According to DIN 8505 hard solders melt
at temperatures above 450 °C; typical hard solders melt
at between 600 and 800 °C, and are processed typically
at temperatures of between 600 and 900 °C. Typical representatives
of this class include hard silver solders based on silver,
copper and zinc. Flux-free vacuum soldering and soldering
under a protective atmosphere in soldering ovens is used for
high temperature soldering at temperatures > 900 °C.
The metallisation requires metals with high melting points
(refractory metallisation) such as molybdenum or tungsten.
Metallised ceramic suitable for hard soldering can be supplied
in this form by ceramic manufacturers.
The most frequent applications are found in machine construction.
The soldering temperatures involved are not usually critical
for the ceramic material. It must, however, be borne in mind
that the materials that are to be soldered have different
coefficients of thermal expansion. Mechanical stresses will
develop in the solder and in the materials that have been
soldered together as the item cools. The stresses must be
held down to acceptable levels through matching temperature
coefficients and through appropriate construction.