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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