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letter 21533
How to qualify a metallic plating-substrate
system regarding its diffusion properties?
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Dear all,
We want to qualify a new bath (cyanide free) for Ag (or Sn)
electroplating on a Cu substrate (electrical device parts). I should
mention that this new bath impede the Ni underplating, which is
harmful in my opinion. But I need to prove it to people responsible
for this project. Furthermore I want to have an in-depth look at the
ways of characterizing platings regarding solid-solid diffusion
properties. In a first approach, I distinguish two problems at least.
DETERMINING THE CONSTANTS EXPERIMENTALLY
For the case of two semi-infinite media in a planar configuration,
the following formula well applies thickness of the "diffusion
affected area"=squareroot(diffusionconstant*time).
Temperature enters through the diffusion constant which obeys an
Arrhenius law. This provides the basis for a time-temperature
correlation. Possibility for this prediction relies on the knowledge
of the two constants involved in the Arrhenius law: the
pre-exponential factor and the activation energy. Either one trusts
values in the tables, if available, or one performs measurements. If
one does the latter, one can let the samples in the furnace (inert
atmosphere), with different temperatures and/or different times of
exposure and then determine the concentration profiles by using a
profilometric technique: SIMS or GDS, for what I know. Then, two
situations have to be distinguished.
1- The elements are not miscible in every proportion
According (more or less!) to the phase diagram, some new layers
are grown. The thickness predicted by the above fundamental formula
is the overall thickness of these new layers. In the connector
industry, it is this way the growth of intermetallic compounds is
mitigated to reasonable low thickness by choosing the mating surfaces
according to their diffusion constants and the required "mean"
operating temperature during the timelife.
2- The elements are miscible in every proportion (e.g. Cu-Ni
The concentrations of the elements are continuous functions of the
distances from the interfaces. Here it is much more tricky. Indeed,
experimental curves are noisy or do not look like a "beautiful"
theoretical one obtained with simple assumptions. Then fitting them
is not straightforward. This is a first problem.
TAKING INTO ACCOUNT THE NON TRIVIAL BOUNDARY CONDITIONS
A second problem, and the most critical in my opinion, is the
following: with the case of layers a few microns thick or less on a
substrate, the physics is more intricate than with two semi-infinite
bodies. However predicting the behavior during the lifetime of the
system requires formulas as well as an experimental measurement of
the diffusion constants! (Even likely less simple than the
fundamental one quoted above!). One faces the first problem also!...
Does someone know guidelines to use diffusion-accelerating
assessments and to exploit them for predicting the behavior during
the lifetime of a system of a plated metallic substrate? Or may be a
simple-minded approach, that is a rough criterion, putting aside any
theoretical considerations?
NB: I am aware that diffusion is not the only phenomenon involved
in the aging of platings. It is on purpose I put aside corrosion
problems on the surface. Diffusion considerations are intended to
ensure that the material chosen from corrosion considerations to be
on the surface will not be replaced by an other one.
Sincerely yours,
Pierre LAURAT, PhD
LEGRAND S.A. - Limoges, Haute-Vienne, FRANCE
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Your question is very deep and rather complex, but perhaps a few
ideas may help you.
Firstly, many years ago I found that the diffusion of one metal
(tin) into another (steel) follows Fick's Third Law of Diffusion.
Secondly, I suggest you plate your substrate with a suitable
diffusion barrier of known thickness. You could do this with a Hull
Cell, so the diffusion barrier thickness will vary across the width
of the plate. Then overplate the barrier layer with a known thickness
of the diffusing metal. Heat treat the plate under known (and
various) conditions and then take metallurgical sections. The
distribution of the diffusing metal can then be determined by using
EDAX on a convenient Scanning Electron Microscope. This will give you
an idea of the effectiveness of the barrier layer and its
relationship to temperature. If you repeat the technique with no
barrier layer, it will tell you a lot about the kinetics of
inter-metallic diffusion. As a rule of thumb, I would not consider a
barrier layer to be satisfactory if it is less than 4um thick.
Also, do not believe the diffusion rate is related to the atomic
concentration on the surface - when I looked at the Sn-Fe system, I
found that whilst there was free tin available on the surface, the
only alloy formed was FeSn2, but as soon as the tin had been
depleted, the FeSn2 became FeSn, Fe2Sn3 and finally Fe3Sn2, all of
them having different rates of formation and activation energies.
(This work was never published - perhaps one day I might get round to
it!). The whole subject is quite complicated, but great scientific
fun.
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Trevor Crichton
R&D practical scientist - UK
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Trevor,
I agree: it is a complex subject. I guess this is the reason why
you have been the only one to risk a reply up to now!
"The effectiveness of the barrier layer and its relationship to
temperature" is related to a time fo exposure and this is precisely
the problem: what is of interest for us is what will really happen
during the timelife at the true temperature of the product.
If I demonstrate that, at a given temperature, an absence of
barrier results in intermetallic diffusion and the presence of a
barrier avoids that, one can always say: "Right, but the "cumulated"
time-temperature effect on the system during its timelife is by far
less than during the heat treatment. So, we do not need a diffusion
barrier."
The correlation time-temperature is really the core problem here.
And to make it, one needs a solution where both the time and the
diffusion constant appear. After that, you feel better because you
can always make the assumption of an Arrhenius behaviour for the
diffusion constant, even if you do not know precisely the regime:
grain boundaries, bulk or surface.
Thickness of a barrier: 4microns
From my own experience, it seems a lot. Between 1micron and 2
microns is usual. Beyond, delamination may occur.
EDAX as a tool
I feel uncomfortable when using EDAX for diffusion studies. The
resolution is quite poor. If intermetallic layers are grown, I do not
know if it is possible to determine the composition of a layer less
than one micron thick. And if the composition is a continuous
function of one lateral dimension, you need to do it slab by slab (as
thin as possible because you make an average). Problems of resolution
again. Do you have some better tricks?
Pierre LAURAT
- Limoges, Haute-Vienne, FRANCE
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There is a lot to cover here. Firstly, Ficks Law of diffusion is
x^n = kT, where x is the diffusion distance, T is the temp and n is
the growth law exponent and K is a constant. Hence the rate of growth
will be expodential to the temperature. The time-temperature
relationship is therefore parabolic for x=2. This will explain why
accelerated diffusion testing is valid by increasing the test
temperature.
Secondly, I stand by my 4um thickness for a barrier layer. If you
are having delamination problems, it is not due to the thickness of
the barrier layer because most common metals will be adherent for an
electrodeposit of only 4um; I would suggest you have problems with
activating its surface and resulting in the loss of adhesion. Using
thin barrier layers results in a discontinuous layer, leaving soome
substrate exposed. I would suggest that your improved adhesion due to
a thinner barrier layer is actually due to the top plating adhering
to the substrate.
Thirdly, I agree that EDAX is not very precise, but it is widely
available. To improve the rsolution, you can take a tangential
microsection of the subtrate/barrier layer/top layer and use a very
low beam energy to minimise electron penetration. By taking a
tangential section, you magnify the thickness by the tangent of the
angle at which you mount the sample. There are other techniques
available, such as scanning Auger and ESCA, but both are rather
sophisticated and not always readily available. You could also take
replica of the tangential section for transmission electron
microscopy analysis and at the same time do a bit of X ray
crystallography. I am sure other surface analysts could thgink of
other methods.
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Trevor Crichton
R&D practical scientist - UK
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