SOME PHYSICAL PROPERTIES OF WIPING SOLDERS 119 



adherence of liquid to solid for different solders, the relative likelihood 

 of porous joints occurring should be thereby indicated. 



Wettability depends upon interfacial tension. As pointed out by 

 Osterhof and Bartell,^" high wetting power of a liquid for a solid means 

 a low interfacial tension between the two, which in turn means a high 

 adhesion tension. 



Furthermore, if the interfacial tension between a liquid and a 

 dispersed solid is high, the formation of large grains, crystals, or 

 flocculates will take place, thus diminishing the amount of solid-liquid 

 interface and the total free energy of the system. This has been used 

 to account for the increased solubility of small particles in contact 

 with a saturated solution, with the consequent growth of large 

 crystals ^^' ^^ and in paint technology to explain the formation of 

 groups of particles or flocculates.^^- ^^ If, on the other hand, the 

 interfacial tension is low, as for solders of high wettability, the particles 

 have a tendency to remain small. This fact has been used to account 

 for the high development of surface in colloidal solutions. 



This would lead to the conclusions that in solder No. 1, where large 

 solid particles exist, the liquid phase wets the lead dispersed within it 

 and hence also the lead of the cable sheath but poorly, while in solder 

 No. 2, exhibiting finer grain formation, the wetting power of the liquid 

 phase for lead is high. These considerations alone would point to a 

 conclusion which has been shown to be the case — that solder No. 1 is 

 more likely to form porous joints than solder No. 2. 



Summary 



An attempt has been made to correlate important qualities of wiping 

 solders with measurements of plasticity, segregation, and particle size. 



A study of six solders has shown that their deformation in the work- 

 ing range is in accord with the theory of quasi-viscous flow, which 

 assumes the shearing stress to be proportional to a fractional power 

 of the velocity gradient. 



It appears that what is required of a solder, in order that it can be 



properly worked, is a low variation in plasticity with temperature. 



This allows the solder to be worked for a comparatively long period 



while it is losing heat to the atmosphere. This requirement appears 



to be satisfied for solders whose resistance to deformation differs most 



markedly in character from that of a viscous liquid. 



i» H. J. Osterhof and F. E. Bartell, /. Phys. Chem., 34, 7 (1930). 

 '1 H. Freundlich, "Colloid and Capillary Chemistry," E. P. Dutton (1922). 

 1- Willows and Hatschek, "Surface Tension and Surface Energy," P. Blakiston 

 & Son (1919). 



'3 Bartell and Van Loo, Ind. &f Eng. Chem., 17, 1051 (1925). 

 inVm. Green, Ind. & Eng. Chem., 15, 122 (1923). 



