I want to qualify electroplatings (Ag or Sn for instance) on a Cu

substrate. (Ni underplated barrier present or not). 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 intrigate than with two semi-infinite

bodies. However predicting the behaviour 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 assesments

and to exploit them for predicting the behaviour 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 phenomenom 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.

Pierre.

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

Dr Pierre LAURAT

Project Manager - Materials Sciences

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

******

*******