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November 2001

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Subject:
From:
"Furrow, Robert Gordon (Bob)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 14 Nov 2001 15:32:33 -0500
Content-Type:
text/plain
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text/plain (319 lines)
Ted,

I'm way out of my area of expertise, but I would think the relative size of
all the atoms (Pb, Sn, Cu) would be one factor, but also the type of atomic
arrangement such as body center cubic, face center cubic, etc. would also
greatly effect migration. I would suggest that the greater the packing
density the more it would inhibit migration. I throw this out as a
supposition only. I would appreciate the input of anyone who could address
this more knowledgably.

Thanks,
Robert Furrow
Printed Wiring Board Engineer
Strategic Supply Global Account Manager
Supply Chain Networks
Lucent Technologies
978-960-3224    [log in to unmask]


-----Original Message-----
From: Ted Stern [mailto:[log in to unmask]]
Sent: Wednesday, November 14, 2001 3:05 PM
To: [log in to unmask]
Subject: Re: [TN] Intermetallic Layers


Eric:

I am also interested in a response to Eric's second to last question
regarding the rate of tin
rich IMC converting to copper rich IMC.

To Dave and George:  in addition to Eric's question, do you have info you
can share regarding
the effect (mechanism) of lead in inhibiting the rate and/or thickness of
tin/copper IMC
growth?  George, your point on lead atomic radii inhibiting its migration
caught my attention.
Are there other factors aside from size encumbrance that inhibit migration
(ie. -  mechanism
whereby nickel prevents/inhibits gold/copper migration)?

Thanks for the great discussion,
Ted Stern

Eric Dawson wrote:

> Hi George,
> One pertinent point here is that the intermetallic layers are compounds
not
> alloys. Thus their oxidising and other chemical reactions will probably be
> different from those of their constituent metals.
> The surface activation energies may be the key, can you give me references
> please George? I would like to follow them up. (See my other reply!)
> One point though, I would expect the copper rich compound to lie adjacent
to
> the copper interface and the copper deficient one next to the solder
> interface. My microsections after solder etching showed that the normally
> visible two layers were still present, so I must have been soldering the
> copper deficient layer, which, as you say, is the easiest one to solder.
> Also, since the melting points of these compounds are higher than
soldering
> temperatures we would always be soldering to the copper deficient layer.
Or
> are we? So, open question, can the copper deficient, aka easily
solderable,
> compound turn into a copper rich layer. Has anybody reading this
discussion
> got access to a well equipped laboratory?
> Regards
> Eric Dawson
> > -----Original Message-----
> > From: Franck, George [SMTP:[log in to unmask]]
> > Sent: Tuesday, November 13, 2001 7:20 PM
> > To:   [log in to unmask]
> > Subject:      Re: [TN] Intermetallic Layers
> >
> > 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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