STRESS SYSTEMS INT THK SOLDKKLKSS ANKAPPKD CONNECTION 1101 



Next the terminal was held at 175°C for various times as shown by 

 Fig. 0, where the stripping forees and the forces to pull the pins are 

 plotted. It is seen that the force re(iuired to pull the ])in decreases with 

 time while the force required to strip the terminal increases with time, 

 'i'lu^ pin force duplicates tlu^ stress n^axation curve initially but departs 

 from this curve more and more as time progresses. This gradual depar- 

 ture appears to be due to some diffusion in the tin which makes partial 

 contact with the terminal. The amount of diffusion between pin and 

 terminal is less than that between wire and terminal for two reasons: 

 (a) the contact area between pin and terminal is not as intimate as the 

 contact area between wire and terminal due to excessive plastic flow in 

 the latter but not the former case, and (b) the contact area between 

 j)in and terminal is much greater than between wire and terminal. 

 Nevertheless, the force recjuired to pull the pin is substantially the same 

 as the frictional force holding the wire on the terminal due to the hoop 

 stress in the wire. Hence, we can conclude from this experiment that the 

 initial stripping force is due to the frictional force resulting from the 

 hoop stress plus shearing forces required to gouge the wire. 



Several experiments have been undertaken to measure the effect of 

 diffusion separate from friction. The most successful of these was the 

 arrangement shown by Fig. 7. Here a wire was pressed against a double- 

 toothed sharp edged block, and a constant pressure was maintained for 

 various times at various temperatures. An attempt was made to produce 

 an indentation of the same magnitude as that developed in the wrapped 

 solderless connection, although, of course, the area of contact increased 

 slightly with time. 



It was found that for aluminum or copper on nickel silver at room 



10^ 10- 



TIME IN MINUTES 



Fig. 6 — Experiment showing difference between stripping force and frictional 

 force due to hoop stress — temperature 175°C. 



