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The effect of underbump metallization (UBM) on electromigration (EM) lifetime and failure mechanism has been investigated for Pb-free solder bumps of 97Sn3Ag composition in the temperature range of 110–155°C. The EM lifetime of the SnAg Pb-free solders with either Cu or Ni UBM was found to be better than the eutectic SnPb (63Sn37Pb) solders but worse than high-Pb (95Pb5Sn) solders. In the test temperature range, the EM lifetimes were found to be comparable for Cu and Ni UBMs but with different activation energies: 0.64–0.72eV for Cu UBM and 1.03–1.11eV for Ni UBM. EM failure was observed only in solder bumps with electron current flow from UBM to the substrate. Failure analysis revealed that EM damage was initiated by the formation of intermetallic compounds (IMC) at the UBM∕solder interface which was found to be significantly enhanced by mass transport driven by the electron current. Under EM, the continued growth of IMC with the dissolution of the UBM and the accumulation of Kirkendall voids resulted in the formation of interfacial cracks and eventual EM failure of the solder bump. For Ni UBM, the IMC formation was dominated by the Ni3Sn4 phase while for Cu UBM, a bilayer of Cu3Sn∕Cu6Sn5 was found. Void formation at the Cu6Sn5∕solder interface was found to be important in controlling the EM lifetime of the Cu UBM solder.

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To relieve the serious saturation in stator teeth of stator‐permanent‐magnet flux‐switching (SPM‐FS) machines due to the co‐existence of magnets and armature windings in stator, this study proposes and analyses a novel rotor‐permanent magnet flux‐switching (RPM‐FS) brushless machine. Different from the conventional SPM‐FS machines, the magnets are removed from stator to rotor, and magnetised in a unique direction, resulting in a significant alleviation of stator tooth saturation level for the RPM‐FS machines. The concentrated armature windings are still wound around stator teeth with an even shorter end‐part winding length due to the absence of magnets. To evaluate the advantages and disadvantages of RPM‐FS machines, a comprehensive comparison between a RPM‐FS machine, a SPM‐FS machine, and an interior permanent magnet machine used in Toyota‐Prius 2004 hybrid electric vehicle, is conducted and the electromagnetic performances of three machines are investigated by finite element analysis. The predicted results indicate the proposed RPM‐FS machine exhibits the largest power density, greatest torque capability, highest efficiency under rated operation, and improved flux‐weakening ability. Therefore, the RPM‐FS machine is a promising candidate for EV and hybrid electric vehicles applications especially for direct driven systems where the superior overloaded performance is crucial.

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Copper (Cu) bond wires are increasingly used in semiconductor components to replace gold (Au) bond wires, and applications for these components are expanding from consumer to high-reliability electronic systems. To assess the impact of this conversion on the overall component reliability, extended reliability testing beyond the typical JEDEC component qualification testing is needed. Additionally, key packaging materials such as molding compound also need to be re-evaluated as they may interact with Cu wire bond differently from Au wires and introduce new failure mechanisms. In this work, we investigate the impact of molding compound chemistry on the reliability performance by varying two material properties, pH level and Clconcentration. A total of 9 formulations are generated for a 16-lead SOP (Small Outline Package) package. The assembled components are subject to two acceleration tests including biased-HAST (Highly Accelerated Stress Testing) and HTS (High Temperature Storage) test. As a control, Au wires are used for some of the cells and are subject to the same acceleration tests as the Cu wire components. The failure rates for all of the experimental cells will be reported. The effects of pH and Cl-levels on the failure rates will be discussed. Additionally, analytical results on the failed components using FIB (Focus Ion Beam), SEM (Scanning Electron Microscopy), and EDX (Energy Dispersive X-ray) will be reported. The reliability testing and analytical results will demonstrate the importance of controlling molding compounds properties and provide guidelines for the selection of these materials. For high-reliability and mission-critical electronic systems, improved and better-controlled packaging material formulations will be needed to ensure the long-term reliability of components using Cu bond wires.

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