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BELL SYSTEM TECHNICAL JOURNAL 



segregation to a large extent. No marked difference seems to exist, 

 for example, between the segregation of solder No. 2, which forms 

 non-porous joints, and solder No. 1, which is a poor solder from the 

 standpoint of porosity. On the other hand, in solder No. 4, an ex- 

 tremely poor solder, segregation is so slight as to be negligible. In 

 Fig. 7 are plotted the data for solders Nos. 1 and 4, the former showing 



1.0 1.25 1.50 1.75 



DISTANCE FROM CENTER - CM 



Fig. 7 — Segregation measurements on two solder samples. 



considerable segregation and the latter practically no segregation of 

 solid from liquid phase. 



It is interesting to note how w^ell the segregation measurements 

 confirm the plasticity data. Solders Nos. 1, 2, and 3, showing clearly 

 the quasi-viscous type of flow, also show marked segregation, re- 

 vealing the reluctance of the solid particles to move and their con- 

 sequent piling up near the center of the sample. In solder No. 4, the 

 flow is of the viscous type, and consequently there is no segregation. 

 The solid phase and the liquid phase do not separate, but flow together 

 as a viscous whole. 



Particle Size 

 Relation to Porosity 



It was thought that in solders forming porous joints the solid phase 

 may, at wiping temperatures, be present as particles relatively large 

 in size, in which case the liquid must be in larger recesses than would 

 be the case if the particles were small. The cohesion of the solder 

 while it is being worked and while it is solidifying must depend to 

 some extent upon the adhesion between solid and liquid. The larger 

 the particles, the greater the distances through w^hich the surface 



