Patent GB-1426873-A (GB)
Title: Methods of pressure bonding a ceramic member to another member
IPC: C04B37/02
Filed: 1972-05-03
Abstract: 1426873 Welding by pressure MULLARD Ltd 4 April 1973 [3 May 1972] 20687/72 Heading B3R In pressure bonding a ceramic member to a ceramic or metal member an intermediate malleable metal layer is interposed between substantially flat surfaces of the members and the assembly is placed in a press under a pressure of 1-5 tons per square inch while maintaining the assembly at a temperature below the melting point of the layer and below the temperature at which any liquid phase would form at the facing surfaces, the pressure and temperature being maintained for not more than 30 seconds to achieve a bond having a tensile strength minimally of 20% of the ultimate tensile strength of the malleable metal layer. The term metal member does not include a semiconductor. The conditions are preferably such that the deformation of the layer is less than 5%, e.g. less than 2%. The facing surface of at least one of the members may be coated with metal, e.g. the surface of a metal member which readily oxidizes is coated with gold before bonding to a ceramic member and if a layer of gold or soft solder is employed a ceramic member is metallized with gold, gold/palladium or silver/ palladium. When using an aluminium layer it may not be necessary to metallize the ceramic member. Preferred bonding conditions when using a gold layer are 2À5-3À5 tons p.s.i. at 300-350‹ C., when using an aluminium layer are 2À5-3À5 tons p.s.i. at 350-450‹ C. and when using a soft solder layer are 2À5-3À5 tons p.s.i. at 75-300‹ C. The soft solder layer may be of lead, silver and tin; lead and tin; cadmium and tin; lead and indium; lead, cadmium and tinor lead, silver and tin. A press for use in carrying out the bonding comprises a fixed steel base 3, Fig. 1, heated by an electric element 6, and having a support member 9 in which a thermocouple is carried, and an upper movable head 3 comprising a steel pressure plate 21 heated by an element 24 and a rubber shock absorber pad 18 sandwiched between an insert 15 on the head 2 and a support 20 for the plate 21. In use a protective gas, 10% hydrogen in nitrogen is supplied via an aperture 12 in an asbestos cover 8, about the work and the pressure between the heated members 9 and 21 is increased gradually over 5-10 sees. until the desired pressure is maintained. In one embodiment of the method of the invention a copper base member 31 and a copper plinth 32 of a transmitting transistor module support assembly are plated with nickel and then gold, a pattern 39, 41 of gold is provided on a beryllia ceramic member 34 by a silk screen process and a gold layer 36 is provided on the opposite surface of the beryllia member. Nickel terminal strips 37, 38 are also gold plated. The strips 37, 38 are then bonded to the beryllia member by placing the beryllia member on the support base 9, placing soft solder (leadindium) foil bodies 40 between the portion 39 and the strips and bonding at 3 tons p.s.i. at 190- 200‹ C. for 10 sees. The beryllia member is then bonded to the plinth 32 and simultaneously the plinth is bonded to the base 31 in the apparatus of Fig. 1, using soft solder (lead-indium) foilbodies 33, 35 between the bodies and bonding at 3 tons p.s.i. at 190-200‹ C. for 10 sees. Subsequently transistor elements are secured to the pads 41. The material of the pads 39, 41 and coating 36 may be silver/palladium. In a further embodiment an alumina lid is pressure bonded to an alumina wall sealed by a glass sealing to an alumina base, the parts forming an envelope for an electronic device. The alumina lid is metallized with palladium and silver, a foil of cadmium-tin soft solder is arranged between the metallized lid and a palladium-silver metallized surface of the alumina wall and the assembly is bonded in the apparatus of Fig. 1 at 165‹ C. using 2À5 tons p.s.i. for 5 sees. The foil may be of lead-silver-tin. A suitable ceramic is zirconia.