3.3.1.2 Brazing, fusing and metallization
In addition to welding, the soldering process is also used to join metals. Soldered joints at soldering temperatures of above 600 °C are used almost exclusively in vacuum technology. In order to eliminate the need for highly corrosive flux when soldering, which often involves high vapor pressure, and in order to obtain oxide-free, high-strength joints, the soldering process is performed under vacuum or in a clean inert gas atmosphere. Soft solder joints are often not suitable for vacuum applications. They typically cannot be baked out, have less mechanical strength and in addition to tin, which has a low vapor pressure, frequently contain other alloy components with a high vapor pressure. Analogously to vacuum welds, a vacuum- brazed connection occurs when the following requirements in particular are met: carefully cleaned surfaces, careful formation of the soldering gap, use of a gas-free solder with a low vapor pressure, good flow and wetting properties of the solder (gap filling), a well-defined fusion zone of the solder and low reaction between the soldering and the base material. The standard brazing alloys can be divided into two major groups: Brazing alloys based on noble metals (mainly silver) and nickel-based brazing alloys. The structured noble metal-based low-melting brazing alloys are significantly more expensive than the higher-melting nickel base. Therefore, it is preferable to use nickel-base alloys, if this is technically possible and if a higher processing temperature is acceptable. The arrangement of the components to each other and the solder gaps between them must match the soldering process. Depending on the nature of the solder used, the soldering temperature and the thermal expansion of the components, the solder gaps (at room temperature) are typically 0.03 to 0.1 mm. The question of when welding and when brazing should be used, cannot be answered in a general and comprehensive way. Except in cases where welding is not possible, brazing can be advantageous if as many connecting points as possible can be produced simultaneously in a batch.
The fusing process is primarily used for glass equipment and for glass-to-metal connections. Glass-to-metal fusings are especially important in the production of vacuum-tight current feedthroughs, for bakeable viewports and in the production of vacuum gauges. To fuse glass-to-metal transitions, the materials must be selected in such a manner that the thermal expansion coefficients of these materials are as similar to one another as possible throughout a broad temperature range. Since this is often not the case, numerous special alloys have been developed for so-called non-adapted glass-metal seals. In the form of a welded lip, they provide an elastic contact between glass and stainless steel for viewports. Fusings are difficult to perform with quartz glass, as it has a very low thermal expansion, which metal and metal alloys can nowhere near achieve.
Ceramic-to-metal connections are used for highly bakeable and highly insulating current feedthroughs. These are used among other things for the production of high-performance transmitting tubes and ceramic vacuum chambers for particle accelerator of the physical large-scale research. Connections with ceramic, e.g. aluminum oxide (92 % to 98 % Al203), are pre-metalized with those points to be joined with the metal. In this connection, it is particularly important to ensure that the thin metal layer (molybdenum or titanium) creates a thorough connection, free of voids and pores, with the ceramic substrate. For the production of electrical feedthroughs. a nickel layer is applied subsequently, to which a metal cap is brazed and a conductor is then soldered to.
