Hi Eric! Sorry but I humbly disagree. There is an abundance of published
industry data which shows that oxidized Sn/Cu intermetallic is one of the
more unsolderable surfaces encountered on an assembly. Problems with poor
plated thru hole fill due to "weak knees" (thin solder causes the  Sn/Cu
intermetallic at the hole rim to become oxidized) and thin HASL coatings
are just two examples of everyday industry solderability issues associated
with Sn/Cu IMCs. What type of flux did you use in your testing? Very
aggressive flux chemistries will allow the soldering of Sn/Cu IMCs but a
fair majority of the industry is moving away from these types of flux
formulations. With all that being said, without the Sn/Cu IMC we would have
no solder joints so it is a critical part of a solder joint.

Dave Hillman
Rockwell Collins
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Eric Dawson <[log in to unmask]>@ipc.org> on 11/12/2001 07:49:56 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>

Sent by:  TechNet <[log in to unmask]>


To:   [log in to unmask]
cc:

Subject:  [TN] Intermetallic Layers


Hi All,

I notice that a few new references have been made to the above subject and
thought I would share some of my early work with you.

In my first post as a graduate, I researched the causes of dewetting on
circuit boards. I came across a solution, the make-up of which I have since
lost, which dissolved the tin lead alloy but left the two intermetallic
layers.

I produced a number of one inch square coupons which I soldered using 63/37
alloy, RMA flux and a Tri Moore Rotary Dip Tester.

These were divided into three categories:

1       Control
2       Remove tin lead and re test in the Rotary Dip.
3       Remove tin lead, Several hours steam ageing and re test.

Microsections showed that I had taken off the alloy successfully and left
the IMC.s.

All of the re tested coupons soldered successfully, including the steam
aged
ones.

This was some years ago so I cannot remember the etchant nor the hours and
conditions in the steam ageing equipment. I do remember that the IMC
soldered very well so I am always dubious when somebody blames the
intermetallic for poor soldering.

I seem to be alone in my faith that the IMC solders but this was the result
of my very own work and I stand by it.

Regards
Eric Dawson, Defender of the IMC.

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