Thanks for the response Loa, but again, I don't
see how spacing can vary in the range of 6 to 28 mils
and still ensure that the leads will sit on the lands/pads.
At MMC, with the minimum possible spacing, this
analysis says that we would have a lead that's 12 mils
wide, shifted over to one side by 7 mils.
The maximum lead overhang allowed by J-STD-001 is
25% of the lead width, which in this case would be 3 mils.
If we take the easiest case and assume that the pads
on the board are positioned dead nuts perfect, then the
pads would have to be 20 mils wide to ensure that the maximum
overhang is 3 mils. And in this case, the electrical clearance
between that lead and the adjacent pad is only 2 mils. If we
allow for any positional tolerance of the pads, you'd have
to make them even wider, which effectively eats up any
pad-to-pad spacing.
This is where I'm having trouble. I think my real question
here is whether we can just subtract the true position
tolerance. The positions of the leads are referenced to
datums, not to each other.
-Joe
-----Original Message-----
From: ALLEN, LOA [mailto:[log in to unmask]]
Sent: Friday, November 17, 2006 10:17 AM
To: (Leadfree Electronics Assembly Forum); Kane, Joseph E (US SSA)
Subject: RE: [LF] Lead Spacing
Joseph,
Not quite sure I understand your question but here goes anyway . . . it's Friday.
For the MO-137 spec they are indicating that max deviation of pitch is .007" at maximum material condition (circle-M modifier in the feature control frame). The pitch is.025" and the leads at 'maximum material condition' (MMC) are .012", therefore the minimum air gap allowed is .025"-.007"-.012= .006". If the leads are at 'least material condition' (LMC), then you are allowed .004" bonus tolerance (max lead size of .012" - minimum lead size .008"). Therefore your max air gap would be .025"+.007"(allowed deviation)+.004"(bonus tol)-.008"=.028".
If you are concerned with pitch only then pitch at MMC is .025"±.007=.018-.032" where pitch at LMC is .025"±.011=.024"-.036".
It's been a number of years since I've exercised the true position brain cells, maybe someone else can smooth the rough edges.
Loa
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Kane, Joseph E (US SSA)
Sent: Thursday, November 16, 2006 9:10 AM
To: [log in to unmask]
Subject: [LF] Lead Spacing
Didn't get any help with this over on TechNet, so I'll try cross-posting
over here.
Guys, need some help. We've got a dispute about how lead spacing
is dimensioned according to the JEDEC specs. The class I had on
Y14.5 was a long while ago, and my geometric dimensioning and
tolerancing
is pretty rusty.
This is of interest because we're trying to figure out when to perform
hot solder dip for whisker mitigation on pure tin components.
Here are some links to the specs for some 25 mil pitch parts, from
JEDEC:
http://www.jedec.org/download/search/MO-137C.pdf
http://www.jedec.org/download/search/MO-118b.pdf
What are the max and min lead spacings? Yes, leads can change width,
being wider at the top of the package, but I'm more interested in how I
can
figure this out for many families of leaded parts.
Some of the answers we've been getting in house don't pass a sanity
check, in my opinion. The pads on the board are dimensioned in much
the same way, and if the lead spacing varies too widely, I don't see how
the leads will sit on the pads, much less meet the 25% maximum overhang
in
J-STD-001.
-Joe
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