Is there a difference in the development of intermettalics between
     their liquid state and solid state?
     Regards
     Edward Brunker


______________________________ Reply Separator _________________________________
Subject: Re: [TN] Intermetallics
Author:  "TechNet E-Mail Forum." <[log in to unmask]>           Guenter Grossmann
<[log in to unmask]> at INTERNET
Date:    09/02/99 09:58


Hi Edward


No, Collin's statement is not wrong. He only points out, that most
tests performed  on the growth do not care at all about the solder
volume.

As usual, one does not care about when reading this stuff. But then,
suddenly there is some person asking you a question about dewetted
solder joints and when analysing the solderings you see that the SOP's
with copper leads ( I mean these are the guys that ought to make the
least trouble ) have extremely thick intermetallic layers.

Phil has mentioned a lot of what happens. It is actually the case, that
the thickness of the intermetallic layer is limited if a large solder
volume is present like when one dips a peace of copper into a solder
bath. Approx. 1um, not regarding the time the copper spends in the
solder bath. The liquid solder actually eats away the intermetallics on
the surface. This means that the remaining intermetallic layers do not
serve as a bareer and new intermetallics are formed. This effect is
accentuated with the use of ultrasonic. The intermetallic layers are
thinner than again. Obviously, as mentioned by Phil, a limited solder
volume, like a reflow operation, results in thicker intermetallic
layers. Naturally this effect is even stronger in the thin gap between
a lead and a pad.

Now, I am not only looking for the effect, but for the reason of the
effect. From What I learned so far

- It is obvious that a movement of the solder decreases the thickness
of the intermetallic layer. This makes sense since the solved
intermetallics are carried away making room for fresh tin to reach the
surface of the intermetallics

- The solder volume surrounding the copper surface has an influence on
the intermetallic's thickness. This results in the fact mentioned
above.

Even if Phil is not convinced that copper enrichment in the solder is a
driving force for this behaviour I do think that it would explain all
we see:

- If you dip a piece of copper in a solder bath a certain thickness of
intermetallics builds up until a equilibrium is reached. This
equilibrium depends on the diffusion rate of the copper solved in the
tin away from the copper surface and the growth rate of intermetallics.
The thickness in the equilibrium state is independent of the time the
specimen is in the solder.

- Agitation of the solder promotes the transport of copper enriched tin
away from the surface of the intermetallics thus reducing the thickness
of the intermetallic layer

- A comparable small volume of solder like fused tin, leads to a
thicker intermetallic layer. The thickness does depend on the time the
specimen is confronted with liquid solder. This would be, because the
copper has no place to go to and the solder builds up a certain copper
content until the dissolution rate of the intermetallics is so slow
that only the growth rate of the intermetallic layer governs the
process. One can even find Intermetallic phases in the bulk of the
solder joint which might be formed out of the copper solved in the tin.



Whoof, I hope I didn't bore you all with my scientific stuff. What do
you folks think of these ideas. Hey Werner this could be something for
us two to entertain the Technet community with a controversial
scientific discussion.


Best regards


Guenter

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