I would also like to add to Werner's comments. The reason side fillets
are not considered especially beneficial is because the sides of most
SMT leads such as SOIC, SQFP, J-leads, etc. are exposed copper and
usually heavily oxidized. Side fillets are therefore not usually
reliable. The primary focus should be to get good wetting on the heel
fillet, where you are soldering to the plated portion of the lead. Toe
fillets are considered to be beneficial if you can get enough solder to
wet to the top (plated) portion of the toe, where the solder volume does
not contribute to the possibility of bridging. This is usually possible
because the toe is usually the lowest point of the lead once placed into
the solder paste. Time spent attempting to obtain solder fillets on the
sides in order to improve the reliability of the overall solder joint is
better spent on ensuring the pad size, component lead solderability, and
the component placement is optimal in forming a good heel and toe
fillet, where the heel fillet is at least higher than the thickness of
the lead. 

-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Monday, May 21, 2007 7:43 AM
To: [log in to unmask]
Subject: Re: [LF] No soldermask between SQFP leads ok?

Hi Lothar,
Solder joint reliability of SQFPs is a non-issue-provided you have good
wetting, because of the compliance of the SQFP leads. While side fillets
add strength to the solder joints, inadequate strength is hardly ever
the reason for solder joint failures, and essentially never with
properly wetted SJs.
In your case, adding side fillets is not warrented and the possibility
of bridging makes this an ill-adviced move.

Werner



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