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January 2000

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Subject:
From:
"Parr, Aric" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 31 Jan 2000 12:44:21 EST
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In a no clean environment, liquid flux should be prohibited (except rosin) on
touch-up. The activators that wick away from the joint don't burn off and you
need to clean. I've worked at several employers over many years where liquid
flux could not be used for repairs. We even used fluxed wire (we know the
flux in the wire sees liquid solder) as a leveling flux with a solder sucker.
We never had a failed joint come back from the field in outdoor coomercial
and automotive applications.

If you show your quality people that adding liquid flux decreases SIR (or
increases ionics), you should be halfway to eliminating the fluxing
requirement.

P.S. Another option is to get a "flux pen". This works like a magic marker
applying a very thin coating of flux. Flux quantity is typically too small
for significant wicking. Most flux manufacturer's can fill pens with thier
fluxes.

Aric Parr
Sr. Process Engineer
Eaton Corporation
1400 S. Livernois
PO Box 5020
Rochester Hills, MI 48308-5020
248 608 7780
[log in to unmask]
---------- Original Text ----------

From: C=US/A=INTERNET/DDA=ID/TechNet(a)IPC.ORG, on 1/29/00 12:12 AM:

Ioan,
See my comments in the body of your memo.

regards,
Bev Christian
Carrier and Wireless Solutions
Nortel Networks
250 Sidney Street,
Belleville, Ontario
CANADA K8N 5B7
613-967-5407

        -----Original Message-----
From:   Tempea, Ioan [SMTP:[log in to unmask]]
Sent:   Friday, January 28, 2000 4:25 PM
To:     [log in to unmask]
Subject:        [TN] Oxidation and solderability

        Hi Technos,

        I'm in the middle of a crossfire here. Everybody, that is our QA
and
customer's QA are concerned about one rework method we are using.
Because one particular connector is very capilarity prone, if we touch
the
leads using liquid flux, the flux ends up on the golden contacts, giving

interconnection problems. I decided that we should touch the connector
without using any flux, except for the one inside the solder wire.

        The problem is that QA is concerned about the reliability of a
joint made
without flux.
Ah, but you are not making joints without flux since you say above you
are using cored wire.  This should provide all the flux you need for the
job.

         Is the heat enough to break the oxide crust?
Only if there is enough solder to be mobile, enough solder that it can
be moved around.  Like moving the dross around on a solder pot with a
spatula to expose the fresh solder.  Even then you need to flux the part
you are going to dip.

         Will
intermetallics be there on both the pin and the pad?
You better hope that there is some intermetallic already on the pin and
pad or you are not going to have a good joint.  Now if your question is
about the amount of intermetallic, this should only be a problem if you
really crank up the heat on your soldering iron and leave it there for
some time.

        My question is: is there any experimental data available on the
degree of
oxidation on HASL surfaces in terms of time?
Yes.  I think you should be able to find that in the better text books
on soldering.  Also, Dr. Chris Hunt of the British National Physics Lab
gives a solderability course where he presents this data.  The short
answer is that oxide formation is immediate.  Sorry I have no hard data
to share, as I am at home and my stuff is at work.

         After how long an exposure to
ambient conditions the oxide layer is thick enough to require flux for
removal?
Tied into your question about crust.  I would never try to solder
without some flux present, unless you start talking about "fluxless"
soldering methods where the parts are pre-treated in various gas/plasma
mixtures, but that's a esoteric topic, don't worry about that now.


Our boards are touched within, at most, 48 hours after reflow. Is this
timing long enough to ruin the solderability of the pad and the
component?
It should be months before there is a solderability degradation,
assuming decent storage conditions.

Once again, some hard data is what I need, since I'm dealing with QA.

        Thank you again for being there,

        Ioan

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