The Four Basic Parts of Every EDA Tool
In the next several weeks, you will have the chance to gain to work with each portion of your Electronic Design Automation (EDA) software. This book is meant to provide a brief overview while you work through the sample projects provided on the EDA tool websites.
2. Pattern Editor
Engineers use the term footprint to describe the outline of the bottom of a discrete component and its pads/pins.
This bottom-side view of a QFN28 package shows the metalized pads that will be soldered to a landing pattern (not shown).
This artistic image of a 14TSSOP package hovering over a matching land pattern. Notice that the lands are larger than the gull-wing pins, but both are equally spaced.
To make things even more exciting/confusing, the land patterns associated with those pads are often incorrectly called footprints by engineers, even though the metalized lands have to be larger than the metalized pads/pins they mate with. Electronics Engineering is terrifically confusing to begin with, the situation is not improved by engineers that use the same words to describe two different things. But that's the world we live in, so let's try to make sense of it.
- Discrete components have metalized pads or pins on their base and sides.
- Printed circuit boards have metalized lands on their outer layers.
These two metalized parts have a similar pattern but are two different sizes. Solder connects the lands to the pads/pins.
Lands are metalized portions of a printed circuit board that are ever so slightly larger than the pads and pins on components.
For a proper solder joint to form, the solder has to form a filet, and the only way that can happen is if the land is bigger than the pad/pin.
A discrete component's metalized pad pattern and a printed circuit board's land pattern are identical in layout and centerline spacing, but the metalized lands are always bigger than the metalized pads.
The size and shape of footprints and lands has been largely standardized into IPC-7351, a document that describes the pin pattern, layout, and naming convention for all standard parts. The document is unfortunately locked behind a paywall. That's okay, it's not a very exciting read, and most EDA programs have incorporated the data from the document into their pattern editor libraries. "Wizards" generate the pattern and pattern names needed in the program. Additional patterns can be constructed manually from information supplied in datasheets.