METALLURGY OF FILLET WIPED SOLDERED JOIXTS 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 re- 

 quirements of a good wiping solder have been frequently listed. The suc- 

 cess 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, wnll end with a concave surface due to solidifica- 

 tion 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. There are thus taking place simultaneously, shrink- 

 age of metal as it becomes solid, shrinkage of previously frozen solid as it 

 cools, and shrinkage of the remaining liquid as the temperature drops. The 

 originally solidified outer cylinder, adhering to the crucible walls remains 

 essentially at its original height. The level of the semi-liquid portion nearer 

 the center of the crucible continuously falls until the precipitated crystallites 

 formed in the body of the melt make a loosely piled mass extending from 

 the upper surface to the bottom of the crucible. Further shrinkage of the 

 liquid then leaves these primar}- crystallites at approximately this level 

 while the liquid recedes, leaving fissures between them. This can be beauti- 

 fully observed by means of a binocular microscope focussed on the surface 

 of a soldifying crucible of wiping solder or, on the top surface of a solidifying 

 wiped joint. 



Further insight into the mechanism of wiping solder solidification may 

 be gained by another simple illustration. If two solder strips are cast by 

 pouring small quantities of molten solder, one on a cold iron surface, and 

 the other on a cloth-covered board and both are then bent cold to produce 

 specimens as shown in Fig. 4, the chill cast sample will exhibit fewer cracks 

 resulting from shrinkage than the slowly cooled one. In the slowly cooled 

 sample primary crystallites form throughout the solidifying mass and pack 

 at a level above that which the final volume of completely solid solder 



