When laying out a circuit board, it's essential to select the proper footprint for each component. This can be complicated because different manufacturers have differing ways of referring to package sizes. An handy example is the simple SOIC-8 surface mount package. The Eagle Microchip library has 3 versions of this package:
Apparently, these are for narrow, medium and wide packages. Which footprint should be used for the op-amp I'm using? The first step is to check the dimensions on the manufacture's data sheet.
Knowing the package dimensions is the first step. To aid in selecting the correct footprint, I've created a Eagle library containing a number of rulers. I can place a possible footprint next to a ruler and quickly evaluate the size. The illustration below shows the above mentioned package sizes.
From the data sheet drawing, I can see that the pads should be about 4 mm apart in the center, and extend to about 6 mm from top to bottom. The SO-08 footprint on the left above looks like the best option.
The next question is the pitch of the leads. The data sheet says the pitch is 0.05" between centers. In the rulers library, I've created some pitch gages at common spacings to quickly check the spacing.
This seems kind of trivial, but it's important to remember that PCB footprints are of varying quality. Many of the Eagle (and other PCB packages too) libraries have been created by users and mistakes do show up from time to time. The TQFP packages shown below should have a pitch of 0.80 mm. The pitch on the smaller package lines up with the pitch gauge but the pitch of the larger package does not; in the span of 16 pads, the spacing is almost half a pad-width off! The footprint would probably work alright but it would be difficult to align and solder.
Errors like this occur when a footprint is created using Imperial measurements when it's based on metric or vice versa. Data sheets show dimensions is both sets of units for convenience; the pitch of the SOIC package above is shown as 0.050" (1.27mm) but the conversion is approximate. Over a number of pads, the rounding error becomes significant!
The illustration below shows the various rulers in the library at this point. It's very much a work in progress. I hope it will be helpful in your circuit board designs. Please let me know of any additions that should be made, or better yet, share your improvements here! The library may be downloaded below.
The library files have been updated as of February 3rd, 2013. The origin of each element has been moved to be the lower left corner to make moving and deleting the element easier.
Please comment if you find this library or idea useful.