The Metallurgy of Fillet Wiped Soldered Joints

Fig. 1--A conventionally wiped joint between telephone cable and sleeve (third size) CABLE Fig. 2--A section taken from a joint wiped conventionally (magnification X). Fig. 3!--A section taken from a joint wiped using the fillet technique (magnification X). METALLURGY OF FILLET WIPED SOLDERED JOINTS 75 ways. The saving in solder and consequently in strategic tin is evident. The field splicing forces find that joints are easier to make by the new method and are less apt to be porous. Several interesting metallurgical considerations which are responsible for the success of the fillet wipe will now be discussed briefly. Much has been written about the wiping process of soldering cable joints and the many requirements of a good wiping solder have been frequently listed. The success of the procedure here described is dependent upon a few fundamental characteristics of lead-tin alloys in the process of freezing which have sound metallurgical explanations. For an understanding of the defects possible in a soldered joint wiped in the usual manner, the simple solidification phenomena of metals may be considered. As is well known, molten metal in a crucible when allowed to cool with free circulation of air will begin freezing near the walls of the vessel and with a few exceptions, will end with a concave surface due to solidification shrinkage. Restricting the discussion to a simple lead-tin wiping solder, solidification progresses as follows: a lead-tin solid solution commences to freeze and forms a rather porous cylinder touching the crucible walls and extending to a height corresponding to the volume of the melt at that time; on further cooling, dendrites of lead-tin solid solution grow inward toward the center of the crucible and at the same time many tiny new crystals form throughout the liquid.