1108 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1954 



copper wire on nickel silver terminals at 150°C and 100°C. The increase 

 in strength does not occur as fast as that for tin plated copper, and hence 

 the standard connection with a tin plated copper wire is better than 

 one formed from })are copper wire. 



PROPERTIES OF SOLDERLESS WRAPPED CONNECTIONS FOR OTHER TYPES 

 OF PLATING 



A study of the factors governing the diffusion strength of the solder- 

 less wrapped connection suggests methods for increasing the rate of 

 diffusion and the strength of the diffusing layer. These methods result 



1 DAY 10 DAYS 



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TIME IN SECONDS 



Fig. 14 — Stripping force and shear strength of bare copper on nickel silver 

 connections as a function of time and temperature. 



in greater mechanical strength and a faster fusion of the wire and ter- 

 minal. Experiments show that there are at least foiu" metals which diffuse 

 faster than tin, but due to the economic problems and the brittleness of 

 some of the alloys formed, none of them are being considered for solder- 

 less wrapped connection. 



The reason the diffusion forces in the tin plated solderless wrapped 

 connection do not cause an increase in strength of more than twenty 

 per cent during the life of the connection is that the hmiting shearing 

 stress in tin is only 3,000 pounds per square inch. If now we substitute 

 for tin plating a plating with a larger limiting shear stress, the strength 

 of the connection should increase by a larger factor. To be of use, how- 

 ever, it is necessary that the diffusion forces shall develop rapidly. 



The data of Fig. 10 show that if we can produce a higher shearing 

 stress on a layer of aluminum, the activation energy can be lowered and 



