What are Footprints and Land Patterns

5. 2-Lead Discrete Components

In Chapter 5 of What is Circuit Design, I introduced E-Series and component value standardization.  What wasn't explored at the time is that those components are also standardized around common sizes.  

Hard Way Hughes

"Diodes, chip capacitors, resistors, and inductors all come in common sizes.  In the United States, it is common to hear engineers mention "0603, 0402, 0201, 01005" sizes, referring to the imperial component dimensions."

Imperial and Metric Component Sizes

A collection of common part sizes as measured in imperial and metric measurements

Unfortunately, in North America, the parts are often listed in catalogs and online part libraries based on their size in imperial measurement (measured in tens of mils) while their footprints and associated land patterns are defined in metric measurements (measured in tenths of a millimeter.)  And to make matters worse, many of the metric values are rounded to make them identical to imperial values for different size parts.  Currently, in America, engineers describe parts based on their imperial sizes, but you must use their metric footprint to make the land patterns.  

  

A select number of parts that have similar or identical values across the different measurement systems.

You might remember that earlier in the course, we selected a 0.1 uF 0603 ceramic capacitor.  The 0603 referenced its length and width in imperial measurements - 60 mils long by 30 mils wide.  However, the footprint we need to use for this part is 1.6 mm x 0.8 mm -- we'd choose a CAPC1608x90

Hard Way Hughes"We put jobs on hold every day because engineers pick parts that don't fit into a particular footprint size."