IGBT Modules - Technologies, Driver and Application (Second Edition) - page 94

82 Low temperature joining
In power electronics, a low temperature connection joins two metallic materials, which
are coated with silver or gold to ensure that they are free of oxide film, using a layer of
granular silver or silver streaks that have been previously treated and correctly shaped.
The joining process takes about 1minute at a temperature greater than 220°C but well
below the melting temperature of any of the materials involved and at a pressure of at
least 40MPa. During the sintering, the volume and porosity of the silver powder are
reduced so that, subsequently, the necessary hardness can be achieved through
surface diffusion between thepowder particles. The diffusion, ormixture, of the surfaces
of the silver layer and the surfaces of thematerials to be joined is alsowhat creates the
final bond.
The aim of sintering is to achieve greater load cycle stability in the bondedmaterials. It
is therefore best used in new developments as an alternative to chip soldering and, if
necessary, to ultrasonic bonding. Formore information, see chapte Diffusion soldering
In diffusion soldering, a layer of solder with a lowmelting point is placed between the
twometal layers to be joined. Themelting temperature of themetal layers is higher than
the melting temperature of the layer of solder. The structure is then placed under
pressure and brought up to a temperature sufficient to melt the layer of solder. The
solder reacts with the surfaces of the metals to be bonded and forms an alloy phase.
The melting point of the alloy phase is higher than the process temperature that was
required tomelt the solder, so it is able to withstand operating temperatures far greater
than the soldering temperature. The difference can amount to several hundred degrees
Celsius. In conventional soft soldering, however, the operating temperature has to be
well below the soldering temperature so that the solder contact does not become
unstable. There are several different solders that can be used for diffusion soldering, the
most common being tin-silver and tin-gold alloys.
A further advantage of diffusion soldering is that the effective connection layer can be
made very thin in comparison to soft soldering, which improves its thermal junction
Like the low temperature joining technique, diffusion soldering is another option that can
be used in place of conventional connection techniques. For more information, see
Fig. 2.21
Comparison of a soft solder joinanda diffusion soldering join
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