What are Footprints and Land Patterns
4. Land Patterns
Parts have metalized pads/pins arranged in a specific pattern called a footprint, and they connect to similarly shaped land patterns on a printed circuit board. Land pads are always a little larger than part pads. The slight size difference allows a fillet to form on the toe, heel, and sides of the part pins/pads that increase the mechanical strength of the connection. It's also worth noting that many designers, including me, are guilty of using the terms footprint and land pattern interchangeably in casual conversation which undoubtedly leads to confusion.
Land patterns generally still have the same geometric layout and centerline spacing as pads. But individual land pads have a greater surface area than individual part pads. The land pads are generally longer than part pads to allow for proper fillet formation
Lands come in three standard density levels that give an idea as to how much larger the lands are than the pads.
- Maximum Land Protrusion. Used in low-density products where there is plenty of room between devices for large pads. Accommodates wave or flow soldering of leadless chips, "J"-formed leads, and leaded gull-wing
- Nominal Land Protrusion. Used in moderate-density products where the components are spaced close to one another, but not so close as to interfere with assembly or rework practices. Wave or reflow soldering of leadless chip and leaded gull-wing type devices is possible.
- Minimum Land Protrusion. Used in high-density products, where parts are stuffed so tightly to their neighboring parts that rework is often impossible and inspection is difficult. These lands have the minimum possible size that still allows proper fillet formation and wetting.
This animation demonstrates fillet formation on what appears to be a MLCC. Original source unknown.
A properly designed land pattern can help to center parts that are slightly misaligned during the pick-and-place process.
This animation shows a misaligned resistor self-centering during reflow. Original Source Unknown
Printed Circuit Boards manufactured today are small, and the components often look like they are stacked immediately next to one other. But a minimum perimeter must exist around the components that allow the part to be placed by an automatic pick-and-place machine and permit inspection and some amount of rework if necessary. If the parts are too close to one another, it's not possible to make repairs to parts that do not bond correctly to the PCB during the reflow process.
Extending beyond the edges of a component body and its associated land pattern is an area broadly referred to as a courtyard. IPC-7351 further subdivides the courtyard area into perimeter boundaries and perimeter areas called pattern boundary, courtyard excess, courtyard manufacturing allowance, and courtyard manufacturing zone. These definitions can best be understood via a diagram.
Image of Courtyard features from Tom Hausherr's Blog at Mentor.com
"If all of those terms and definitions seem confusing to you, don't worry about it too much -- I work for an assembly company and the only word I ever hear is 'Courtyard' when it is used to describe the general buffer-zone around each component. For now, leave a minimum of 1.25 mm (50 mils) of space between lands. If you have a design that places components less than 0.5 mm (20 mils) from neighboring components, and you expect to repair/rework your product, you might have to worry about the terminology a bit more. Otherwise, just understand you need a 'courtyard' and move on."