Hmmmm....  This is getting interesting. I think I'll troll a little and
see what I get .....

These comments refer to trimming wave soldered leads, NOT parts put on
at touch-up/add on. In general,  we as an industry have developed our
post-wave cleaning processes to the point where they are predictable and
repeatable, and meet the cleanliness standards we set for ourselves.
When the boards come out of the post-wave cleaner, they are "clean".
Ideally, all leads are cut to length *before*  wave solder, and they are
all solder coated at this point. No trimming is necessary and
deformation / fracture is not an issue.

Parts put on at touch-up/add on, on the other hand, are frequently done
there because the parts are not compatible with our cleaning processes.
So they are installed on a bench, by an operator with a soldering iron,
a plunger pump full of flux remover, and a brush or other device to
apply the flux remover.  At the point the part is added, flux is already
on the board from soldering the part in, so by all means, re-solder the
clipped end of the lead if you wish. The operator dependent cleaning
process at add-on  may be <g> much less predictable and repeatable than
the post-wave cleaning operation.

 I think flux residues left on the board from touching up trimmed leads
might be more of an issue than the bare copper, particularly if you're
going to conformally coat the finished assembly.

II remember some articles advocating leaving rosin flux on the board -
the benign rosin encapsulates all the "bad" contaminants.  If you try to
locally clean, i.e. by brush and plunger pump, you decapsulate and
spread these contaminants across the board.  Also, the process is  time
sensitive.  If you don't clean the boards within an hour or so after
soldering, a molecular bond between the resin and the FR4 epoxy forms,
and you *CAN'T*  effectively clean after that.

Anyway, back to the thought -  Fab shops do SMOBC boards every day,
"conformally coating" bare copper traces with -usually- an acrylic or
epoxy material we call 'soldermask'.  What's the difference in
reliability between that and coating the trimmed bare copper lead ends
with -usually- a silicone or a urethane or (again) an acrylic  material
called 'conformal coating'?  If none, then we shouldn't spend the time
to re-solder wave soldered /cleaned/ trimmed leads, and we certainly
shouldn't contaminate with flux the boards we just cleaned in our
predictable, repeatable post-wave cleaning process.

 What do you think??

Mike Matson
The usual disclaimer applies - I'm on my own with this one.

"Give me ambiguity or give me something else."
 ----------
From: [log in to unmask]
To: [log in to unmask]
Subject: Re: ASSY: Lead Trim After Wave
Date: Thursday, November 14, 1996 2:43PM

 Randy,

        As you are likely aware, IPC-A-610, section 4.2.2 states that
"When  lead cutting is performed after soldering, the solder
terminations shall either be reflowed or visually inspected at 10X to
ensure that  the original solder connection has not been damaged, (e.g.
fractured),  or deformed."

        My research has revealed that experts are divided on the need to
 re-solder the joint after cutting.  In practice, our company opts not
to re-solder and has built well over 200,000 assemblies without any
reliability issues.  The assemblies are thoroughly tested as well.

  - David Bruni
______________________________ Reply Separator
_________________________________
Subject: ASSY: Lead Trim After Wave
Author:  [log in to unmask] at cc_gate
Date:    11/13/96 5:51 PM

Is there a consensus one way or another on the practice of clipping
component leads after wave in regard to the effect on solder joint
reliability and the effect on reliability of leaving the clipped ends
untinned?  The level of reliability expected is Class 3.

Thanks

Randy Johnson
Reliability Engineering
Woodward Governor  Company
[log in to unmask]

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