A CAD librarian has two avenues for part generation.
What to do when a datasheet is unsatisfactory.
The CAD librarian’s mark on the design beyond creating land patterns.
In any cataloging system, organization is key. The ability to order a list allows for quick searches and referencing of items. Computer-aided drafting (CAD) librarians are similar to librarians who shelve books, but with a key difference—at some point, CAD librarians produce the content they intend to later reference and use. Unlike a physical library, duplicate files pose a huge design integrity issue: the nature of the media makes duplication unnecessary and could cause confusion as to which file is in circulation. This is technically not an issue if both files are identical, but there is a chance that elements of a design may diverge and lead to issues during fabrication.
A CAD librarian renders a service of utmost importance—ensuring the files in use by the design team are correct and unique as to prevent an incorrect unchecked land pattern being falsely waved through. Failures during fabrication will not only require redesigns at prior levels until the source of the issue is resolved, but may also lead to the scrapping of otherwise correctly designed and fabricated boards. Below are the specifics of a CAD librarian’s importance to the design team as well as a guide for troubleshooting some common pitfalls a CAD librarian might encounter.
What Does a CAD Librarian Do?
The role of a CAD librarian invokes a symbiosis between the librarian and PCB layout designers; in smaller layout divisions, the divisions between the two may be entirely nonexistent. For the times when there is a clear distinction between the two job titles, the librarian provides a crucial service. The heart of their work revolves around maintaining a parts library, ensuring first that all new part requests are not already present in the library, and generating accurate pads and overall land patterns. The first fork in their design flow arises based on how standard the part is in question as well as the type of component:
- “Wizard” tool: For simple and common part designs, a semi-automated program can provide pad and part files after user selection of package style and entry of a few common measurements.
- User-generated: Less common package styles, pad layouts/shapes, and connectors will need to be designed within the toolset itself. Care must be taken when reviewing design documents, e.g., is the pad layout diagram from a top or bottom view perspective.
Depending on the toolkit used, pads may be saved as their own files or simply as elements of the greater part file. There are notable advantages to the former storage method. There is a marginal time investment checking pad dimensions against existing entries, much like one might check the part output of a wizard tool to ensure there are no duplicates in the library. However, the librarian and all of the layout designers downstream can be confident that the pad, once initially approved, can be reused indefinitely. Converse to the time spent checking pads against the library, pads ascertained to be correct from prior designs can bypass additional checks.
How to Address Datasheet Shortcomings
Missing datasheet information – or entire missing datasheets – can lead a librarian to question how best to address the issue
The ability of a CAD librarian to perform their job rests on the whims of documentation. More often than not, companies that have been designing ICs and other board components have been doing so for some time and have developed strong internal standards for document preparation and preservation. However, there are few experiences more taxing to a librarian than having to scour the internet for the piece of information necessary to complete their work. Alternatively, documentation may be spotty, heavily artifacted to the point of illegibility, or otherwise incomplete. In these situations, a CAD librarian should progress through the following steps to ensure the integrity of the design and prevent pad-related failures during manufacturing.
- Check the manufacturer’s website - Some of the aggregating services for datasheets may miss smaller or more regional manufacturers. Oftentimes, this information is readily available and the manufacturer may simply prefer to keep their files within their ecosystem as a means of lead generation.
- Contact the manufacturer directly - Most websites will have a form to email a sales representative; in certain cases, datasheet information may be intentionally placed behind this point of contact.
- Speak with the engineer - The engineer responsible for the schematic/BOM files may have access to the relevant datasheet or have a line of contact with a member of the manufacturer in question. Failing this, the engineer can make a change order to a suitable replacement, though this may open a new can of worms with current global supply chain challenges.
- Online ECAD databases - This step should be considered with extreme caution. Unless provided directly by the manufacturer, unattributable land pattern files should be taken with a whole shaker of salt. There is simply no way to tell what information the librarian used to generate the part or whether that information is accurate and up-to-date. Incorporating an unattributable part into a library or design opens the board up not only to failure but also liability during failure analysis.
How a CAD Librarian Influences Manufacturability and the DRC
Perhaps it is obvious to state that a CAD librarian’s work has downstream effects on the layout designer as well as the final product, but the influence of the librarian extends beyond the part files they are providing. Manufacturability will shape the layout designer’s design rule check (DRC) as well as tweak the elements of parts from the manufacturer’s recommended specifications. Take thermal pads for instance; the assembly team may call for maximum dimensions instead of nominal values on thermal to increase solderability as well as thermal capabilities. Though technically belonging to the part itself, the accuracy with which the librarian takes down a ball grid array’s dimensions will ultimately influence layout DRC constraints such as via size and track width. PCB design is a field of constant revision from multidisciplinary teams and the CAD librarian’s role in ensuring the success of the finished board is not to be understated.
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