Failure detection methods

4.1.2.1 Electrical and thermal measurements

Before starting the test and at the end of it, power modules are electrically and thermally characterized by measuring the out and transfer characteristics, the leakage currents, the resistance RDSon and the thermal impedance Zth. The goal is to evaluate the healthiness of modules after cycling.

4.1.2.1.1 Electrical measurements

Out and transfer characteristics, leakage currents, as well as the RDSon are measured by the Keysight Technologies, Inc. B1505A Power Device Analyzer and Curve Tracer (Figure 4.4). It is a single box apparatus which can accurately evaluate and characterize all types of power devices. It has a fast pulsing capacity (10 µs) and a µΩ level RDSon measurement resolution and sub-pA level current measurement capability [Key14].

The out characteristic of the MOSFET is recorded. It consists in measuring and plotting the drain to source current IDS versus the drain to source voltage VDS for different gate to source voltage VGS. Curves obtained are characterizing the linear region also known as ohmic mode (Figure 4.5).

Figure 4.5: Out characteristic of one MOSFET for different values of V_GS

The transfer characteristics of the MOSFET at low and high current VDS were recorded as well (Figure 4.6).

Here, IDS was measured and plotted versus VGS at low and high IDS. This allows verifying that the switching operation occurs correctly and thus that the MOSFET is still electrically functional. A damage of a single cell in the MOSFET would lead to an increase in IDS at low VGS. This leakage current would be in the range of 10e^-6 A.

Figure 4.4: The Keysight Technology Inc. B1505A Power Device Analyzer and Curve Tracer[Key14]

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Figure 4.6: Low and high transfer characteristics of one MOSFET

Semiconductors conduct a small amount of current even when they are turned off, it is refer to as leakage current. This leakage is a quantum phenomenon where electrons tunnel through an insulating region. Other than tunneling via the gate insulator or junctions, carriers can also leak between source and drain terminals of a MOS device. This is called subthreshold conduction. Drain to source leakage current IDSleak as well as the gate to source leakage current IGSleak were measured at respectively VGS=0 and VDS=0. Increased leakage is a common failure mode resulting from non catastrophic overstress of a semiconductor device, when the junction or the gate oxide suffers permanent damage not sufficient to cause a catastrophic failure. On the Figure 4.7 the drain leakage curve IDSleak has an exponential form due to its conduction properties, and the gate leakage curve IGSleak can be explained by the tunnel effect for VGS ≤ -20V and VGS ≥ 20V, whereas the rest is noise. The large variations in the gate leakage curve are due to measuring incertitude as values are really low, around 1e^-11 A.

Figure 4.7: Drain and gate leakage of one MOSFET

The RDSon is a characteristic of a power device. It corresponds to the drain to source resistance and is temperature dependent. Thus a pulse test is performed at room temperature to measure this parameter minimizing heating of the junction. During this test IDS and VDS are measured while applying the maximum VGS. The RDSon is then deduced with the ohmic law: RDSon =VDS/IDS (Figure 4.8). When the chip is degraded it leads to an increase in the RDSon.

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Figure 4.8: R_DSon of one MOSFET

4.1.2.1.2 Thermal measurement

The thermal impedance Zth is measured respecting the norm JEDEC-JESD51-14 [JESD10] via an equipment from MentorGraphics and its T3ster-Master Software that enables the interpretation of measurements (Figure 4.9). This equipment has a 1µs time resolution, registers up to 64 000 data points and has a high resolution of 12 bit in ΔU=50mV [Men00]. The Zth measurement is performed by loading the body diode of one power MOSFET of the package. The body diode conducts a loading current of approximately 5A for 10s, the time needed to reach the thermal steady state. Then the load current is switched off and the cooling behavior is measured during 10s until the junction temperature cools down to the environment temperature. The temperature rise is measured on the device by electrical means through the forward voltage Vf of the body diode which is a TSP. The TSP is turned into temperature via a sensitivity factor K, which is a characteristic of the device. For our module the factor K was determined to be approximately equal to 2,2 mV/K. Once the measurement performed, the T3ster-Master software enables us to visualize the Zth curve and to interpret this curve as a cumulative or derivative structure function, already described in the chapter 3. Degradations in module’s layers generate an increase in the Zth curve.

Figure 4.9: Test bench to measure the Z_th [Men00]

4.1.2.2 Optical analysis

Once the device tested and electrically and thermally characterized, some microscopic observations are carried out. First of all, the module was carefully examined at naked eyes to detect any big deformations, or visible cracks in the package. Then a Scanning Acoustic Microscope (SAM) is used to detect delamination at the interface mold/ Cu lead frame or degradation at solder joints area. A SAM analysis is a non destroying control technique that allows scanning the entire package. Thus it gives an overview of the module’s degradations and help to decide if it is worthy to investigate the device more in details and with which

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method. For example based on SAM observations one can define where a cross-section could be cut.

Moreover, SAM allows an approximated measure of delaminated surface at solder joints (Figure 4.10).

Sometimes, X-ray was also performed on modules, highlighting as well degradations in solder joints but also cracks in mold.

Figure 4.10: SAM picture of the entire B6 Bridge on the left, and measurement of the non degraded area in the top solder, on the right.

After that, one or two cross-sections were cut in the module at degraded area, and are observed under optical microscope and Reflection Electron Microscope (REM) (Figure 4.11). With the optical microscope one can see distinctly deformations and cracks of layers, while the REM allows a more detailed analysis of microcracks, voids and grains structure. In order to highlight delamination in package, the cross-section was observed with the dye and pry method. It consists in coloring the specimen with UV oil, which will penetrate in voids, cracks and delaminated zone. Thus by observing the metallographic specimen under UV light microscope, cracks are appearing in white color whereas the rest of the module is dark.

Figure 4.11: Cross-sections of metallographic specimens

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