IGBT Modules - Technologies, Driver and Application (Second Edition) - page 91

Fig. 2.16
Solder connections: DCB toDCB (b) and thealternativewith bondwires (a) Ultrasonicwelding
In ultrasonic welding, the electrical connection is created by a welding process that
produces long-lasting joint and contact zones on the DCB substrate to bond
components made of the same material. This connection technology was first used for
IGBT modules only a few years ago, but has already proved to be one of the most
reliable types of bonding in power electronics. The load and auxiliary terminals, which
are both made of copper, are welded directly to the surface of the DCB using a
sonotrode, a tool that enters into resonance vibration when exposed to high frequency
mechanical vibrations in the ultrasonic range. The sonotrode establishes the connection
between the ultrasonic generator and the workpiece and adjusts the ultrasonic vibration
to the task.
Very little heat is transferred into themain or auxiliary terminal, so that this process can
be used with electrical connections that have already been set into the plastic case. In
ultrasonic welding, the ultrasonic vibration is applied parallel to the welding surface and
is therefore perpendicular to the welding force. The vibrations caused by the ultrasonic
sound wave make it possible to open and displace most of the layers of contamination
and oxide on the metal surfaces. Displacement of these layers enables direct contact
between the surfaces to be bonded, which are now clean, and it means that thewelded
parts are firmly bonded. Compared with frame bonding aluminium to copper, ultrasonic
welding of copper to copper halves the thermal and electrical resistance, since the
specific resistance of copper is only half that of aluminium. The advantages are greater
current handling capacity and/or reduced thermal load.
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