There are three fundamental points to understand the wettability of molten soders. Namely,
(a) The magnitudes of surface energy in mother metal or molten solder and interfacial energy of mother metal/molten solder are closely related to the propriety of wettability.
(b) The alloying tendency between mother metal and molten solder or the magnitude of diffusion power of molten solder to mother metal is related to propriety of wettability.
(c) The magnitudes of alloying energy or potential energy due to gravity are related to propriety of wettability.
Generally, wettability varies with chemical composition of soft solders, fluxes, mother metals and surface condition, heating temperature or atmosphere. And necessary condition to get perfectly wet is the following equation.
σs>σ
LS+σ
L⋅cosθ
where σs; surface tension of mother metal.
σ
LS; interfacial tension of solder/mother metal.
σ
L; surface tension of solder.
θ; equilibrium contact angle of solder/mother metal.
Accordingly, in this test, authors have measured the spread area of solders combined with various fluxes and investigated the surface tension of mother metal, interfacial tension of solder/mother metal, surface tension of solders and viscosity of fluxes which have effect on the magnitude of spread area. As fluxes, ZnCl
2, ZnCl
2-SnCl
2⋅2H
2O, ZnCl
2-NH
4Cl, ZnCn
2-NaCl, ZnCl
2-KCI binary system fluxes and ZnCl
2-SnCl
2. 2H
2O-NH
4Cl ternary system flux are used. As solders, Sn-Pb, Pb-Cd, Sn-Bi, Sn-Cd, Sn-Zn and teernary solders are used.
The results obtained from above investigations are as follows.
(1) The spread area of eutectic solder at 350°C is little affected by flux such as ZnCl
2-NaCl or ZnCl
2-KCl.
(2) The spread areas of Sn-Pb, Zn-Cd and Pb-Cd solders are comparatively satisfactory with ZnCl
2 flux at 350°C. And in the case of Sn-Pb, Sn-Bi, Sn-Cd, Pb-Sn and Pb-Cd, i.e., two component solders, the spread area with ZnCl
2:NH
4Cl=2:1 binary flux at 350°C has a comparativey larger value than with ZnCl
2 only.
(3) With regard to the effects on spread area of the 3rd alloying metals in eutectic solders, the effects of these metals are not a negligible factor; especially, the spread area is increased by addition of As to eutectic solders. As the quality is deteriorated by addition of As, Bi and Zn, addition of these metals is to be avoided.
(4) When Sn70Wt%-Zn30Wt%, Sn70Wt%-Bi30Wt%, Sn70Wt%-Pb30Wt% and Sn70Wt%-Cd30W% i.e., two component solders combined with ZnCl
2;SnCl
2⋅2H
2O=2:1 and ZnCl
2:SnCl
22H
2O:NH
4Cl=2:1:1 system fluxes are used, the temperature dependency of the spread area of solders is relatively high in range from 300 to 400°C. The spread area of above solders combined with ZnCl
2:SnCl
2⋅2H2O:NH
4Cl=2:1:1 ternary flux has a larger value than ZnCl
2:SnCl
2⋅2H
2O=2:1 binary flux.
View full abstract