METALLURGY OF FILLET WIPED SOLDERED JOINTS 79 



rigidly practiced. During the development period of the fillet wiping tech- 

 nique examination of the few fillet type wiped joints that were found to leak 

 showed quantities of solder present much in excess of that required. Under 

 the microscope such joints showed the tell-tale sponginess where the eutectic 

 had been drawn away from the junction in the course of final solidification. 



Physical tests on joints made using the fillet wipe between sections of 

 telephone cable and sleeving, have demonstrated that fillet joints similar 

 in size to that shown in Fig. 2 made with 38% tin, 0.1% arsenic, balance 

 lead wiping solder are stronger in tensile strength, creep and fatigue than 

 the cable itself. 



The application of the new technique has gone much further toward 

 saving tin than any known permissible change in the composition of solder. 

 Using the old technique, a reduction of only one per cent in the nominal 

 tin content of a lead-tin wiping solder resulted in widespread occurrence of 

 leaky joints, indicating that little tin could be saved by a simple change in 

 solder specification. This observation was to be expected since many 

 studies had been conducted over the years to reduce the tin content in 

 wiping solders to the minimum consistent with the production of satisfac- 

 tory joints. Tin has always been much more expensive than lead and for 

 large users of solder a reduction of one per cent in the tin content might 

 result in savings of many thousands of dollars annually. 



While the use of the fillet wipe results in large savings in tin other avenues 

 for conserving this strategic metal are available such as the substitution of 

 ternary and quaternary alloys containing less tin than that required by the 

 binary lead-tin wiping solders. A satisfactory alloy of this type was de- 

 veloped which contains 13% tin, 23% bismuth, 0.1% arsenic, balance lead. 

 Though readily available a short time ago, bismuth now has become too 

 restricted to be used extensively in solders. A wiping solder is now being 

 introduced into service in which, through the inclusion of a small quantity 

 of antimony, it has been possible to reduce the tin content. This material 

 appears suitable for fillet wiping although it requires more skill to use than 

 the 38% tin, 0.1% arsenic, balance lead wiping solder. Other compositions 

 may be usable that contain less than normal tin, but on the whole, the 

 savings accomplished by composition modifications will be small compared 

 to those produced by the new wiping technique that has been described. 



In Summation 



By virtue of its small solder volume the fillet wipe reduces tin consump- 

 tion and produces joints less liable to leakage than the conventional wiped 

 joints. The reasons for the success of this type of joint are based on the 

 sound metallurgical principles herein described. The use of the fillet wipe 

 promises to survive the period of restricted tin consumption. 



