Kinetics of wetting Ag and Cu substrates by molten 60Sn40Pb
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I.
INTRODUCTION
IMPORTANT in the manufacture of electronic packages is the soldering of components and leads. For the efficient production of such joints and for the modeling of the soldering operation, a knowledge of the nature, kinetics, and mechanisms by which molten solder wets the solid substrate is needed. Although Pb-Sn solders have been employed in industry for many years, only little is known regarding these features of wetting. Physical properties important in the spreading of a liquid drop on a solid substrate are the pertinent surface and interfacial energies and the viscosity of the liquid. For larg2e drops, gravity may also be significant; i.e., when R > (Ytv/Pg), where R is the drop radius, Yl,, is the surface tension of the liquid, p is its density, and g is the acceleration due to gravity. Young's equation ~u gives the balance of surface and interfacial tensions at equilibrium: %v = 3'~, + 3'l,.cos 0e
[1]
where %v is the solid surface-vapor (or flux) surface energy, 3% is the liquid-solid interface energy, Ylv is the liquid-vapor (or flux) surface energy, and 0e is the equilibrium contact angle. When the drop is small so that gravity can be neglected, and 0 < 90 deg, the geometry of the drop generally takes the shape of a spherical cap, giving 0 = 2 tan_, ( H ) ~ __2HR
[21
R =
3V I
111/3
[tan z 0 (3 cosec 0 - t a n ~ )
[3] ~H 7"rHR2 V = - - (3R 2 + H e) --~ - 6 2
[4]
where R and H are the drop radius and apex height, respectively, and V is its volume. When a liquid drop is placed on a solid substrate, the drop will usually spread with time; i.e., the radius R will increase, and the contact angle 0 and height H will decrease. The kinetics of the spreading of a liquid drop on a solid surface can be classified into two types: t21 (a) nonreactive (class I), where the kinetics are determined solely by the unchanging liquid and solid properties, normally the viscosity and interfacial energies, and by gravity or thermal effects if these apply; and (b) reactive (class IlL where chemical effects are involved and the liquid reacts with the substrate, producing changes in the system during the spreading process. The major driving force F for the spreading of a small, weakly interacting liquid drop on a flat, solid surface is considered to result from the imbalance of the surface and interface energies, i.e., F = Y,v - (Yt, + Trycos 0)
[5]
or, considering Eq. [1], XIAO HONG WANG, Research Fellow, and HANS CONRAD, Professor Emeritus, are with the Materials Science and Engineering Department, North Carolina State University, Raleigh, NC 276957907. Manuscript submitted January 11, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
F = Yt~ (cos 0e - cos 0)
[5aJ
It is clear from Eq. [5] that F decreases as 0 becomes smaller, i.e., as equilibrium is approached. An important parameter in Eqs. [1] and [5] is Yls, the VOLUME 26A, FEBRUARY 1995--459
energy of the interface between the molten metal and the substrate. The smaller Yl,, the larger will be F and the smaller 0 at equilibrium; i.e., an increas
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