LEAD-TIN-ARSENIC WIPING SOLDER 263 



not only does the wiping process itself not admit of scientific measure- 

 ment but also the basic properties related to the process are difficult 

 to determine and are of restricted practical bearing. These difficulties 

 arise because a complex solid-liquid system is involved, and during 

 most of the time of wiping the joint the system is far from being in an 

 equilibrium condition. 



Experience has shown, however, that a wiping solder should possess 

 certain general characteristics which are enumerated below, although 

 in many instances the characterization cannot be extended beyond a 

 qualitative statement. 



1. The temperature at which the solder begins solidification should 

 be lower than the temperature of beginning melting of the cable sheath 

 and sleeve. The temperature of beginning solidification for the 38-62 

 tin-lead solder is 240° C. while the lead alloy sheath begins to melt at 

 approximately 310° C. Since with this solder no trouble is encountered 

 with melting sheath it appears that a 70° differential is satisfactory. 



2. The solidification range of the solder should be such as to provide, 

 during cooling, an ample forming period between the time when enough 

 primary lead has precipitated to give sufficient body to permit forming 

 to begin until the mass is too solid to manipulate. In the 38-62 

 wiping solders the solidification range is approximately 60° C. while 

 the forming range is about 40° C. 



3. The tendency for the joint to drain or slip and break apart during 

 wiping should be a minimum. These properties are associated with 

 surface tension and plasticity. The desired condition is sometimes 

 referred to as a "buttery" texture. 



4. The solder should readily wet and alloy with the parts to be 

 joined. This implies a freedom from non-reducible oxides in the solder 

 and a minimum in the tendency to form reducible oxides. Suitable 

 non-corrosive fluxes are used to clean the surfaces being joined and 

 reduce the oxides which cannot be entirely excluded from the melted 

 solder. To facilitate handling, the solder should not adhere to the 

 splicers' wiping cloths. 



5. The solder should be such that the strength of the joint formed 

 should be equal to or greater than that of the parts being joined. The 

 joint must also be gas-tight. This property is secured through a fine 

 texture in the solder and freedom from draining of the lower melting 

 constituents. The test for porosity of a joint is simple. A positive 

 gas pressure of from six to nine pounds per square inch is applied 

 inside the sleeve and soap-suds are then painted on the joint. Ob- 

 servation of the soap-suds will then show whether or not the joint is 

 porous. 



