In order to meet the IPC-610 spec and the J-STD-001 spec for heel solder
fillet, the heel fillet must extend up the back side of the lead greater
than or equal to 1/2 of the lead thickness. If the heel is off of the
pad such that you do not get a heel fillet that meets this spec, it is
too far off and is a reject condition. The heel fillet is the most
important part of the solder joint, as it is the portion of the gullwing
lead that is forming an intermetallic compound with the finished portion
of the lead. The side fillet and the toe fillet form an IMF with only
the exposed copper on the cutaway portion of the lead. While having a
toe and side fillet is good, the heel fillet is of primary importance.
Without a heel fillet, the toe and side fillet presence is irrelevant,
the heel fillet must be present.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mary Jane Chism
Sent: Monday, March 06, 2006 12:00 PM
To: [log in to unmask]
Subject: Re: [TN] Heel Overhang
Kevin,
This is one of those "cumulative" conditions where in order for you to
evaluate "heel overhang", you would need to evaluate "Side Joint Length"
and "Minimum Fillet Height".
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kevin Glidden
Sent: Monday, March 06, 2006 11:29 AM
To: [log in to unmask]
Subject: [TN] Heel Overhang
Hello again.
Question on IPC-610 (class 3) criteria. For SMT solder joints, there
appear to be criteria set for TOE overhang, but not HEEL overhang. Is
this an oversite or a purposeful omission? Is there a limit on how much
the heel of an "L" shaped SMT lead can be off the pad lengthwise? The
obvious and defined limit is "does not violate minimum electrical
clearance", but does anyone else have info on this criteria?
Thanks again,
Kevin Glidden
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