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May 2002

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Subject:
From:
Guy Ramsey <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 3 May 2002 07:10:54 -0400
Content-Type:
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I agree with you, as far as trimming leads before soldering. I agree, a
solder - cut - solder process should be avoided if possible.

But, I can't find any place in the J-STD-001C or IPC-A-610C which bans
exposed copper. In fact, I can only find places where it is specifically
acceptable, even including nicked leads unless the damage is greater than
10% of the lead diameter or thickness. Unless the damage is the result of
careless or repeated bending.

Is this a topic worthy of discussion? Have I misinterpreted the standards?

> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of
> [log in to unmask]
> Sent: Thursday, May 02, 2002 8:05 PM
> To: [log in to unmask]
> Subject: Re: [TN] Wave solder re-runs/re-work
>
>
> One thing I've learned in my 20+ years in electronics
> manufacturing is that
> one should pre-trim leads prior to soldering, not trim them afterwards,
> especially with  the plier type lead cutters. The shock to the solder
> joints when the leads part with that snap is very high, and joints, too
> numerous to mention, have been damaged in the process. Even
> high-speed lead
> saws aren't much better, and often tend to smear leads  rather than cut
> them. I've also known these cut right through a joint and into a
> board when
> the board is a bit bowed (bowed boards weren't so critical back then with
> plated-through-hole components).
>
> Also, on class 3 boards anyway, exposed copper is banned, so all trimmed
> leads had to be put back through the wave solder to cover them up. This is
> an additonal and unnecessary thermal excursion that should be
> avoided. It's
> detrimental to the life of the board and takes extra time and cost for the
> additional processes involved. It's better to trim the leads to length
> prior to soldering then wave solder only once. Clinch or half-clinch two
> opposing leads on each component before trimming if you're worried about
> the components rising in the wave, or put a pad over the
> components to hold
> them in place.
>
> One holding technique I saw years ago is to vacuum-form thin plastic sheet
> over the populated board that can be peeled or lifted off after the
> soldering process. We never tried it, and I never saw it used "for real",
> so I don't think it caught on. Another technique was to secure a foam pad
> over the components (wave resistant, of course)
>
> Anyway, in your current curcumstances, I would opt for doing a second wave
> solder pass, in spite of what I just said, especially if you have a lot of
> leads to do on a lot of boards. It will be kinder to the board
> that locally
> heating lots of areas, though your no-clean residues will build up. Maybe
> our TN cleaning experts can help with that one.
>
> Good luck
>
> Peter
>
>
>
> David White <[log in to unmask]>      02/05/2002 11:14 PM
> Sent by: TechNet <[log in to unmask]>
>
> Please respond to "TechNet E-Mail Forum."; Please respond to David White
>
>              To: [log in to unmask]
>              cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
>              Subject: [TN] Wave solder re-runs/re-work
>
>
>
>
>
>
>
>
> Advice please.  We have had a problem here we have cut in the
> solder fillet
> while trimming leads (pretty deeply, we have also seen cracked solder
> fillets) which is a violation of IPC.  There are several hundred of these
> asm.  I suggested to Re-run them through wave.  Has anyone else had to do
> this and what have the results been, this sure beats touch-up on every
> lead.  I am not re-fluxing  the asm, we spray no-clean and some
> gets on the
> top the 1st time around which is acceptable, but 2nd time it is just to
> much.  Anyone have any comments?  Any help would be great.
> Dave
>
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