SOME PHYSICAL PROPERTIES OF WIPING SOLDERS 117 



forces must operate in hindering the free motion of the Hquid phase. 

 The liquid phase in a two-phase system where the particles are large 

 should then be only loosely bound to the mass, and though there 

 might be no gross tendency toward segregation, this liquid phase should 

 in working be squeezed out, leading to cavity formation and consequent 

 porosity. 



Experimental 



To test this explanation of porosity two samples of solder No. 1 

 were quenched from 210° C. and two of solder No. 2 from 200° C, 

 at which temperatures these solders exhibit practically the same 

 plasticity. Though the data are too meagre to allow generalizations 

 to be made, the results in both sets of samples were well in line with 

 the theory. Microscopic examination showed a much coarser particle 

 structure in solder No. 1 than in solder No. 2, as illustrated by Fig. 8. 

 In this instance at least the solder which forms porous joints tended 

 to form solid particles at wiping temperatures larger than those 

 formed in the solder which is acceptable from the standpoint of 

 porosity. 



Particle Size and Porosity as Related to Wettability 



It appears that this test may have a significance beyond that just 

 attached to it, and since it may conceivably be used again in the 

 study of solders, it is perhaps well to point out a further explanation 

 based on the theory of interfacial tension. 



As was stated earlier in this paper, one demand upon a wiping solder 

 is that it shall wet the cable covering readily. This is necessary if 

 porosity is to be avoided, since areas of imperfect wetting will form 

 tracks between the solder and the lead allowing passage of air or liquid 

 along the interface. Further, the liquid phase of the solder must wet 

 the solid phase of the solder; otherwise passages will exist in the body 

 of the joint. There is some question as to whether porosity in a 

 joint is more commonly due to lack of adherence between the sheath 

 and the solder or whether complete air passages may exist throughout 

 the solder itself. At any rate, there is no assurance that either type 

 of failure does not exist. These two types of porosity are, however, 

 very similar phenomena from the standpoint of wettability, since both 

 the cable sheath and the solid particles of the solder are composed 

 chiefly of lead. If the liquid phase of the solder will wet the sheath, 

 it will also wet the solid particles of lead within the solder, and neither 

 type of failure will be likely to occur. The problem resolves itself into 

 one of the wettability of lead by the liquid phase of the alloy. There- 

 fore, if it is possible to determine, at equal plasticities, the relative 



