OK, I know I’m supposed to spend my days pondering the imponderables pertaining to programmable logic, but FPGAs end up on circuit boards, and I just ran into a problem with regard to PCB terminology…
My mind bounces from one topic to another with the agility of a mountain goat (my body takes a little longer), which may or may not explain why I suddenly started to wonder as to the “official” name for the graphics associated with the pads for components when a layout designer is laying out a circuit board. Thus it was that I sent the following message to Jack Olson, who you may have seen commenting on various articles here at Programmable Logic Designline:
Hi Jack, Quick question, when a PCB layout designer is laying out the board – the graphics associated with the components – are these called “footprint graphics” or land patterns”? Could one use both terms? Are there other terms one should use? Cheers -- Max
Jack quickly responded as follows:
Well, a little discussed but (when you think about it) important facet of the Electronics Industry is for people to gradually converge on a common interpretation of terms. I don't know if you've noticed, but if you ever visit isolated pockets of engineering groups, they almost always develop their own language subset which speeds up communication among themselves, but visitors or new members need some "translation time" to get up to speed with them.
Nothing wrong with that (cultures have been evolving language for quite some time, right?) but now that we have design groups that can stretch internationally, and wide-audience communications like your blogs and internet email forums and such, it’s important for most of us to use the same terms in the same ways. That is one of the side-benefits of an organization like the IPC, they have members from hundreds of different companies trying to discuss (and reach consensus) on many related topics, and for that to happen effectively, they need to have a common understanding of the terms.
So, to answer your question, each CAD system developer picked their own names for what you are referring to; footprints, geometries, land patterns, cells, etc. but moving forward into the future we (the IPC) have found it useful to refer to them as "land patterns" and use the name "footprint" in a different way. You can think of a footprint as what you would see if you picked up a component and pushed it into sand or clay, the imprint it leaves would be its footprint, which is its actual physical size. The land pattern on the other hand describes the size of the pads needed to solder the leads to the board, which have wildly differing dimensions depending on what you are trying to do (wave soldering, reflow, high-density, etc.). So for a given part it will have one footprint but maybe several land patterns. And, for the record, even though many of us are trying to use standard terminology, MANY are NOT, so you will still hear people talk about footprints as the pattern you see on a circuit board, probably forever! Hope this helps, Jack
Actually this was very useful. For example, it had never struck me that the pads on the board might vary depending on the way in which one wishes to attach the component to the board. Hmmm… interesting. So all was now clear, until Jack sent a follow-up message as follows:
Funny, after all that philosophy I went checked the IPC terminology document, and it still has it the old way. Don't think I told you wrong though; I stand by my explanation and will submit a request for correction.
Wow! Just think that the humble musings by yours truly have set the ball rolling to instigate changes at that august organization. Even now, those who don the undergarments of authority and stride the corridors of power at the IPC are doubtless sending manic memos to each other asking “Who is this man known around the globe as Max The Magnificent?” To which I reply: “Be afraid, be very afraid…”
Some time ago, when I did PCBs by hand with a light table, we said "dolls", corresponding to Mr. Olson's footprint; the land pattern was implied. These were imprinted plastic sheets, die-cut to the outline of the components, which were adhered to a large clear plastic sheet. Another sheet was overlayed, and black tape used to complete the circuit routing.
It is possible that the name was a derivative of "models", shortened to "dels", then vulgarized to "dolls".
It is also possible that these plastic stick-ons were similar to the Barbie dress-up toy, and a number of other games. In those toys, plastic cut-outs adhere to a background, allowing the child to complete a picture. Afterwards, the stick-ons could be easily peeled up and stored for later play.
Freud could have a field day with this one.
"So far no one has called me an accented immigrant!" I'm glad to hear it -- if they did they would have me to answer to! :-) [By the way, I'm an immigrant myself -- I'm from the UK -- and I still have my English accent (grin)]
The definition Max explained is technically correct. However I have used the term "footprint" for both the component and the PCB land pattern this way:
"the components footprint is..." or
"the PCB footprint for this component is..."
So far no one has called me an accented immigrant!!
Yes, it makes sense to distinguish footprints (component outlines) from land patterns (solder pad layouts), but in practice, when using PCB layout tools these are drawn together as an entry in a library of footprints (or other name!). I've not heard of any PCB design software that can handle the component outline and the pads separately.
Oh, and the PADS software package calls them "decals".
I've also heard the terms "library parts" and "library components" used in reference to the actual code in the CAD software. Those along with "land pattern" and "footprint" seem to be pretty universally used interchangeably in my world.
Generally, when I've been speaking with someone who looks at things more from the perspective of the chip, it's a foot print or land pattern. From those in the EDA world, it tends to be library part or library component. It drives me nuts and thus I do like the idea of standardizing on one term. I pick "footprint." - just kidding. "Land pattern" sounds good to me. I'll do my best to promote that as the standard.
I find it an interesting phenomenon how companies frequently end up with their own vocabulary. It's not just in engineering. Many areas of the business do this. Spin-offs seem to follow the original company's dialect. You'd think we would all be taught the same vocabulary in college and would take that with us, but in practice, that idea seems to fall flat in the face of reality.
Yes Max, with our subtle influence and occasional well-directed questioning,
"We Control the Language of Electronics"!
p.s. No, I refuse to list any other terms for land patterns, because we want to converge on the term
L A N D P A T T E R N S ! ! !
All else simply falls short of our struggle towards a common terminology.
Get with the program, my friend. (smile)