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November 2001

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Subject:
From:
Ted Stern <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 14 Nov 2001 14:05:15 -0600
Content-Type:
text/plain
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text/plain (238 lines)
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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