Kevin, 
 
A heel fillet is required for class three. It plays an important part of the mechanical bond for the SMT device. Additionally, minimum side joint length could be violated. J-STD-001 rev D for class 2 & 3 states, when the length of the toe is greater than three times the width of the lead, the minimum side joint shall be three times the width of the lead or 75% of the toe length (whichever is longer). When the length of the toe is less than three times the width of the lead; the minimum side joint shall be 100% of the toe length. That portion of the toe (hopefully all of it) is a mechanical bond to the conductive pattern on the PCB. The side joint length is visual evidence that solder flowed under the lead and bonded the lead to the land pattern. The goal is to bond entire toe portion of lead to land. There is an exception call toe down configuration where the toe portion of the lead doesn't make adequate contact to form a good mechanical bond to the land pattern. Hence, the minimum heel fillet shall extend to the mid-point of the outside lead bend at a minimal. This provides more surface area to bond to.
 
Eva Jones
Instructor
Quality Engineer

________________________________

From: TechNet on behalf of Kevin Glidden
Sent: Mon 3/6/2006 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


---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------



---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------