First evaluation of lifetime prediction

To be able to compare tests results in function of the different tests parameters, an End-of-Life (EoL) criterion has to be defined. In a first approach, an increase in Vf of 1% is taken as EoL criterion. This increase of 1% in Vf corresponds for our module to an increase in 5% in Vce, which is a typical EoL criterion used for standard IGBT module. This criterion corresponds actually to a wire lift-off in a standard module. As our module does not have wire bonds but a Cu clip, this criterion does not correspond to a specific failure mechanism occurring in the module and thus may be too severe. But this criterion is here used as a basis to help us determining a suitable criterion for our module.

The number of cycles to failure with the EoL criterion Vf +1% is determined for every tests and is plotted for different sets of test parameters (Figure 4.33). Sometimes, the quality of the test was not optimal as some problems occurred for example with the cooling system, thus leading to strong variations in the Vf monitoring.

For these tests it was not possible to determine the number of cycles to failure. For some other samples, no increase in Vf is observed thus the number of cycles to failure was not reached before stopping the test. In this case, the number of cycles to failure is reported to be greater than the number of cycles reached until the end of the test. On the histogram these cases are marked with a red arrow on top of the bar.

It appears on the histogram that increasing the ΔTj reduces the lifetime. So, it confirms what was already seen with the metallographic analysis. It is also important to remark that for all tests performed with ΔTj =60K, the EoL criterion is not reached even though devices have already been submitted to several millions of cycles.

For a higher ΔTj of 120K the maximal number of cycles to failure reached is 445 000 cycles. Thus a huge difference of lifetime exist for tests performed with field loads ΔTj=60K, and accelerated tests with ΔTj=120K. Actually a device under field load can endure about 2,5 times more cycles than with ΔTj=120K.

This rise the question to know if the failure mechanisms are the same for field loads and high accelerated tests. Simulations will give us more information regarding this issue. Determine the influence of Tjmin and of ton is more difficult: more tests have to be performed in order to have more data to compare. Thus it can only be deduced from the experiments that large ΔTj can be critical for our module in terms of lifetime.

In order to better characterize the influence of test parameters, the number of cycles to failure is plotted in function of each parameter. Here also the tests which did not reached the EoL criterion are indicated by a red arrow. First we study the influence of the ΔTj, with the Figure 4.34. Even though not enough tests were conducted to be able to make some statistics, a trend line appears quite clearly out of our few results. This shows that there is an exponential relation between the number of cycles to failure and ΔTj.

Figure 4.33: Histogram showing the number of cycles to failure N_f with the EoL criterion V_f +1% for different sets of test parameters

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Figure 4.34: Nf cycles to failure with EoL Vf +1% in function of ΔTj

The Figure 4.35 shows the influence of Tjmin on the device lifetime. Here 4 exponential trend lines are plotted for 4 different orders of magnitude of ΔTj.The 4 trend lines are not parallel to each other. The 2 trend lines obtained for ΔTj=60K and ΔTj=80K to 95K are indicating that the lifetime stays constant whatever Tjmin value.

But for the 2 other trend lines obtained for ΔTj=100K to 130K and ΔTj=155K to 180K, the lifetime increases with an increasing Tjmin. Thus, no clear trend line emerges out of the experimental results concerning the influence of Tjmin on the lifetime, as it also strongly depends on ΔTj.

Figure 4.35: N_f cycles to failure with EoL V_f +1% in function of T_jmin

Finally the influence of the pulse width ton on the device lifetime is studied on the Figure 4.36. Here 4 trend lines emerge for 4 different orders of magnitude for ΔTj.The 4 trend lines are not parallel to each other, but 3 of them are decreasing with a ton increase. Thus trend lines indicate that the longer the pulse width the shorter the lifetime.

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Figure 4.36: N_f cycles to failure with EoL V_f +1% in function of t_on

Then, results obtained for our module were compared to the ones coming from the literature (cf. chapter 1).

The same three graphics were plotted showing respectively the influence of ΔTj, Tjmin andton on the device’s lifetime. Data coming from the literature include all kind of devices (IGBT, MOSFET, diodes...), with all kind of internal structure (with solder or Ag sinter or with wire bonds or ribbons). On the Figure 4.37, one can see that our module is robust as our results are in the upper limit of the scatter chart. Moreover for some points on the chart, our module did not even reach the EoL criterion (points with a black arrow). This supposes a lifetime even higher than what is plotted.

Figure 4.37: N_f cycles to failure in function of ΔT_j for the B6 Bridge with EoL V_f+1% in comparison with data from the literature

For the Figure 4.38, results of our module are also in the upper limit of the scatter chart, once again with some points that did not reach the EoL criterion. It appears also that we were the first to perform some tests with a negative Tjmin of -30°C. Thus, for those points no comparison with existing results is possible.

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Figure 4.38: N_f cycles to failure in function of T_jmin for the B6 Bridge with EoL V_f+1% in comparison with data from the literature

Finally the Figure 4.39 shows that results of the B6 Bridge are following the same trend as the ones from the literature. Results of our module are more scattered than on the other charts, due to the strong influence of ΔTj

on the results. Thus here the results of the B6 Bridge are not in the upper limit but in the average of the results from the literature. We can notice also that we have done a few tests with short pulse width of 0,2s which was not made until now in the literature.

Figure 4.39: N_f cycles to failure in function of t_on for the B6 Bridge with EoL V_f+1% in comparison with data from the literature

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