At 10-03-04 18:36, Ray Mitchell wrote:
I'm sure this is a repeat, but is there a simple specification readily available that gives the "commonly accepted" (if there is such a thing) dimensions for 0402, 0603, ..., SIOC-14, etc., and all the other "standard" footprints? I don't really want to wade through a bunch of technical stuff to derive all of this myself and I certainly don't want to trust a priori the patterns that come with Protel or any other product. It's really annoying when part manufacturers don't provide these footprints, assuming they are common knowledge.

Others have already refrenced the s'standards' (IPC). I'm a little more sceptical.....


To my knowledge, there is no such thing as a standard footprint..
Was it not the CEO of Sun Micro Systems who said at one time "I just LOVE standards, because there are so many of them".


In short, even a simple shape such as a 0805 is implemented differently by different manufacturers.
Weird? I'd say so. Surprising? Not really: OUR wheel is MUCH rounder..... New and improved...... New formula...... You know the buzz.


I use a spreadsheet (not ready for the public yet, but I'll post it sometime in the not-so-distant future to the Yahoo list) to calculate a footprint from the dimensions and tolerances of the component given by the mfgr. The differences are sometimes quite large (as in: HUGE); in fact so large that a component from anoher mfgr will not fit on the same footprint (we lost a complete production run on that one).....

I'd still say to look up the mfgr datasheets (whenever available) and derive your own footprints from that information using a combination of guidelines (IPC, JEDEC) and common sense, seasoned with a tuch of practicality (you don't want 73 mils distance on 0805 pads when you're using a 5 mil grid).
Hey, I even round off pad sizes that were designed in mm to an integer number of mils.....and I'm in the metric part of the world ;-)
Find values (IPC, JETEC, your own fab house) for things like placement accuracy, heel and toe solder fillets and ask your board stuffer what component clearances (and directions) are needed for what processes and machines they use.
All of these (and then some) will influence the final optimum footprint......
Last but not least: small series (50+) can most of the time get away with minor deviations from optimum, big series (500+) usually can not, huge series (10k+) never can.


A lot of work, yes, but one only uses so many different components......

I found it worthwile to invest the effort: I have no production failures yet on my boards (apart from that one that was designed for Philips TSOP and could not accomodate Fairchild).
BTW, I am designing boards for just 10 years now, so I really don't know much about the subject yet. In fact, I'm still learning, every single day.....


To qoute another frequent poster to this list: "I hope this helps".

Regards,

Leo Potjewijd
hardware designer
Integrated Engineering B.V.

[EMAIL PROTECTED]
+31 20 4620700



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