Hi Chris,
Let me give you a couple scenarios.
1) You work for a large company that manufacturers many design, in
large volumes. One thing you don't want is scrap. Scrap = dollars,
volume = large, volume+scrap= $$$. Understood? So, what these companies
normally have is a manufacturing engineering department. These folks
help come up with the formulas for the land patterns so they don't get
(as many) scrap boards. It can sometimes take 10-20 days to get an
approved land pattern through these folk, especially if you're using
some "new fangled" part type (uBGAs as an example). Also, the land
patterns don't take into account the product market of that board. You
might be doing a simple test board for the company but you still use the
"volume manufacturing" footprints.
2) You work for a small, product driven company that needs boards turned
out to show profit, but there isn't much volume for your product (yet).
These companies "trust" the pcb designer to give them a land pattern to
design the boards. The designer either has the experience to "make" land
patterns or he/she outsources this part (they might even use the
software tool's inherent library). Some might use IPC, some might use
SMTPlus, etc. These companies normally don't have a manufacturing
engineering department, but will hire individuals that have experience
to make up for it. Mostly it falls on the designers and project managers
to handle the footprints.
I've worked both sides of the fence, and both sides (seem to) work. Your
problem, what standards do you use. I've used the IPC specs off and on
since they came about (SM782). They were great when they "told" you what
size to make the patterns (pre rev A). Problem was, with all the
manufacturing variations, and new parts, and new product variations,
there was just too much IPC would have to account for to give everyone a
"complete" standard (as they're trying to do now with the footprint
variations). What I found at the time Rev A came out was SMTPlus, and
their land pattern guide. I've used it since, and have gauged it against
companies footprints that have used "manufacturing formulas". SMTPlus
has worked for me, and continues to work fine. I don't claim it to be
the greatest, but I also don't like to waste time (should I use .003"
tolerance, or do I need to go .005" tolerance, BLAH!) in making a damn
footprint.
In the end, unfortunately, it's the individual designers (usless you're
with the large companies) like yourself that has to go out and "qualify"
footprint sources. There are a few to choose from, and nowadays, they
will even provide you these land patterns per your software application.
'Course then you have to go about making sure the silkscreens are right,
the padstacks are what you like, the text is how you want it, etc, etc,
etc. (Get my drift?) The footprint again is a small part of the puzzle.
I've often wondered if a part of my title should be "Footprint
Designer". Not!
Have a great day, and "design on". :)
Mitch
---------Included Message----------
> Date: Wed, 15 Aug 2001 07:55:24 -0500
> From: "Chris Robertson" <[log in to unmask]>
> Reply-To: <[log in to unmask]>
> To: <[log in to unmask]>
> Subject: [DC] SM footprints and standards
>
> I have several links to surface mount standards, but what I would
like to
> know
> is who actually makes the standards for these footprints?
> Is it IC makers and they are adopted such organizations as IPC?
> Are the IPC standards THE standard or are they a collection of
> others?
>
> I understand there are some European standards as well. Who
> controls these? (These may be a Gary questions)
>
> I've found standards or actually descriptions at several
different sites and
> want to ensure all are correct.
>
> Same question with package types....
>
>
> CR
>
---------End of Included Message----------
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