I've watched , I've waited and now I have to add my two cents.
IPC-782 is a great place to start. These land patterns work (though not
always perfectly) and are simple to understand as a good starting place for
well documented library creation. The difference between the standard and
the calculator is, I think that the standard is made up of process proven
design where the calculator is just that, a calculation of acceptable (key
wording - acceptable not preferred) extension from termination size to allow
an usable solder joint. My experience has been that the calculator usually
creates a slightly smaller footprint than the standard recommends.
Back to the original subject:
I have always recommended that for library creation the following inputs in
order of importance:
1. Process proven footprints
modifications of existing footprints that best fit the process being
used (determined by trial and error/DOE)
2. IPC-782
this is an industry standard
3. SMT+
the folks at SMT Plus have done a good job but don't always fit all
processes
4. Purchased library sets
some are good , some are good starting places, some are bad
5. Creations
using the IPC calculator or calculating from scratch using IPC-782
recommendations and DOE.
6. Software libraries
a starting place to create better symbols from
7. Manufacturers data sheets
I don't trust these well enough to use except as a verification of
my calculations. Unfortunately data sheets are all to often WRONG.
Almost done here. I adamantly agree the most important step while finding a
good land pattern is to document what was done. All to often a change in
personnel, vendors or suppliers can cause an existing footprint to become
non-optimal and only good documentation will keep mistakes from having to be
repeated.
Lastly, though I personally think good PCB design is an art more than a
science I also agree that great PCB design requires that that art be a blend
of good science, communication, common sense and documentation.
Hope this makes sense and helps.
Thanks,
FNK
Frank N Kimmey, C.I.D.+
Senior PCB Designer
Powerwave Technologies
PH. 916-941-3159
Fax 916-941-3195
-----Original Message-----
From: Jack C. Olson [mailto:[log in to unmask]]
Sent: Wednesday, April 17, 2002 5:27 AM
To: [log in to unmask]
Subject: Re: [TN] IPC SMT FOOTPRINT DESIGN GUIDLINES VS SUPPLIERS
Yes, I agree, and I have a confession to make as well...
I asked an IPC dude (Gary) about the difference between the
registered pattern dimensions and the calculated results,
and he said the calculator was in the process of a significant
re-design, but then I never heard anything about it since then.
(Does anyone know if there is a newer version on the way?
or is that what we are using now?)
The confession is, sometimes I am lazy and just look it up
in my SMTplus library, since they have every SMT part under
the sun. (my original post said "If I was starting OVER again")
After being frustrated by seeing 4-5 "suggested" patterns
for the same darn part from every vendor and data sheet,
I still maintain it is preferable to START with a standard.
As for your statement about "permutations in board design,
materials, assembly processes, component package styles,
materials and contents, operating conditions..."
well, this getting into the area where your knowledge and
experience is leading you towards a modification, which is
GREAT!, All I say is DOCUMENT THE REASON so the next bozo
to come along won't change it and lose all the benefit of
your work.
And I still stick by my statement that if you DON'T know
what you are doing, USE A STANDARD, don't use a component
data sheet.
Jack
[log in to unmask]
16Apr2002 08:02 PM
To: "TechNet E-Mail Forum." <[log in to unmask]>
"Jack C. Olson" <[log in to unmask]>
cc:
Subject: Re: [TN] IPC SMT FOOTPRINT DESIGN GUIDLINES VS SUPPLIERS
Retain Until: 05/16/2002 Retention Category: G90 - Information and Reports
Caterpillar Confidential: Green
Jack,
I had great difficulty in getting land patterns to "conform to IPC". If you
have a registered land pattern and it's in IPC-SM-782, but you then
check/compare it using the calculator/formula in the same spec, you get two
different results. Where is the conformity, I ask? Also, there are more
instances of no-land-pattern-information-availability than there are
guidelines or recommendations. As was said, component manufacturers do
their own thing, maybe loosely centred around "standard" package types, but
they often differ enough from the standard to make one suspicious about
using the "standard" land patterns for them.
As I said (provocatively) in my last reply posting, there are too many
permutations in board design, materials, assembly processes, component
package styles, materials and contents, operating conditions and so on, to
have a concise "standard" for all occasions - unless each parameter in the
overall CCA universe becomes standard/fixed. What we have are workable
compromises that suit most groups of parameters. It's towards the more
extreme CCA applications and environments that these compromises start to
break down.
I believe there's still a ways to go before 'art' changes to 'science'.
Peter
"Jack C.
Olson" To: [log in to unmask]
<OLSON_JACK_C cc: (bcc: DUNCAN Peter/Asst
Prin Engr/ST
@CAT.COM> Aero/ST Group)
Sent by: Subject: Re: [TN] IPC SMT
FOOTPRINT DESIGN
TechNet GUIDLINES VS SUPPLIERS
<[log in to unmask]
ORG>
04/16/02
10:53 PM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
"Jack C.
Olson"
Great question!
We have wasted SO much time designing custom libraries
for specific vendors, its to the point of being ridiculous.
Here is what I would do if I was starting over:
- Design to IPC unless I had a GOOD reason not to.
- NEVER use a component data sheet. can't trust 'em.
- If a vendor had a reason supported by SCIENTIFIC
DATA to suggest a change, DOCUMENT IT!
The ideal (in my opinion) would be to say "All footprints
conform to IPC with the exception of..." and for those
exceptions explain WHY YOU CHANGED IT. (including
high density versions, wave solder alternates, whatever)
best wishes,
Jack
-----Original Message-----
From: Earl Moon [mailto:[log in to unmask]]
Sent: Sunday, April 14, 2002 2:03 PM
To: [log in to unmask]
Subject: [TN] IPC SMT FOOTPRINT DESIGN GUIDLINES VS SUPPLIERS
Folks,
Just a simple question really. How many folks use IPC SMT footprint
guidelines compared with supplier recommendations? There often is quite a
difference. As everyone on this planet uses IPC acceptance criteria, why
shouln't everyone use IPC design guidelines but for component supplier
liability issues?
MoonMan
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