IGBT Modules - Technologies, Driver and Application (Second Edition) - page 98

oil-free threading. If a screw connection is to be lubricated or locking paint is to be used,
the bolt torquewill be different from that stated by themanufacturer. Furthermore, screw
connections in IGBT modules do not represent a robust support in the application.
Manufacturers permit only limited tensile, pressure and torsional loads on their products.
It is not uncommon for the failure of an IGBT module to be traced back to inadvertent
mechanical problemswhereby the load or control terminalswere subject to inadmissible
vibrations or forces. Solder connection
Solder connection is a conditionally separable electrical joining technology and remains
by far the most commonly used connection technology in the small to medium power
range. For IGBTmodules with reverse blocking voltages of ≤1.7kV and electric currents
of ≤200A, the most common solder connections on the power electronics market are
those inprinted circuit boards.
There are two groups: Hard solders and soft solders. Soft soldering is divided into
leaded and lead-free solders. The processing temperatures for hard soldering are
between 400°C and 1000°C and therefore not applicable in electronics, those for soft
soldering arebetween 230°Cand 400°C.
Soldered connections pose some potential problems from the point of view of reliability.
They are sensitive to temperature shock and mechanical distortion and are at risk of
snapping. In addition, they are potential local element
and there is always a risk of
incorrect processing. The introduction and implementation of the RoHS Directive has
made this even more of an issue. The Directive prohibits the use of lead in solder,
stipulating silver as a replacement. However, silver has a higher melting temperature
than leadand is hardandbrittle in comparison.
Solder connections are used to join IGBT components to printed circuit boards. The
soldering is done by selective/non-selective soldering robots or wave soldering, but
IGBT components are usually still soldered in manually and without any monitoring of
the process. IPC-A-610DE
sets out theessential quality criteria for soft soldering.
Transitioning to a lead-free soldering process, many manufacturers are moving from
SnPb (tin-lead) solder to other solders, including Sn60Pb40, SnCu (tin/copper) or
SnAgCu (tin/silver/copper). This makes the process window smaller and increases the
process temperature. While lead solder was soldered at 190°C to 250°C, lead-free
solder is solderedat 220°C to260°C.
A local element is a corrosion element smaller than 1mm
, such as the crystals in an alloy, which are
connected so that they conduct electricity. Pitting corrosion is often causedby a local element.
IPC-A-610 DE is the German version of the standard for soft soldering connections. The February 2005
version, RevisionD, replaces theRevision 2000 version, and includes lead-free solder connections.
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