PLATED QUARTZ CRYSTAL UNITS 243 



small percentage of the plates be used as a control sample to which mcjunt ing 

 wires are attached and pull tested. Satisfactory spots should withstand 

 for a few seconds a force of at least two pounds. The average pull-off 

 strength of commercial attachments using 6-mil hooked or headed wires is 

 between three and four pounds. Unsatisfactory plates can be reclaimed at 

 this stage by stripping off the spots by means of aqua rcgia and ammonium 

 hydroxide, and reprocessing in the manner described. 



13.42 Sihcr Plating 



At this point of the process the surfaces of the plates are coated with 

 silver electrodes by the evaporation process previously mentioned. Four 

 milligrams per square inch of silver is the weight of coating generally em- 

 ployed, which amounts to a thickness of .024 mil. Except for harmonic 

 and other special types of crystal units, these coatings are required only on 

 the major surfaces of the plates. However, during the evaporation procees, 

 the silver is deposited to some degree on the minor surfaces or edges as well, 

 and it is necessary to remove it. This process called "edge cleaning", is 

 done by lapping the edges of the crystal on a flat plate covered with a 

 mixture of jiumice or a finely divided abrasive such as No. 600 carborundum 

 and water or kerosene in the form of a paste. Rubbing the edge of the 

 plate lightly over very fine abrasive cloth is also satisfactory. The pumice 

 is preferable, however, since while it readily removes the silver, it is much 

 softer than quartz and consequently does not remove any material from the 

 plate. The use of harder abrasives has a tendency to chip the edge of 

 the quartz unless the operation is performed very carefully. After the edge 

 cleaning is completed the plates are washed, dried and inspected by testing 

 for insulation resistance at 500 volts d.c. to make certain that no conducting 

 material remains between the silver coatings on the major surfaces. 



13.43 Dhision of Coating 



For circuit reasons all but a few types of crystal units require a balance.! 

 pair of electrodes on each side of the plate. Division of coatings along the 

 longitudinal axis is essential on flexure mode crystals in order to make the 

 plate vibrate in flexure. Typical divisions of coating can be noted on the 

 crystals shown in Figure 13.2. In the case of the flexural crystal the divid- 

 ing line is carried around the wire attachments in such a manner that each 

 of the divided surfaces is connected to one of the wires. One method for 

 dividing coating involves the use of a low voltage (two to three volts) 

 impressed between the coating to be divided and a stylus.'^ When the fine 

 point of the stylus is brought in contact with the silver plating and moved 

 along the desired line of division, the silver is burned away, leaving a smal' 



' W. L. Bond, Pat. M 2,248,057. 



