Jason ,
just to add the little possible to Ed's ;
ages ago we came across the same strings you described :
When we disassembled the pump and the tunnel , we found a lot of dross
drawn & balled on the pump impeller , when we chipped it all off ;
scrubbed , blasted , etc. > the strings disappeared = the hidden dross
"reservoir" on impeller has been "depleted" . I'd put my bet on the
maintenance , considering the extra drossing stress you have to tackle .
Paul Klasek
http://www.resmed.com
PS
Sorry Ed ; didn't catch Jason's original
> ----------
> From: Ed Holton[SMTP:[log in to unmask]]
> Sent: Friday, 26 June 1998 22:35
> To: [log in to unmask]
> Subject: Re: [TN] dross in the solder pot
>
> Have you checked what the O2 level is at your solder pot? You are
> absolutely right, if you have an inerted system, you should not have
> this
> issue, nor should you have to de-dross daily, but not knowing whose
> machine
> you are using or what your volume is, the frequency of de-drossing is
> different. Dross is chiefly produced by the burning of oxides. More
> dross
> is generated running in open air than in nitrogen, eliminate the
> oxygen and
> you eliminate (mostly) the dross. If you are de-drossing the pot
> nightly
> you have too much oxygen in your system. An O2 monitor at the wave
> area
> should tell you what is happening (Note: not with the contour wave,
> because the N2 blanket is not complete until after the board is over
> the
> wave and trapping the N2, the O2 monitor will only work with a full
> tunnel
> system)
>
> Here is my hypothesis, based on something I have seen:
> The conditions: increased levels of O2 in the tunnel
> The second wave only spills over the front, not over the
> back
> (this is common in some inerted wave systems). The
> back
> gate is set high so solder does not spill over the back(factory
> setting).
> The second wave has a standby mode, only runs at full speed
> when
> the board is present.
>
> What happens: the smooth wave gets a thin layer of dross (or a skin)
> over
> the surface of the solder while it sits in standby mode waiting for
> the
> next board, due to the increased amount of O2 in the system. When the
> wave
> activates, solder spills over the front. Some of the dross "skin"
> moves
> with the solder as it falls over the front, but not all. As the board
> passes over the wave and exits, the component leads (especially
> connectors)
> grab this "skin" of dross and drag it off the pot, and voila! you have
> the
> solder short! What is not happening, that is normal with most open
> air
> system, is that as the board passes over the smooth wave, the leading
> edge
> of the board acts like a plow and pushes any dross that accumulated on
> the
> smooth wave over the back gate. My inerted machines have the back
> gate
> raised on the smooth wave so the solder will only fall over the front.
> (some wave solder manufacturers have the A wave instead of the smooth
> wave,
> that basically negates the above issue)
>
> The flux issue shouldn't be the problem, though in this situation, in
> open
> air, insufficient flux will also lead to bridging. The stuff that was
> mixed in with the flux, I would think has been burned off by now and
> should
> not contaminate the solder pot
>
> Ed Holton
> Hella Electronics
>
>
>
>
> [log in to unmask] on 06/25/98 04:07:38 PM
>
> Please respond to [log in to unmask]; Please respond to
> [log in to unmask]
>
> To: [log in to unmask]
> cc: (bcc: Ed Holton/Hella North America Inc.)
> Subject: Re: [TN] dross in the solder pot
>
>
>
>
> Experts, I am currently getting alot of failures at my functional
> tester.
> At my post solder station, I am noticing strings of dross across the
> pins
> of connectors. We are having to dedross the pot daily....for a
> no-clean
> process and inerted atmosphering, I think this is a bit extreme.
> Once, an
> operator managed to accidently put stencil cleaner in the
> flux....could
> this cause a reaction unable to be corrected without dumping the pot
> and
> cause excess dross. I initially thought we had too much exhaust
> drawing
> all nitrogen away from the wave. Our flux manufacturer did analyis
> and has
> determined that it's not the residues left which is causing my bridges
> at
> functional. Is a type 'C' solder analysis going to tell me anything
> of
> significance to my problem from any of your experiences? Any
> suggestions????
>
> Jason Smith
> Process Materials Engineer
> Lexmark Electronics
>
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