Screaming Circuits: Foot Prints


Foot Prints

I write a fair amount about component foot prints / land patterns / CAD library components - whatever you call them. What are you supposed to call them anyway? I've heard them called all three of those things and a few other names as well. Seems like a good case for some terminology standardization. Or maybe I just need to do some more research on that specific subject.

PCB123 cut out capture I'm curious as to how common it is that a designer needs to create a custom footprint for a part. I seem to have to create at least one for every project I do and I rarely use new parts or complex packages. Of course, I don't use CAD packages that cost gobs of money either. Just Sunstone's PCB123 and Cadsoft Eagle. PCB123 has all of the NXP parts so that's good, but it's not just the big components that get you.Eagle library design

In a board I 'm just finishing up - 22 BOM line items, I had to make a custom library part for a crystal and modify a library part for my microcontroller. I had to create some custom footprints for a couple of connectors too, but half way through the layout, I upgraded Eagle and discovered that someone else had just done the same thing I needed for the same connector, and uploaded it to the Eagle site, so I used their work. Hopefully I haven't made any of the mistakes I always write about.

I really am curious as to how many designs typically require at least one custom footprint and how many custom footprints per design would be a typical rule of thumb.

Duane Benson
Aquatone

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Comments

Awesome post! You bring up an interesting point for discussion. I work with the McGraw Hill Sweets Network, who offers a large cad library for all types of use.

Why is it that there's not a standard file format for packages and schematic symbols?

I often have to make new parts, but it seems to me this is something the manufacturer should do. They're providing detailed descriptions of the land pattern in the datasheet anyway.

I have used the terms "footprints" and "land patterns" interchangeably for years, but was recently corrected by one of the IPC grand masters who I respect.

If you want to get on board with the IPC, the footprint is the actual physical dimensions of the component itself, so think of it like you picked up a component and pushed it into the sand, the imprint in the sand is the component footprint.

The land pattern however, is the conductive areas on the surface of the board, which has much larger dimensions to allow for proper solder fillets and to cover tolerance range.

I always think of components as the actual parts that are going to be placed and soldered, but the term "CAD component" should probably cover the symbol and land pattern AND the mapping information (pin assignments, gate/pin swapping info, refdes, part number, etc) for packaging syms to parts; enough info to generate the BOM

just my take on it,
(onward thru the fog!)
Jack

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