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...
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…”