120 



BELL SYSTEM TECHNICAL JOURNAL 



Determinations of the rate of compression of solders at wiping 

 temperatures between parallel plates have been found to conform to 

 the equation: 



dh 

 dt 



= W 



in which dJi/dt is the rate of compression, h the sample height, and k 

 and b are constants, the latter depending only on the character of the 

 material. For viscous materials b = 5.0; the larger the value of b, 

 apparently, the more does the flow differ from that of a viscous liquid. 

 Solders having large values of b (15-25) have low temperature gradients 

 of plasticity and good workability, in agreement with the conclusions 

 noted above. 



It has been shown that the plasticity cannot be predicted from the 

 solid-liquid ratio. 



Evidence is presented that segregation is not responsible for the 

 porosity of wiped joints, and that this defect may be related to the 

 particle size of the solid phase at wiping temperatures. The relation 

 of particle size to cavity formation and to the cohesion and wetting 

 power of the solder is discussed. 



Appendix 

 As stated in the body of this paper, a series of runs were made on 

 solder No. 2 at 183° C. to determine how closely the results of such 

 tests agree with the theoretical expression for quasi-viscous flow given 

 as Equation (4) above. Only five test runs were made, the minimum 

 sufficient to indicate the existence of such agreement. The sample 

 volumes and loads used in these runs are listed in Table IV. For each 



TABLE IV 



b = 16.156 



a: = 0.906 



a = 1.656 



c = 6.875 



log iC = - 31.359 



run there was prepared a plot of sample height (//) vs. time (/), and 

 values of dh/dt were determined from the slopes of tangents drawn as 



