Anatomy of PCB Footprint


Any respectable pcb design software will come with a handful of footprints to get your pcb design off to a quick start. However there is no doubt that you will need to know how to quickly build your own custom library of footprints.

A footprint breaks down into 4 main parts. Pads, Obstacles, Text, and Origins.

Pads

Every component has a pin associated with it.  Where this pin comes in contact with the pcb, is called a pad.  This pad is formed from several different variables which we’ll discuss in detail as we go.  In most cases every pin will have the same pad, however you can vary pin1 or a high current output with a different pad on the same component.

The top and bottom layers of the pad will need a height and width or a diameter defined. If a hole is needed the drill size of the hole should be specified in finished hole size. You don’t have to account for plating, etc. Most programs also have an option to simply add thermal reliefs, whether you want a plated hole or not, and clearance areas.

Using the Free Landing Pattern Viewer you should easily be able to get the necessary vales needed to define the pad including, copper pad, solder mask, solder paste.

Obstacles

An obstacle is a graphical characteristic that you want to add to the footprint. These could be component outlines shown on the silk screen or mechanical layers. These obstacles can be used to define keep out and and keep in areas when using the autorouter.

Text

One of the most important pieces of text on the entire printed circuit board is the reference designator. This are displayed on the silkscreen layer and ussually the mechanical layer. Text is also used for serial numbers or design revision numbering.

Origins

When each footprint is placed onto the circuit board for layout you can place it on the board by using its origin. The origin could be the pin 1 center or the body origin depending on how you define it in the footprint. This is critical for precisely placing components or sensor arrays that must be aligned to an optics system.

This is just a quick overview of pcb footprints. It should be enough information to get you started off in the right direction. If you can get the pad defined with the proper spacing, a component outline, reference designator and orgin you’re probably in good shape.