Extensive fatigue investigation of solder joints in IGBT high power modules

IGBT (Insulated Gate Bipolar Transistor) power modules are more and more used in traction applications, where they replace thyristors and GTO (Gate Turn Off thyristors). Unlike the press-pack packaging of the latter, these modules contain numerous IGBT chips in parallel, and thus numerous solder joints which play a key role in the reliability of this stack packaging. Accelerated aging tests have been carried out with more than one hundred representative samples divided in seven batches. They differed in the type of solder (two compositions including a lead-free one), cooling rate after the reflow, and metallization of the ceramic substrate. Most cycled samples have been extensively analyzed to make a ranking of the different technological choices. Moreover, the influence of the main technological and manufacturing process parameters has been evaluated. Results have shown that the solder composition and the cooling rate after the renew have a strong influence on the thermal fatigue resistance of the tests specimens.