During my PWB education I learned that there are at least three "alloys" of
copper and tin that are formed as the Copper diffuses into the solder. The
first alloy formed (and I don't remember the ratios) has the highest tin to
copper ratio, and has surface activation energies within the range of most
fluxes. Therefore, this first tin copper alloy, or IMC, is solderable. All
the others alloys, with higher amounts of copper in them, are not.
Activation energies reflect how strongly attached the Oxygen atom are on the
surface of the metal. The more copper on the surface alloy, the tighter the
oxygen is attached, the stronger the flux needed to break the oxygen from
the surface.
While the copper is diffusing into the Solder, the Tin is also diffusing
into the copper surface, lowering the % tin in the solder alloy, which
raises the melt temperature of the remaining solder. It is all quite an
interesting solid state diffusion system. By the way, for all intents and
purposes, the lead is just too big (physical size of the atom) to
participate in this diffusion system much, except to get in the way.
It is possible that Eric only exposed the "solderable" tin-copper
intermetallic in his tests, i.e. the one with the least amount of copper in
it.
What was the magic chemical I played with in my old PWB shop to make boards
solderable, Thiourea hydrochloride? Wasn't that the active chemical in
Tarnex also?
George Franck, CID+
who recalls his "PWB as Black Magic" roots.
--------------------
Disclaimers:
1) If Dave Hillman says I am wrong, then I probably am.
2) My company will neither confirm nor deny the existence of any of my
opinions.
3) Tin-copper alloys are commonly referred to as Bronze.
-----Original Message-----
From: David Hillman [mailto:[log in to unmask]]
Sent: Tuesday, November 13, 2001 12:25 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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