The Prius That Shook.. - VFAQ Prius Site

Yasuho pondered. "I'll have to design something much
more advanced than the `igniter' so that it can work. Now,
how can I do that?" What he needed was the world's best
IGBT module that would withstand the heat cycle, but
there was no such thing in the world at the time.
Looking for the material
Meanwhile, Electronics Production Engineering
Division's Staff Leader Yoichiro Baba had also noticed
the heat cycle problem. The IGBT chip was on top of a large
ceramic board, which was bonded to a copper plate. Copper
was at the bottom, the ceramic board was soldered on it,
and the IGBT chip was soldered again on the ceramic,
forming a three-layer construction. Ceramic worked as the
insulator.
The problem here was that the expansion rate of the
copper was four times greater than that of the ceramic.
The expansion rate of the aluminum nitride used for the
ceramic was 4.5 PPM / degree C. The copper's expansion
rate was 16.5 PPM / degree C. The chip was 4 PPM / degree
C.
In other words, the expansion rates of the ceramic and
the chip were roughly the same, whereas the copper expanded
four times as much. If the copper should expand by itself as
the temperature increased, the solder
222