Method for estimating the service life of a power semiconductor component

A method which exploits the phenomenon that an increase of the collector-emitter voltage U.sub.CE of a semiconductor power component during a load change test can be correlated with the service life expectancy of the relevant IGBT module. A method for estimating the service life of a power semiconductor component, comprising the steps of subjecting the component to a periodic load change, measuring an electrical parameter P of the component that serves as an indicator for reliability or durability against the number N of load changes, calculating a derivative dP/dN of the electrical parameter P according to the number N of load changes; and comparing the derivative dP/dN with a target value representing a determined service life. The characteristic "longer-lived than a reference module" or, respectively, "more resistant to failure" can be allocated to a test module if the derivatives of the respective voltage U.sub.CE satisfy the condition (dU.sub.CE /dN).sub.Test <(dU.sub.CE /dN).sub.Ref after the number N of load changes.

BACKGROUND OF THE INVENTION 
Due to their excellent electrical characteristics and comparatively simply 
constructed trigger electronics, IGBT (Isolated Gate Bipolar Transistor) 
power semiconductor modules increasingly are being used, even in 
failure-critical systems. Since traction control units of track-bound 
vehicles (locomotives, streetcars and subway trains, etc.) are often 
subjected to large mechanical and thermal stresses during their often 
decades-long service life, the users place the highest demands on the 
reliability and service life of the IGBT modules used. 
Currently, there is no standardized method for testing the reliability of 
IGBT power semiconductor modules. A group of research institutes, 
manufacturers and users has concerned itself with this problem, and has 
discussed a corresponding test method. See, Proceedings of the 20th 
International Symposium for Testing and Failure Analysis, Los Angeles, 
Nov. 13-18, 1994, pp. 319-325, incorporated herein by reference. 
According to the recommendations of this group, an IGBT module should be 
subjected to a plurality of load changes under defined conditions, and, in 
particular, should be rated as defective or faulty when the 
collector-emitter voltage (U.sub.CEsat), measured after several thousand 
cycles, deviates from an initial value by more than a predetermined 
percentage (20% divided by the number of chips in the module). Even if 
this condition is relaxed and a drop off of 10% below the initial voltage 
is still tolerated, in long-life modules the corresponding degradation 
first arises after some hundreds of thousands of load changes. Since the 
duration of the cycle (t.sub.on +t.sub.out) is typically in the range from 
about 10 to 20 seconds, the test of reliability/service life requires a 
measurement time of several weeks or even months, which is far too long 
for routine control in production. 
A method for testing the reliability of electronic components specified in 
H. Koschel, et al., Zeitraffende Zuverlassigkeitsprufungen an 
Transistoren; offprint from Nachrichtentechnische Zeitschrift, No. 5 
(1964), incorporated by reference, rests on the assumption that an 
increase in the loading of a test object results in an acceleration of the 
failure mechanism. As long as the failure mechanism is maintained, the 
test time can then be shortened corresponding to what is called an 
acceleration curve. The individual points of the acceleration curve are 
obtained from a plurality of failure distribution curves, measured 
respectively from samples of test objects, using a mathematical model that 
approximately describes the expected distribution of the service life. 
SUMMARY OF THE INVENTION 
The present invention provides a test method that makes it possible to 
carry out an early estimation of the service life of a power semiconductor 
component or, respectively, to allocate the characteristic "longer-lived 
than a reference element" or "shorter-lived than a reference element" to 
the component after comparatively few load changes. 
In an embodiment, the invention provides a method for estimating the 
service life of a power semiconductor component, comprising the steps of 
subjecting the component to a periodic load change; measuring an 
electrical parameter P of the component that serves as an indicator for 
reliability or durability against the number N of load changes; 
calculating a derivative dP/dN of the electrical parameter P according to 
the number N of load changes; and comparing the derivative dP/dN with a 
target value representing a determined service life. 
This method can be used in particular in the area of development and 
manufacturing of IGBT power semiconductor modules. It makes it possible to 
determine after a relatively short time which of the technologies examined 
offers the greatest advantages with respect to the reliability/durability 
of the modules, and which process parameters still require an optimization 
if warranted. Inserted into the quality control, samples taken from the 
running production can be tested more rapidly than before, and the 
corresponding batch can be released for delivery to the customers. 
In an embodiment, the invention provides a method wherein the derivative 
dP/dN is compared with a derivative (dP/dN).sub.Ref, measured for a 
reference element under the same conditions and within the same interval 
of N load changes. 
In an embodiment, the invention provides a method wherein the component is 
rated as longer-lived in relation to the reference element as long as the 
condition 
EQU dP/dN&lt;(dP/dN).sub.Ref 
is satisfied within the interval of N load changes and in that the 
component is rated shorter-lived in relation to the reference element as 
long as the condition 
EQU dP/dN&gt;(dP/dN).sub.Ref 
is satisfied within the interval of N load changes. 
In an embodiment, the invention provides a method wherein at least a second 
derivative d.sup.2 P/dN.sup.2 is calculated and compared with a 
corresponding second derivative (d.sup.2 P/dN.sup.2).sub.Ref of the 
reference element. 
In an embodiment, the invention provides a method wherein a warning signal 
is produced if the derivative dP/dN is larger than the target value. 
In an embodiment, the invention provides a method wherein the parameter P 
is selected from the group consisting of a potential difference between 
two terminals of the component, an ohmic resistance between two terminals 
or an ohmic resistance of a component terminal. 
In an embodiment, the invention provides a method wherein the power 
semiconductor component comprises an emitter terminal, an auxiliary 
emitter terminal and a collector terminal, a resistance (R.sub.EH) between 
the emitter terminal and the auxiliary emitter terminal, a resistance of 
the emitter (R.sub.E) or a resistance of the auxiliary emitter (R.sub.H), 
or a collector-emitter voltage (U.sub.CEsat) is measured against the 
number N of load changes. 
In a further embodiment, the invention provides a method for estimating the 
service life of a power semiconductor component, comprising the steps of 
subjecting the component to a periodic load change, measuring an 
electrical parameter P of the component that serves as an indicator for 
reliability or durability against the number N of load changes, 
calculating a derivative dP/dN of the electrical parameter P according to 
the number N of load changes, and comparing the derivative dP/dN with a 
target value representing a determined service life, wherein the 
derivative dP/dN is compared with a derivative (dP/dN).sub.Ref, measured 
for a reference element under the same conditions and within the same 
interval of N load changes, wherein at least a second derivative is 
calculated, wherein the parameter P is selected from the group consisting 
of a potential difference between two terminals of the component, an ohmic 
resistance between two terminals or an ohmic resistance of a component 
terminal, and wherein the power semiconductor component comprises an 
emitter terminal, an auxiliary emitter terminal and a collector terminal, 
a resistance (R.sub.EH) between the emitter terminal and the auxiliary 
emitter terminal, a resistance of the emitter (R.sub.E) or a resistance of 
the auxiliary emitter (R.sub.H), or a collector-emitter voltage 
(U.sub.CEsat) is measured against the number N of load changes. 
In an embodiment, the invention provides a method as set forth above 
wherein the component is rated as longer-lived in relation to the 
reference element as long as the condition 
EQU dP/dN&lt;(dP/dN).sub.Ref 
is satisfied within the interval of N load changes and in that the 
component is rated shorter-lived in relation to the reference element as 
long as the condition 
EQU dP/dN&gt;(dP/dN).sub.Ref 
is satisfied within the interval of N load changes. 
In an embodiment, the invention provides a method as set forth above 
wherein a warning signal is produced if the derivative dP/dN is larger 
than the target value. 
These and other features of the invention are discussed in greater detail 
below in the following detailed description of the presently preferred 
embodiments with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
Since the electrical terminals, usually made of aluminum, of an IGBT 
component and the silicon chip to be contacted have thermal expansion 
coefficients of different magnitudes, mechanical shear forces arise in the 
region of the bond wires during operation. As a consequence of these shear 
forces, fine hairline cracks can form in the metal, which grow 
continuously and gradually detach the bond wire from its base. The bond 
wire can then finally disengage from the metallization, and thereby 
interrupt the current and voltage supply of the IGBT component (see FIGS. 
3-6 in Proceedings of the 20th International Symposium for Testing and 
Failure Analysis, Los Angeles, Nov. 13-18, 1994, pp. 319-325), 
incorporated by reference. This mechanism most often leads to the failure 
of the IGBT module, which consists of several components, whereby the 
auxiliary emitters not loaded with current during operation mostly 
disengage, at least at small currents, before the respective main 
emitters. 
In accordance with the invention, the degradation of the bond wires leads 
to a change in the bond wire resistance, so that this resistance can serve 
as an indicator for the reliability/durability of the respective terminal, 
and thus for the service life of the component or, respectively, of the 
relevant IGBT module. In the context of corresponding investigations, it 
has turned out that the faster the bond wire resistance increases with 
time or, respectively, with the number N of load changes carried out, the 
earlier a component of the module fails due to the disengaging of a bond 
wire. FIG. 1 shows the results of the measurements carried out on three of 
the total of six components (10A, 600 V) of an IGBT module. The resistance 
R.sub.H of the respective auxiliary emitter is shown plotted against the 
number N of load changes. The test parameters are indicated next in Table 
I: 
TABLE I 
______________________________________ 
Test Parameters 
______________________________________ 
Input/output time per cycle 
t.sub.on = 10 sec.; t.sub.off = 20 sec. 
Temperature spread .DELTA.T = 100 K 
Number of cycles N .apprxeq. 10.sup.4 
Gate voltage U.sub.G = 15 volts 
Collector current I.sub.c = 3 A 
______________________________________ 
The strongest increase was shown by the resistance R.sub.H of the auxiliary 
emitter, measured at component M2C5. This component also failed, due to 
disengagement of the emitter bond wire, already after about half the 
planned number of load changes. The component M2C1 was somewhat 
longer-lived. At the beginning, the resistance of its auxiliary emitter 
already had approximately the same time dependence as the auxiliary 
emitter resistance of the component M2C4, which was the longest-lived in 
the load change test that was carried out. The failure of the component 
M2C1 made itself known in advance, after about half the measurement time, 
through a stronger increase in the auxiliary emitter resistance. The 
resistance of the auxiliary emitter measured at component M2C4 did not 
reach failure-critical values until much later. 
The correlation between the curve of the auxiliary emitter resistance 
R.sub.H and the expected service life of the respective IGBT component can 
also be established on the basis of FIG. 2. There, the slope dR.sub.H /dN 
of the respective auxiliary emitter resistance R.sub.H is shown plotted 
against the number N of load changes. It can be seen clearly that an IGBT 
component fails earlier the more strongly the resistance R.sub.H of its 
auxiliary emitter increases with time. Through comparison of the 
derivative dR.sub.H /dN, measured at a test element, with the 
corresponding value (dR.sub.H /dN).sub.Ref of a reference element, the 
characteristic "shorter-lived than the reference element" (dR.sub.H 
/dN&gt;(dR.sub.H /dN).sub.Ref) or "longer-lived than the reference element" 
(dR.sub.H /dN&lt;(dR.sub.H /dN).sub.Ref) can thus already be allocated to the 
test element after comparatively few load changes. If the test element and 
the reference element are respectively the most failure-susceptible 
components of their unit, then the relevant test module is also 
shorter-lived/longer-lived than the reference module. 
For the measurement of the resistance R.sub.H, the load change test is 
broken off short and the IGBT component 1 is connected with a controllable 
source of direct current 2 in such a way that the current I.sub.CH, 
respectively fed into the collector 3, flows via the semiconductor chip 4 
and the upper metallization 5 to the auxiliary emitter 6, and from there 
to the source 2 (see FIG. 3). The gate terminal 7, separated from the 
semiconductor chip 4 by an insulator, remains without current, as does the 
emitter terminal 8. After recording the voltage drop U.sub.EH across the 
emitter 8 and the auxiliary emitter 6, the current strength I.sub.CH is 
altered, in order to carry out at least one additional measurement of the 
potential difference U.sub.EH. The slope of the U.sub.EH /I.sub.CH 
characteristic curve, obtained for example by means of a linear 
regression, then supplies the sought resistance R.sub.H. 
As explained above, the bond wire 8, serving as the emitter terminal, is 
also subject to degradation. Hairline cracks, caused by strong shear 
forces, influence the contact resistance, so that through measurement of 
the emitter resistance R.sub.E or of the resistance R.sub.EH between the 
emitter 8 and the auxiliary emitter 6, the bond wire quality can be judged 
and the service life of the relevant component or, respectively, IGBT 
module can be estimated. Thus, a component 1 has a longer service life 
expectancy than a component 2 if the derivatives satisfy the condition: 
EQU (d.sub.RE /dN).sub.1 &lt;(dR.sub.E /dN).sub.2 
or, respectively 
EQU (dR.sub.EH /dN).sub.1 &lt;(dR.sub.EH /dN).sub.2. 
The corresponding measurement constructions are shown schematically in FIG. 
4 (R.sub.E measurement) and FIG. 5 (R.sub.EH measurement). 
Larger IGBT power semiconductor modules usually have no auxiliary emitter 
bonded immediately to the semiconductor chips. In order to estimate the 
expected service life of these modules, it is advantageous to examine the 
degradation of the collector-emitter saturation voltage U.sub.CESat during 
a load change test, and in particular the slope dU.sub.CEsat /dN. FIG. 6 
shows the saturation voltages U.sub.CEsat measured respectively for two 
IGBT modules (300 A, 1600 V) plotted against the number N of load changes 
and the allocated third-degree compensation polynomials (solid curves). 
The two modules can be distinguished from one another essentially in that 
different technologies were respectively used during their manufacture. 
The load change test was broken off when the saturation voltage 
U.sub.CEsat of module No. 1 deviated from its initial value by 10%. On the 
basis of the slope dU.sub.CEsat /dN of the saturation voltage U.sub.CEsat 
or, respectively, the slope of the allocated compensation polynomial, it 
can be predicted with high probability, after about 2/3 of the testing 
time at the latest, that module 1 will satisfy the failure criterion (10% 
deviation of the U.sub.CEsat voltage from its initial value) earlier than 
module 2. The relevant time region for the prognosis of the service life 
expectancy is shown in FIG. 7. In contrast to FIG. 6, in the calculation 
of the compensation polynomials only the data already present after about 
half the measurement time were taken into account. The greater slope of 
the polynomial allocated to module No. 1, after a starting phase, can be 
seen clearly. Since the second derivatives also satisfy the condition 
EQU (d.sup.2 U.sub.CEsat /dN.sup.2).sub.1 &gt;(d.sup.2 U.sub.CEsat 
/dN.sup.2).sub.2, 
module 1 can also be allocated the characteristic "shorter-lived than 
module 2" with high certainty after about half the planned load changes. 
If the service life expectancy of a module is carried out through analysis 
of the U.sub.CEsat curve already after comparatively few load changes, 
this necessarily leads to an increased uncertainty of the prognosis. Thus, 
in addition to the first and second derivative of the saturation voltage 
U.sub.CEsat the recorded degradation curves should, if necessary, be 
subjected to another time series analysis, trend analysis or spectral 
analysis. Corresponding methods are known from the field of statistics. 
The method specified above is used particularly in the fields of research, 
development and quality assurance. The possibility of classifying IGBT 
components and IGBT modules according to their service life expectancy 
through analysis of the recorded degradation curves (R.sub.H, R.sub.F, 
R.sub.EH and U.sub.CEsat curves) can however also be used advantageously 
in traction systems. The control unit, computer-controlled in any case, 
can for example periodically query the U.sub.CEsat voltages of the 
individual components, analyze the recorded curves and produce a warning 
signal if one of the voltages U.sub.CEsat or, respectively, its slope 
dU.sub.CEsat /dN reaches a critical value that signals imminent failure. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventors to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of their contribution to the art.