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. 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