IGBT Modules - Technologies, Driver and Application (Second Edition) - page 87

75 Chip soldering
In standard IGBT modules, the metallic backside of the semiconductor chip is
connected electrically and thermally to the DCB substrate in a vacuum soldering
process. The vacuum in the soldering furnace prevents air pockets forming between the
chip and the DCB. The insufficient linking caused by these air pockets can increase the
thermal resistance of the junction toDCB region, whichwould inevitably cause the IGBT
module to fail at a later date. Due to the RoHSDirective, most manufacturers now use
lead-free solders. System soldering
System soldering involves joining theDCB to the baseplate by a soldering process. The
process differs according towhether AlN or Al
DCB substrates are used. Most of the
solders used for Al
are now lead-free, but for AlN DCB substrates in combination
with copper or AlSiC baseplates, the soldersmostly contain lead. Here, too, it is critical
that the solder be fully wetted between the DCB substrate and the baseplate. As with
chip soldering, air pockets, also known as cavities, increase the thermal resistance,
which causes prematureagingand even failureof the entire IGBTmodule.
Fig. 2.9
System soldering Ultrasonicbonding
Speaking of electrical bonding technology in the internal construction of IGBT
components, one must not forget ultrasonic bonding. Ultrasonic bonding is a wire
bonding process that is typically used in all power electronic chips. In most cases, for
IGBT modules, an aluminium wire is micro-welded to a contact pad with the exclusive
use of ultrasound at a frequency in the range from 40kHz to 100kHz. The ultrasound
wave removes the top oxide layer from the metal, which enables the wire to attach
firmly. The thermal stress of the component is low. There are two different types of
ultrasonic bonds for power electronics components: Thin-wire bonding, in which a bond
wire of 99% aluminium and approximately 1% silicon, and a gauge of 17µm to 100µm is
used (more rarely, gold is used as a bond wire in IGBT modules), and heavy-wire
bonding, inwhich thewire is 99.99%aluminium. Thewire gauge is 100µm to 500µm.
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