SCHOTT Technical Glasses

54
Because fillers reduce the flow capability during the soldering
process, they can be added to the batch only in limited
amounts.
Taking into consideration the properties of the materials to
be joined, suitable solder glasses are mainly selected under
the following aspects:
1. highest permissible soldering temperature,
2. thermal expansion of the materials to be joined,
3. maximum application temperature of the solder zone,
4. chemical behavior.
Processing
To achieve satisfactory soldering, the solder glass must flow
sufficiently and wet the parts to be joined well. Flow and
wetting are temperature- and time-controlled; the higher
the temperature, the less time is required for sufficient flow,
and vice versa.
Thus, soldering at high temperatures may take only a few
minutes, whereas at low temperatures (i.e. at viscosities
≥ 10 7 dPa · s) reaching sufficient flow takes very long and
usually can be achieved only under additional mechanical
load.
Properties
As with all sealings involving glass, adapting the thermal expansions
of the components to be joined with solder glass
α (20/300)
––>
12
10
8
6
4
Composite
Vitreous solder glass
Devitrifying
solder glass
2
400 450 500 550 600 650 700
Soldering temperature in °C ––>
Fig. 44. Relationship between soldering temperature and linear
expansion α for solder glasses. The curves describe the lowest limits
for the respective solder glass types.
Spherical lense (center) sealed to a metal casing with a sintered solder
glass ring
is a necessary prerequisite for stable, gas-/leak-tight joins.
As a rule, the coefficient of thermal expansion of the solder
glasses should be by Δα = 0.5 – 1.0 x 10 –6 /K smaller than the
expansion coefficients of the sealing partners.
The relationship between soldering temperature and thermal
expansion is schematically shown in Figure 44.
With devitrifying solder glasses, one must consider that the
expansion coefficients given in the respective lists of properties
are only valid on condition that the specific soldering
program defined for each solder glass is complied with.
Changing the soldering program, in particular the soldering
temperature and the soldering time, may affect the relationship
between glassy and crystalline phase and thus change
the coefficient of thermal expansion, with the possible result
of a mismatch.
Sealing joints produced with vitreous solder glasses can be
loaded to approximately 50 K below the transformation
temperature of the respective solder glass. The maximum
service temperature of devitrifying solder glasses depends
on the type and melting point of the precipitated crystals
and on the amount and properties of the residual glass
phase. Up to maximum service temperature, solder glasses
are moisture- and gas-proof. Their good electric insulating
property is superior to that of many standard technical
glasses. They are therefore also suitable as temperatureresistant
insulators. The chemical resistance of solder glasses
is generally lower than that of standard technical glasses.
Therefore, solder glass sealings can be exposed to chemically