Hi, Earl,
We went through some pain with this a while back and decided to adopt the
following heirarchy:
1. Where supplier reccomendations exist, use them.
2. Where no supplier recommendation exists, but where there are fottprints
given in IPA-SM-782, use those.
3. Where neither of the above are specific, calculate your own. There is a
formula given in IPC-SM-782 near the beginning, but it's also on-line from
the IPC website. There are, however a number of other on-line calculators
available that give slightly differing results, so the pattern you wind up
with can often be based on best guess and experience, given the surrounding
board features and the assembly processes to be used.
Determining land patterns/footprints is still something of a black art
rather than a science, and as with all recommendations, they aren't always
applicable to specific cases - they have to be decided by the man on the
spot.
Not a lot of help perhaps, but then there isn't much to be had in this
area, I found - too many permutations that affect a given pattern for a
given component on a given class, layout, construction, operation, etc.,
etc., etc. of PCB.
Best of Luck
Peter
Earl Moon
<[log in to unmask] To: [log in to unmask]
M.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: Aero/ST Group)
TechNet Subject: [TN] IPC SMT FOOTPRINT DESIGN
<[log in to unmask] GUIDLINES VS SUPPLIERS
ORG>
04/15/02
02:02 AM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
Earl Moon
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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