Thanks Guenter for an offline reply. Don't mean to drag you into this
never ending fray on line. Not trying to alienate Guenter or anyone else as
a soldering expert. Simply continuing on the theme: "Haven't you got it
right yet?" comment/statement/quote - after 2,000 years of metal joining as
soldering - not brazing or welding.

Guenter's following comments and factual analysis - together with some
relatively basic and simple physical formulae says much as did Dave, Werner,
and so many other's examples.
Hi Earl

Well well. Stubborn or not you are one of these guys I like most as students
cause they keep me working in asking on and on and on.
Let me try to explain wetting in the way I understand it. Maybe it's kind
too simplified, for mor profound explanations you might have to look into a
textbook (also good ol' Klein Wassink has a lot in it), maybe it's too long.
I don't know.

First of all wetting has nothing to do with intermetallics.
Wetting describes the spreading of one substance on another.
Formation of intermetallics is an alloying process.
Diffusion is the travelling of one substance trough another one.


Wetting:

As you see a drop of a liquid on a substrate you see that it reaches above
the level of the substrate. This height varies, depending on the combination
of liquid and substrate. To do so there must be a difference of pressure
between the inside and the outside of the drop very much like one of these
soft rubber balls kids use in the swimming pool. the more air you blow into
the ball the rounder it will lay on the surface of a table. Laplace thought
about this problem and introduced a measure called surface tension.

dP = g (1/R1 + 1/R2)

dP= Pressure difference
g = surface tension
R1; R2 = radii of curvature

In my opinion this is not a good name since I always feel that surface
tension is something with force per area. However, surface tension is not
this it is a measure for the state of energy per area the surface atoms are
in. But who am I to correct Laplace?
Where does this surface tension come from? Each atom has a certain total
internal energy. In the bulk, each atom is surrounded by other atoms mostly
of its own kind and is somehow bound to these neighbours. Now imagine the
atoms on a surface in vacuum. Those fellows have only neighbours below and
on the side. Thus some bindings are missing which brings these atoms to
another ( higher) potential energy than those in the bulk. Enlarging the
surface means enlarging the potential energy since more atoms are becoming
part of the surface.
If the substance we look at is not in a vacuum the atoms at the surface have
bonds to  the atoms of the surrounding medium. This means the surface
tension at the interface of the two is a function of the combination of the
two substances.

Looking at a drop of liquid on a surface one sees several interfaces:
- Liquid- environment
- Liquid. substrate
- Substrate- environment

Each having its own interfacial energy. The total surface energy of this
system is:

E= g1x (area of free substrate surface) + g2x (area of drop surface) + g3x
(area of drop-substrate interface)

g1= surface tension substrate / environment
g2= Surface tension liquid / environment
g3= surface tension Liquid / substrate

Since every system wants to minimise its free energy all three interfaces
define the shape of the droplet with their interfacial energy. Assuming that
the drop has the shape of a spherical cap this shape can be defined with the
angle the surface of the drop forms at the point where it meets the surface
of the substrate. This wetting angle can be calculated after Young ( Yes,
the Young, seems as if them clever fellows before us did a whole lot of
things):

g3 +g2 x cos a = g1 or cos a = (g1- g3)/ g2

g1= surface tension substrate / environment
g2= Surface tension liquid / environment
g3= surface tension Liquid / substrate
cos a = cosinus of wetting angle

This means, the smaller g3 and g2 the smaller the wetting angle.


Formation of Intermetallics

The same bonds between two substances that determine the wetting angle are
also responsible for the alloying behaviour or the solubility of two
substances in each other. The better two substances are soluble in each
other the smaller is the interfacial tension between them. Generally, if two
liquid metals A and B are mixed together, one lets say A, will be solved in
the other  ( B ) until saturation occurs. Again this is an energy  minimum
of the system called the eutectic alloy. If the liquid metal mixture
solidifies A and B form mixtures of mixed crystals composed of A and B with
the maximum amount of A in B and the surplus B as pure B. Some metals
however do form substances that haven't got pure metallic bonds.  They have
something called intermediate bonds that incorporate metallic bonds as well
as covalent  bonds and ion bonds. In this manner chemical compounds are
created having complex stoechiometric composition and are called
intermetallics.


Diffusion

As diffusion one assigns the movement of atoms through some environment. In
context we are looking at this is the movement of atoms of copper atoms
through the intermetallics  to the tin and movement of tin atoms to the
copper. How many atoms per area and time pass through a layer depends on
- Temperature
- Difference of concentration of copper in tin or tin in copper respectively
- Substance diffusing
- Substance of the layer
- Thickness of the layer

This is described with the 1st. law of Fick extended with a bit of
Arrhenius:

dm / dt= - D ( dc/dx ) S x e exp ( - Q / R T)

m = Amount of diffusing substance
t = Time
D = Diffusion coefficient ( Material constant )
c = Concentration
x  = Diffusion way
S = Area through which the diffusion takes place
Q = Activation energy
R = Gas constant ( 8.32 J / mole K )
T = Temperature in K


All in all you might see that in soldering
1st : Wetting takes place
2nd : Solder alloys with copper ( or nickel, depending on the surface
finish)
3rd : Intermetallics are formed
4th : By diffusion of tin into copper and copper into tin through the
intermetallic layers the intermetallics keep growing

Phew I'm getting a stiff neck of writing. Sorry it got so long and sorry I
didn't get more into depth but then I would get even longer.


Have a great day

Guenter

Again, thanks to Guenter and many others, I have added to my limited
understanding of factual elements regarding soldering. In school, I always
had a hard time with LaPlace transforms as intended for use developing
electronic circuits. Went to mechanical engineering instead but he kept
showing up.

On the surface, all this wetting is simple stuff. It's the surface, and its
definition and its characteristics, I question. Still not trying to stir the
pot anymore, but trying to get this thing is perspective so - cant' we all
just get along while understanding Rodney is stuck in his ways, in jail
again, and I'm not stuck anywhere but trying after my 2,000 years, it seems,
on earth still trying understand the question - can't we all just get it
right - or at least - make it easier.

MoonMan

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