Hi Dave,
Whilst I realise that there is an abundance of published data I am not
convinced that any of it that I have seen has been the result of a properly
controlled experiment. It is usually subjective comment. If there is such a
published work I would be glad to see it and change my views accordingly.
However, as I said, I cannot dismiss my observations. The flux used was pure
rosin, no activators.
There is, obviously, some mechanism which causes poor soldering and
dewetting but currently, I am not convinced that it is the tin copper
intermetallic or its oxide. If it is, what is the difference between the
structures and properties of an oxidised IMC and the individual oxides of
tin and copper? Even mild flux will aid the soldering in the presence of the
latter two; that is its job.
The other mechanism that I investigated was the possibility that impurities
in the copper or the solder were rejected ahead of the advancing
intermetallic layer thus building a concentration which eventually caused
problems. I had access to Auger and SEM/EDAX but found nothing significant.
The fact remains that I soldered oxidised IMC very easily under ideal
conditions, so if it fails in other conditions then there is some
explanation that we are all missing.
Please don't get me wrong, I will not defend the IMC to the death, but I
need good, solid evidence which is better than mine.
My duty these days is the prosaic one of maintaining quality on a modern
surface mount line, one day I will take up where I left off and nail this
one.
Regards
Eric Dawson
> -----Original Message-----
> From: David Hillman [SMTP:[log in to unmask]]
> Sent: Tuesday, November 13, 2001 5:25 PM
> To: [log in to unmask]
> Subject: Re: [TN] Intermetallic Layers
>
> 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
> [log in to unmask]
>
>
>
>
> 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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