240 BELL SYSTEM TECHNICAL JOURNAL 



13.4 Fabrication of Wire Supported Unit 

 13.41 Silver Spotting 

 13.411 Application of Silver Paste 



Starting with the crystal plate, the first step in manufacture is to apply- 

 silver spots to the surfaces of the plate. These spots serve as footings to 

 which the supporting wires are ultimately soldered or sweated. They are 

 placed on the nodal points or along the nodal lines of the plate in order to 

 detract as little as possible from the intrinsic characteristics of the plate 

 itself. The areas of the silver spots cover the range from about 40 to 90 

 mils in diameter depending upon the amount of solder to be used in attach- 

 ing the wire to the plate. Before spotting the plates it is essential that 

 they be free from any contamination such as grease or organic material 

 that might affect the fusion of the silver spots into the surface of the quartz. 

 One of the best methods to ensure cleanliness is to boil the plates in aqua 

 regia, followed by copious rinsing in water. Detergents such as sodium 

 meta-siUcate are also employed followed by a rinsing. The plates may 

 finally be boiled in distilled water and carefully dried. Throughout the 

 subsequent processes the plates should be handled with clean tweezers or 

 gloved fingers and kept away from any source of contamination. Prefiring 

 of the plates at 950°F prior to spotting has also been used as a positive way 

 of ensuring freedom from any contamination that would affect the fusion 

 of the spots, but this process is not necessary if the first mentioned process 

 is properly controlled. 



In spotting, small quantities of a prepared silver paste are placed on the 

 areas of the plate to which the wires will ultimately be attached. The paste 

 consists of a compound of finely divided silver and low melting point glass 

 (lead borate) thoroughly mixed with a suitable vehicle to facilitate appHca- 

 tion. For spotting purposes it has been found that a paste having a specific 

 gravity of between 2.3 and 2.6 gives best results. In use, the materials 

 must be constantly agitated or stirred in order to prevent the solid ingredi- 

 ents from settling out. This is important, for, unless the concentration of 

 silver is maintained around 90 to 95 per cent of the solid matter, it will 

 not be possible to obtain good wetting of the solder in making the wire 

 attachment. 



The placement of the semi-liquid material on the plate is accomplished 

 by means of a small stylus, the crystal plate being held in a clamp or vise 

 and the stylus guided so as to place the material at the exact location on the 

 plate as desired. A typical tool for doing this work is shown in Figure 13.3. 

 The point of the stylus should have a slightly rounded end. With the 

 rounded point the tendency of the paste to spread out is minimized and 

 consequently the diameter of the spot is substantially the same as that of 



