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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