IGBT Modules - Technologies, Driver and Application (Second Edition) - page 517

at the corners of the DCB, the cycle capability of system solder can be increased. The
reason is the increased proportion of borderline between ceramic and copper region of
the DCB and, hence, the area provided to pick up themechanical stress caused by the
load cycles. Another way to increase the cycle capability is given by the fact that the
elasticity of the DCB can be increased by a special doping of the ceramic portion. The
CTE value itself remains largely unaffected. However, theDCB can now absorb a larger
proportionof themechanical stress and thus reduce the delaminationover time.
In addition, or by itself, the cycle capability can be improvedwith respect to delamination
of the copper layer, by the introduction of defined indentations in the edge area of the
copper coating of the DCB. This technique is referred to as "inserting dimples" and was
introduced byElectrovacCuramik.
Fig. 14.15
DCBwith dimples
14.5.3Low temperature joining
Load changes and the associated changes in temperature will always stress the solder
layers and bond connections. An alternative to soldering and bonding connections are
so-called low temperature joining (LTJ) connections. Twomaterials are joined bymeans
of a pre-treated layer of granular silver or silver strips which are suitably shaped. As
described in more detail in chapter
the connection process takes place within
about 60s at a temperature above 220°C but far below the melting temperature of all
materials involved (hence the term low temperature joining) and a pressure of 40MPa.
The structure of the individual layers and the connected materials A and B is depicted
schematically i
Examples of the use of low temperature connections are listed i
shows an example in which the chip-to-DCB solder joint has been replaced. In the
example i
the chip-to-DCB bondwire connectionwas replaced too, using a
silver strip. The first modules with the variant depicted in
are already
available on themarket as standard products. The variant i
, however, has
not been established so far.
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