MANUFACTURING DEVIATIONS IN CRYSTAL UNITS 271 



14.24 Etching 



The surface condition of the quartz plates also has some effect on crystal 

 characteristics. This surface condition is determined in large part by tlie 

 lapping operation used to obtain final dimensions. As described in Chapter 



XIIIi", the plates are given a final lap with 400 or 600-mesh carborundum. 

 This, in turn, is followed by an etching bath which removes foreign particles. 

 A short etch, about eight minutes in 47% hydrofluoric acid, has been found 

 adequate to ensure firm adherence of the metal coating to the quartz. On 



i" "The Mounting and Fabrication of Plated Quartz Crystal Units," R. M. C. Green- 

 idge, this issue of the B. S. T. J. 



