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BELL SYSTEM TECHNICAL JOURNAL 



warrants. The sample cast on the cold plate starts to freeze at its surface 

 in contact with the iron plate and, because of the rapid extraction of heat 

 by the cold iron, it continues to freeze in a rapidly advancing smooth front 

 until the last liquid at the top is solid. Because of the steep temperature 

 gradient there is little opportunity for nucleation and dendrite formation in 

 the upper liquid. The surface of this melt is therefore smooth and free 

 from the fissures that are caused by the shrinkage of the eutectic away from 



Fig. 4 — Bent strips illustrating the effect of variations in cooling rate on the structure 

 of wiping solders are here shown. The upper strip was chill cast and shows a sound 

 ductile surface. The lower strip of the same solder was slowh' cooled and upon bending 

 exposes fissures between the crystallites at the surface (magnification 3 X). 



the dendrites in the slowly cooled sample. Recession of the liquid in the 

 slowly cooled sample leaves a multitude of shrinkage channels which, if 

 they occurred at the critical portion of a wiped joint, would cause leaks. 



Another illustration may be useful in demonstrating the processes taking 

 place in connection with joint wiping. Solder may be allowed to solidify 

 in a crucible until its surface is quite firm to a probe. If, at this stage, the 



