With many QFP's, and other fine-pitch components, there's usually no side
fillet worth speaking of either. The problem with not having a heel fillet
as well really shows up if the board is to be used in a harsh envirnment -
there, the heel fillet is all-important, as it's where the major stresses
are focused. No heel fillet and a measure of vibration, and you'll find
cracks promulgating rapidly through what solder joint you do have, with
obvious consequences.
Replacement pads are not an ideal solution, as they depend on the skill of
the operator to fit them, but the procedure to do so, and the materials
replacement pads are made from, are compliant with IPC-7721 for board
repairs, even class 3's. Circuit Technology Center, Inc do kits of parts
with full procedure manuals.
If you don't do so already, you could bond the affected components to the
board to support them against vibration, adn raise that waiver Kathy
suggested.
Peter
Kathy Kuhlow <[log in to unmask]> 13/07/2002 04:58 AM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Kathy Kuhlow
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: Re: [TN] part/pad mismatch
you might be able to find an after market adapter. I forwarded your
original post to a company qfp-adapter.com and asked them to help out.
When you mentioned the heel hangs off of the end of the pad, did you mean
that the heel overhangs the front or back of the pad? If the pad overhangs
the front, well, you are in a pickle. But if the heel is the only part of
the solder joint that can not be achieved then why not determine the amount
of joint that could be maintained to section 12 and then go for a deviation
from any heel joint requirements due to an incorrect pad layout.
Kathy
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