Thermoelastic Stresses in Balanced and Unbalanced Seals

In the electronic industry ceramic-to-metal seals are of increasing importance in the fabrication of encapsulations for electron tubes and semiconductor devices. The most common method for sealing ceramics to metals is to first "metallize" the ceramic by sintering onto it a thin coating of metallic powder; common brazing materials such as gold-copper alloys, silver-copper alloys, etc. are then used to fasten the metallized ceramic to the metal. Temperatures in excess of 500°C are required for the brazing operation. As the assembly cools from the set-point of the brazing material to room temperature, thermoelastic stresses are induced in the bonded components by the differences in their thermal contraction rates. These stresses must be controlled if physical distortion and the resultant possibility of bond failure or fracture of the metal or ceramic are to be avoided. 1519 1520 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , S E P T E M B E R 19f>2 Their magnitude depends on the choice of metal, ceramic, and brazing material as well as the applied heat treatment. If it is assumed that these parameters are held fixed, the magnitude of the thermoelastic stresses will still vary widely as a function of the structure geometry. This paper presents the results of a two-part analytical study of the stresses produced when ceramic cylinders are butt sealed to metal washer plates. The problems considered are represented by the electron tube envelope assembly depicted in Fig. 1. At position A the top ceramic cylinder is used to balance the effect of the ceramic-metal seal on the other side; it serves no other purpose.