There are some folks that bet the farm and lost by not reflowing the solder
after cutting or "solder cut" instead of "solder cut solder" processes.
J-STD-001C requires reflow or inspection with magnification (Paragraph
7.1.3) after cutting, but residual stress in the solder connection does not
normally appear as a fracture until after a couple of thermal cycles or
several operational cycles and then is evidenced as an intermittent
connection.
The copper issue was intentionally eliminated from the standards as opposed
to the very old mil specs (WS and the like). The concern under older
criteria was justified by increased opportunity for metal migration with
copper over tin/lead, and corrosion possibilities. Copper corrosion tends
to be self limiting, but over time with environments can look pretty ugly.
Mel Parrish
Soldering Technology International
102 Tribble Drive
Madison, AL 35758
256 705 5530
256 705 5538 Fax
[log in to unmask]
www.solderingtech.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Guy Ramsey
Sent: Friday, May 03, 2002 6:11 AM
To: [log in to unmask]
Subject: Re: [TN] Wave solder re-runs/re-work exposed copper
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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