What are Footprints and Land Patterns

6. Package Name Resources

6.1. Land Pattern Determination

The Old Way

The size and shape of discrete components, including chip resistors and their associated land patterns, have been standardized for a number of years.

Component and Land Pattern

These values are copied from IPC-SM-782A rev 2 from 1999.  This data precedes the subdivision into maximum, nominal, and minimum density classifications.  Measures C and Y are redundant.

You'll notice that IPC dictates both the size of the part to the manufacturer and the recommended land pattern to the PCB designer.  Take package 3216 (1206), for example.  The length is 3.2 mm ± 0.15 mm and the width is 1.6 mm ± 0.15 mm.  The nominal length and width of the footprint, measured in tenths of millimeters, are used to generate the metric package number 3216.

The New Way

Things have become more complicated though, since the introduction of the maximum, nominal, and least density specifications.  Now each part has three land patterns associated with it. Each pattern describes the board density, land pad size, and courtyard.  The maximum variant offers the greatest capability for cleaning, rework, and inspection.  Components that use the minimum variant are packed so tightly next to one another that there is often no opportunity for repair or rework.  Many discrete components (inductors, capacitors, resistors, diodes, etc...) have similar land pattern dimensions and similar naming schemes.

Maximum Nominal and Least density patterns.

Land-patterns (including courtyard) for a molded 3216 polarized capacitor are CAPPM320X150X180L80X120M, CAPPM320X150X180L80X120N, and CAPPM320X150X180L80X120L

You should be able to find land patterns for your discrete parts without too much trouble.  Remember, you can use your footprint wizards, or copy the pattern from another part if you have to add parts to your library.