Failure Analysis of Aluminum Wire Bonds in High Power Igbt Modules
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Fig. 1: Cross-section of a typical IGBTpackage (schematically)
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Mat. Res. Soc. Symp. Proc. Vol. 390 0 1995 Materials Research Society
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Fig. 2: Estimated T-loads for 30 years of service at 360 days a year and 26 trips a day on the Gotthard south ramp
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EXPERIMENTAL PROCEDURES A range of thermal cycling experiments has been taken into consideration. The results considered were obtained from commercially available IGBT-modules in the 400 A range as well as single-chip test devices. The test parameters differ in: STcase and cooling unit mounts. • Time constant r depending on the package geometry, temperature distribution and package materials (thermal properties such as thermal conductivity, specific heat and density). Typical values are T = 0.5 ... 1.5s. • Power losses are a function of the switching frequencies in the range of t = ton + toff = 5 ... 100 s, currents and on-state voltages. All these parameters determine AT - 60 ... 200 K. Failure can be defined as voltage drop AVCE or an increase in the thermal resistance. After failure the modules were opened and the silicon-gel cover removed for inspection by Scanning Electron Microscopy (SEM) and metallographic methods. Fig. 3 shows the SEM secondary electron image of a fractured wire bond and Fig. 4b depicts a cross-section through an emitter wire bond after failure. The microstructures of the failed parts were also compared to those of wires after bonding in order to correlate the crack location to microstructural features, that might be dominating.
Fig. 3: SEM-image of a fractured wire-bond
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b) Fractured bond
a) Welded interface after bonding (etched)
Fig. 4: Metallographic cross-sections In order to investigate the thermal conditions (temperatures and temperature distribution) on the chip surface infrared thermal imaging was employed (Fig. 5). These measurements were performed on open single-chip modules built up for these experiments. For measurement of the thermomechanical displacements and the resulting strains an optical image analysis method was used. This method yields the displacements which are obtained by cross-correlation of two images at a temperature difference AT. The measurement field was restricted to the bond area with the image covering a field of about 1.1 mm by 1 mm (Fig. 6). The readings from the IR-imaging were taken to correlate the displacement measurements with theory.
Fig. 5: Test chip with IR-thermal image
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Fig. 6: Visualization of the displacement field 4,41I
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RESULTS AND DISCUSSION Materials analysi Metallographic cross-sections of the fractured wire bonds show the crack location (Fig. 4b). This example shows crack propagation mainly along grain boundaries within the wire material. Other samples exhibit crack propagation
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