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Increasing the lifetime of power modules by smaller bond wire diameters
Keywords: Lifetime, Power Electronic Modules, Different Bond Wires
The main failure cause in state-of-the-art power electronic modules is the bond-wire lift-off. Therefore new wires like aluminum-copper composites [1] or pure copper wires [2] are in development. The copper-wire bonding process is still not state-of-the-art [3] and all other alternatives to aluminum wires are not mature. To evaluate the lifetime of different bond wire diameters, power cycling tests (test system described in[4]) were performed with IGBTs soldered to direct-copper-bonded substrates. The dies were bonded with two groups of wires. The first one consisted of ten samples with 125 µm diameter aluminum wires and the second one of ten devices with 375 µm diameter aluminum wires. The results showed that all samples with the thicker wires failed due to bond wire lift off. In contrast the life limiting factor of the test group with the smaller wires was the solder layer. To prove that, cross-sections were analyzed with light microscopy. The results were also evaluated by a finite element analysis (FEM), which showed that the thermo-mechanical stress was much lower in the smaller wires. The FEM-model was simulated undergoing a temperature cycle ranging from 77 °F to 311 °F. The plastic strain on the underside of the bond foot, which resulted from the thermal expansion mismatch at the interface to the semiconductor, was then calculated. By taking a higher number of small wires instead of a few big ones, a low cost alternative (compared to copper wires) was shown to improve the lifetime of the power modules. Combined with new technologies like silver sintering and SiC devices the whole potential of the power modules can be used.
Aaron Hutzler,
Fraunhofer IISB
Nuremberg, Bavaria

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