Footprint accuracy is paramount. The best assurance is human backup.
Can third-party libraries be trusted? This question arises occasionally during design team meetings. Third-party libraries are default libraries in the library tools, the standard libraries included with design software, and any libraries provided from a customer or vendor.
This is not an ad for or against library tools. There are a few well-written tools for managing and building libraries.
Footprint accuracy is the single most destructive mistake a layout designer or engineer can make. What are some of the factors that lead to footprints being built slightly differently?
- Component specifications are not standardized.
- Footprint drawings are not always to scale.
- Specifications drawn from a bottom view as opposed to a top view.
- Components that do not fit the manufacturer’s recommended footprint.
- Unclear controlling dimensions.
These same factors raise the risk of human error. Even the best layout designer eventually will overlook an offset or cryptic note concerning a pin on another page.
If all libraries were built the same, and there was not a chance of human error in the creation of the parts, the libraries would remain untrustworthy. At the bottom of every part spec rest, there is a phrase to cover any changes the manufacturer chooses to make: “Information in this document …. [insert company name] reserves the right to make changes, corrections, modifications or improvements to this document without notice.” Package changes from the manufacturer are rare, but they do occur. It also is possible that the recommended footprint does not fit.
A safe policy is to verify every footprint type on the design before sending the final files for fabrication. Peer reviews always are recommended since this allows a person who is not familiar with the footprints to inspect them. There are tools to overlay component outlines for verification as well, but the price tag typically will keep the task in the designer’s wheelhouse.
Figure 1. All too often, footprint drawings are either not to scale, or the parts do not fit the manufacturer’s recommended footprint.