing in its tracks if the wrong part numbers in the bill of materials result in bad component purchases. Incorrect component and net connectivity data can also result in a bad PCB layout that will have functional problems once it is built or it may even not work at all. In some cases, the schematic is so bad that it won’t even synchronize properly with the PCB layout database causing a slowdown during the design cycle.
As a document, it is also important to realize how an unorganized schematic can affect manufacturing. Documents must be readable and if data is incorrect or sized incorrectly so that it can’t be read correctly, a whole new set of problems could be experienced. These could include everything from miss-filing a schematic to legal problems if important company information is left off.
To avoid all of these problems then, let’s take a look at some tips for creating a usable and readable schematic.
A good schematic is the beginning of a well-designed circuit board
Schematic Optimization Tips for Design Success
In order to avoid the problems that can come from a badly created schematic, take the time to make sure that it not only looks good, and also contains complete and correct design data. To do that it is best to develop a standard workflow for your schematic capture process that is complete with design checkpoints. Here are some other general tips that can help:
- Work with up-to-date and correct schematic library symbols and components.
- Develop a systematically organized logic flow for your design.
- Make sure that your circuit paths are clearly defined for the layout team.
- Use schematic hierarchy to help with repetitive circuits if your tools are so equipped.
- Spread your work out to give yourself more room, it’s OK to use more than one page.
- Work with your CAD systems design rules and don’t ignore them.
- Add notes to clarify what you expect from the PCB layout team.
- Have others review your schematic, it’s always best to get another set of eyes on your work.
Lastly, don’t be afraid to ask for help if you need it. There is a lot that needs to be set up in both the schematic and the layout tools in order to correctly design a PCB for error-free fabrication and assembly. Here is where the expertise of a PCB contract manufacturer can save you both time and money by helping you to understand what some of these manufacturing requirements are for your PCB design.
At VSE, we have been working with design engineers like you for over 30 years now. We understand the challenges that you face and the questions that you need to be answered, and we are looking forward to working together with you on your PCB design.