IGBT Modules - Technologies, Driver and Application (Second Edition) - page 95

2.3.2 External bonding technology
In bonding technology, a distinction ismade between separable, conditionally separable
and inseparable electrical bonds. While chapter
described inseparable electrical
connections such as bonds and welding, this external bonding technology chapter will
discuss separable electrical connections (screws and spring-loaded contacts) and the
conditionally separable electrical connections (soldering and press-in). Bolted
connections predominate in medium to high power IGBT components, while solder
connections aremore likely to be found in low power IGBTmodules. In recent years, in
addition to soldered connections, new solderless technologies have been incorporated
in IGBT components. These include spring-loadedandpress-inmounting technologies.
All these technologies are based on connecting two contact partners that play an
important role in the reliability and field of application of the electrical bond. Current,
voltage, temperature, contamination and mechanical influences such as vibration load
and shock are key factors in the selection of the correct bonding technology. On this
basis, users need to find the correct contact partner for their IGBT module. Many
manufacturers produce extensive application information and assembly instructions for
using power electronics components. The next chapter describes the fritting effect,
which is seldom described by power electronics engineers but increasingly prevalent
where electronics and sensing technology is integrated in power electronics
components. The frittingeffect
This effect can be described as an electrical breakdown in an insulating path. It occurs
in separable connections in which the surface between the two contact partners is
contaminated or becomes contaminated over time. The fritting effect does not occur in
conditionally separable electrical connections, i.e. soldering and press-in, or in
inseparableelectrical connections.
The contamination between the separable contact partners usually involves oxidation of
the surfaces. Preciousmetal contact partners such as gold (Au) or palladium (Pd), when
contaminated with organic molecules or with contaminated or humid air, form only a
minimal oxidation layer of approximately 5nm. Basemetals like copper (Cu), nickel (Ni)
and tin (Sn) , on the other hand, are very reactive and form oxidation layers several
10nm thick. While this means that precious metals are very good electrical contact
partners, the basemetals aremore susceptible to contact contamination.
The fritting effect cleans the contacts in two ways. If a voltage is applied to the contact
pair that has been insulated, and if this voltage reaches a critical value, known as the
fritting voltage, and therefore a critical field strength, field emission and tunnelling
occur, injecting electrons in the insulating layer. An electrically conducting path is
created at the place with the greatest electrical field strength E. To initiate this physical
process a field strength
V 10 E
has to be reached in the basemetals described with
an insulating top layer D of, for example 10nm. For the separable contact technologies
described in the following chapters, this represents a voltage of at least U = 1V, which
has to beapplied over the individual contact points.
The tunnel effect is a quantum mechanical effect that enables the electrons to overcome a finite potential
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