In a perfect world, I absolutely agree of course. Given a
choice between no heel fillet (with proper toe and sides
evident) and the reforming of leads? I would take the lack of a
heel fillet any day.
Some of the other solutions are interesting as well, and the
preferred one would be to acquire properly designed boards, but
when stuck between a rock and a hard place - analyze the
intended purpose of the board and make the best quality,
reliability decision.
Jan Pelchat
______________________________ Reply Separator _________________________________
Subject: Re: [TN] reforming a SOIC
Author: Jack Crawford <[log in to unmask]> at 0UTG0ING
Date: 8/26/98 2:43 PM
Please refer to current documents J-STD-001B and/or IPC-A-610. The heel fillet
is
the most critical part of the solder joint on a gull lead device. Insufficient
heel
fillets even with max to the top side/toe fillets are NOT acceptable for any
class
product. There is high probability of cracking of the side/toe fillets with
minimal
normal thermal excursions if there is not heel fillet. Conversely, a proper
heel
fillet will survive a lot of environmental stress screening even if there are no
side/toe fillets. This topic has been discussed as great depth in IPC technical
meetings in the past. There are no plans to change these requirements in the C
revisions now out for proposal on both the documents. Jack
IPC HAS A NEW PHONE SYSTEM!! EXTENSIONS HAVE CHANGED!!
Be sure to listen to the prompt for new directions. 8/20/98
IPC/SMTA Electronics Assembly Expo
Technical Committee Meetings, Conference, Exhibits
Providence RI October 24-29
More info at http://www.ipc.org
Jack Crawford, IPC Project Manager - Assembly
2215 Sanders Road, Northbrook IL 60062-6135
[log in to unmask] NEW DIRECT NUMBER 847-790-5393
fax 847-509-9798
>>> PELCHAT_JM <[log in to unmask]> 08/26/98 07:18AM >>>
If I were the Customer, I would rather leave the leads alone and
rely on the toe and side fillets than even attempt to reform the
leads. Particularly on Class 3 product.
The reforming would require buyoff from the Customer and if
they're going to buy off on anything, they should buy off on the
lack of a heel fillet.
If you get good wetting on the toe and sides, that is by far the
preferred scenario.
Depending on how many boards you have with this situation
(hopefully, not too many otherwise it should have shown up in
prototyping) it may be better to have the components hand placed
and soldered for optimum pad to lead interface and to give every
advantage to good solder filleting.
Jan Pelchat
______________________________ Reply Separator _________________________________
Subject: [TN] reforming a SOIC
Author: "Collins; Graham" <[log in to unmask]> at 0UTG0ING
Date: 8/26/98 8:54 AM
Technetter's
Let's say a customer designed a board with the wrong pad size, too far
apart, such that a gull wing lead placed on the pad doesn't have a heel
fillet. Obviously a bad thing. A redesign is in progress, but in the
interim it has been proposed that the part be reformed to spread the feet a
bit further apart (by about 20 thou). Component is a 20 lead SOIC.
My questions and concerns are:
- the bend will have to occur at the point where the lead enters the
component body, otherwise there will be no clearance between the part and
the board. How dangerous is this from a reliability perspective (class 3
product, conformally coated)? Should I take the next few weeks off? Flee
the country? Anyone have job openings?
- do any technetters have suggestions of a half decent way to reform these?
I'm currently faced with doing it to about 100 to 150 parts, and doing it by
hand does not appeal to me due to the variability.
Any other suggestions? I did consider borrowing some of the Freeze-it that
Steve's using on his diodes, and shrinking the boards, but the reflow
process might disagree with that...
thanks!
Graham Collins
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