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
################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following
text in the body:
To subscribe: SUBSCRIBE TechNet <your full name>
To unsubscribe: SIGNOFF TechNet
################################################################
Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional
information. For technical support contact Hugo Scaramuzza at
[log in to unmask] or 847-509-9700 ext.312
################################################################
################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe: SUBSCRIBE TechNet <your full name>
To unsubscribe: SIGNOFF TechNet
################################################################
Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################
|
