Ray

I know your pain, I started off life as a designer, then a layout
engineer and after working with a lot of sub contractors did my bit in
process engineering, then in service/manufacturing doing failure
analysis and feeding that back to design where I ended up (again) and
due to the experience I had stayed there doing a better job (IMO anyway)
than I did in the first place.

But I think your frustrations are getting to you a bit, or my long
response tipped you over the edge (sorry), but as you mentioned in your
reply below, you had issues with data sheets and also manufacturing,
because of library/footprint issues, so as I said, a library has to be
more than it looks. And you cannot rely on the data sheets 100% unless
it has a report attached to it with all manufacturing details, Motorlola
and NSC do a lot of this, but most passive companies do not.


Best Regards

John A. Ross

RSD Communications ltd
Email  [EMAIL PROTECTED]
WWW    http://www.rsd.tv
==================================   

> -----Original Message-----
> From: Ray Mitchell [mailto:[EMAIL PROTECTED] 
> Sent: Thursday, March 11, 2004 7:48 PM
> To: Protel EDA Forum
> Subject: Re: [PEDA] Common PCB footprint specifications
> 
> Everyone,
> 
> Thanks for all the responses on footprints.  This whole issue 
> is pretty sickening actually.  Since we produce low 
> quantities of diverse products we have no dedicated PCB 
> layout people.  All engineers do their own circuit designs 
> and parts specification and ultimately are expected to do 
> tiny PCB layouts of everything and get them to work.  The 
> thing that gets me is that it seems like it would be 
> extremely simple for parts vendors to provide land patterns 
> for their parts along with the mechanical drawings of the 
> parts themselves.  Some do but most don't.  I just talked to 
> Maxim about this and they said they simply don't provide this 
> information.  They recommended IPCSM782.  Of course a good 
> percentage of the parts you need are not listed in this 
> document and a lot of them that are there do not match the 
> recommendations of the vendors of the parts.  I asked Maxim 
> how they layout their own eval boards since they provide no 
> guidelines and no guidelines exist in IPCSM782.  They didn't 
> have an answer but I suspect they rely on rules of thumb and 
> intuition, which is what we end up doing with our designs 
> here most of the time.  After enough bad yields and scolding 
> from our PCB fabricators we manage to stumble into something 
> that seems to work.  I did find what I thought was a good 
> layout for 0402, 0603, etc. from AVX capacitors.  Upon closer 
> inspection, however, I found that their recommended 
> footprints violated their own guidelines given on a 
> different page of the same document.   Go figure!
> 
> Ray Mitchell
> 
> 
> At 04:59 PM 3/11/2004 +0000, you wrote:
> > > -----Original Message-----
> > > From: Ray Mitchell [mailto:[EMAIL PROTECTED]
> > > Sent: Wednesday, March 10, 2004 5:36 PM
> > > To: [EMAIL PROTECTED]
> > > Subject: [PEDA] Common PCB footprint specifications
> > >
> > > Hello,
> > >
> > > 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.
> >
> >Ray
> >
> >I have accumulated quite a library of such footprints but 
> most of them 
> >will have been optimised to suit our in house processes more than 
> >following the IPC standards.
> >
> >The supplied Protel IPC land patterns are not too bad, they are 
> >certainly a good basis to build your own on. But most 
> libraries stop at 
> >the land pattern stage, which is what the IPC are looking to change.
> >
> >A lot of the way the IPC are trying to structure library conventions 
> >are along the lines of what I was already doing for years 
> anyway, not 
> >because it is good, but because it make life easier for us 
> internally 
> >if the naming conventions for footprints already match 
> vision library 
> >footprints on placement machines (which then relates to mechanical 
> >dimensions as well, as a Murata 16V X7R 0603 will have different 
> >dimensions to a Kemet 16V X7R 0603 in same voltage) and other EDA 
> >packages we use etc.
> >
> >I especially like the way the new IPC recommendations take 
> account of 
> >things like, 0 deg positions in tape or tray, if Protel 
> could also make 
> >allowances for rotation on non-polarised chip parts (only 
> use 0,90) to 
> >reduce un-necessary head rotations on turret head placers 
> that would be 
> >even better as I currently use an in house utility to parse the P&P 
> >files and check for string matches on footprint & part number to 
> >identify non-polarised parts and it will replace 360 or 180 
> values with 
> >0 and 270 values with 90.
> >
> >In DXP I planned to use a parameter for that at SCH level, so I only 
> >need to check for one match, but that's another story, no time for 
> >documenting or agreeing how this should be done internally yet.
> >
> >Same with pad sizes, I slightly oversize SMT pads in some 
> cases against 
> >IPC recommendations (not much) to allow for place tolerances when 
> >reducing Z height & down pressure, Vac release and place speed, 
> >especially on Chip r/c's as well as wave flow direction and so on.
> >Same for connector placement, especially for IDC and connector rows 
> ><2.54mm, I sometimes enlarge the pads beyond IPC 
> recommendations in one 
> >direction to get the best out of the features on our wave soldering 
> >equipment (Vitronics-Soltec with Select-X debridging).
> >
> >If Protel could assign a different footprint for rotation, or side, 
> >based on some sort of logical system, then it would make 
> life so much 
> >easier to define DFM rules even at SCH level.
> >Perhaps that's worth a new feature request on the DXP forum :)
> >
> >To me a library has to be more than just a symbols 
> collection, or the 
> >manual pre-processing required diminishes its value, very 
> little third 
> >party libraries do this, so IMO are not worth it.
> >
> >I like the IPC new offerings for library recommendations 
> very much, and 
> >would like to see it adopted, even although some of the naming 
> >recommendations may choke some placement machines offline 
> programming 
> >software or optimisers software a bit like white space, characters, 
> >case sensitivity and a lot of other things that should be 
> non-issues in 
> >this day/age. I prefer a direct import approach to programming these 
> >machines, Gerber import, pattern search and processing is 
> alright, but 
> >takes to long and can be error prone.
> >
> >If anyone wants me to split & upload the library contents I 
> have here, 
> >ill do it as a part time job, but I guess most people will 
> have these 
> >things already, or prefer to use their own in-house libraries.
> >
> >John
> >
> >
> 
> Ray Mitchell
> Engineer, Code 2732
> SPAWAR Systems Center
> San Diego, CA. 92152
> (619)553-5344
> [EMAIL PROTECTED]  
> 


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