While the term “library” traditionally invokes images of quiet rooms containing thousands of books organized on shelves, it’s also used to describe a method of computer storage. In the case of CAD tools used for designing printed circuit boards, libraries store many different types and formats of data for electronic schematics and PCB layouts. However, where a misplaced book could earn a stern glare from a librarian, a misplaced or outdated CAD part can result in far worse.
One of the most common causes of problems during the manufacturing of printed circuit boards is outdated or incorrect library data. These problems can delay manufacturing, or worse, completely upend the project until it can be redesigned. Design teams need to ensure that their library data is up-to-date and ready to avoid costly and time-consuming delays. Let’s look at some good CAD library management practices and ideas to guarantee that your PCB libraries won’t let you down.
PCB Libraries, Precision CAD Part Models by the Book
Printed circuit boards are designed using two different types of CAD software systems. These systems include schematic capture tools for the logical representation of the circuitry and PCB layout tools for the physical design of the board. In a schematic, the function of the components is represented by industry-recognized symbols containing pins for electrical connectivity. These pins often carry their own intelligence to represent electrical inputs, outputs, power, ground, or many other functions important to the schematic. Each schematic symbol and its associated pin and part information is stored as a CAD model for easy retrieval and instantiation on a schematic drawing.
Another important library element used in the schematic is circuit simulation models. Schematic circuitry is often simulated using a “Simulation Program with Integrated Circuit Emphasis” system known as SPICE. A SPICE model must represent each schematic symbol to run the simulation. However, circuit simulation isn’t always used during PCB design, and therefore some PCB libraries won’t contain these models.
Once the circuit logic is captured in the schematic, the physical board must be designed based on the schematic’s net connectivity using PCB layout tools. Like a schematic symbol, the layout tools rely on models for the physical components to be assembled on the board. These models, also known as land patterns or footprints, allow a single component part number to be replicated as multiple instances for use throughout the board. The most important aspect of a footprint is that it contains the exact size, shape, and configurations of the PCB pads to be etched in metal during circuit board fabrication for soldering the components. Additionally, PCB footprints will also contain some or all of the following information:
- Thru-hole or surface mount configuration
- Drill information for thru-hole pads or mechanical fasteners
- Height, width, and length data
- Keepout zones and component outlines
- Silkscreen, solder mask, and solder paste graphics and information
- 3D STEP (Standard for the Exchange of Product data) model
Component footprints aren’t the only elements stored in a PCB library. Here are some of the other CAD models you can find there:
- Vias
- Mechanical parts
- Documentation pieces
- Drawing formats
All of these library elements require careful management, as outdated or incorrect library information can create many problems.
Library Containment Requires Good Library Management
To best serve the circuit board designers that depend on it, a PCB library needs three things for foolproof operation:
- Configuration: The corporate PCB library needs to be located and structured to make sense to the users. Many CAD systems create their own structure, and the library just needs a location. However, in older systems, it may require some thought on how to best structure the different folders for work-in-progress, unapproved parts, and legacy data.
- Process: Users need a set procedure to submit new part requests. Once in the system, that procedure needs to continue through the final notification that the completed part is ready to be used. It is also important to establish a process for managing the library, so all users know what they can do and who is responsible for maintaining it. It is also essential to secure the library against unauthorized access that can corrupt or delete data.
- Librarian: The library process also needs a designated librarian. This user, or users, will have access permissions for library management and be trained in the new part request process.
What doesn’t work in PCB library management is the “hands-off” approach. This method generally leads to an unmaintained corporate library that users don’t trust or use. Users will often keep personal libraries without a set procedure to work from, and the resulting consequences can be devastating to the PCB design process.
The Consequences of Bad PCB Library Management
Without a set library process, the users often try to maintain their own PCB libraries. This approach creates a situation where library work is duplicated repeatedly, and bad parts are built and used due to the lack of a formal review. Parts kept in a central “bucket” instead of a structured PCB library typically don’t have any security control. Each user can change these parts as desired or move or delete them. This lack of process consistency often results in many redesigns to find and correct library part problems. It also creates an impossible scenario for implementing component changes as a normal part of routine library management.
As you can see, using a controlled library management process is essential for eliminating design and manufacturing errors to reduce the overall time and cost of circuit board production. One way to help yourself is to use the third-party PCB librarian services available for PCB design. These services work together with component manufacturers to provide current approved symbols and footprints to PCB designers. There is also another resource available, and that’s working with the PCB contract manufacturer building your boards to maintain your libraries.
PCB Library Services Available from Your Contract Manufacturer
Your PCB CM can offer you some very valuable library services to help organize the design process and keep you from designing with outdated or bad library parts:
- When you submit a design to your CM for manufacturing, they review your bill of materials and the parts in your design compared to their component resources. This process will tell them if you are using older and outdated parts in your design.
- Next, the CM will update the libraries in your project to the currently available standard. This library maintenance lets you upload these corrected parts into your corporate library to keep it current.
- PCB CMs also offer subscription services to continually review their customer’s BOM and AVLs on production boards to identify component life cycle problems. This service will alert you when a redesign of a legacy board is needed to prevent the next production run from being ruined with bad parts.
At VSE, we have offered all of these services to our customers for over 35 years. These services guarantee that legacy boards will be built correctly and new projects can be designed without concern. Our component engineers and procurement teams have built an incredible reputation of persistence and quality with their work, and we would be happy to help protect your designs as well.
