|With the miniaturization of electronic products, power devices sustain more and more operations under high power, high frequency, and high integration. Power electronic devices are subjected to high operating temperature and current density, which poses a serious challenge on the packaging materials. An open-pin failure was found on a metal-oxidesemiconductor field-effect transistor (MOSFE) device after operated for 3-4 a under high current density condition. The lead frame material is C194 alloy, belonging to Cu-Fe-P alloy, which is a copper base alloy strengthened by dispersion. The operating temperature of the MOSFET device is in the range of 88-120°C at the full working current of about 15 A from the drain to the source, while the gate current is relatively small in a milliampere level. Failure analysis is conducted on the open-pin MOSFET device. According to the investigations by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), two intermetallic compounds (IMCs), Cu3Sn and Cu6Sn5, were formed at both the C194 alloy/Sn-Pb and the Cu/Sn-Pb solder interfaces. However, the crack is always initiated at the C194 alloy/Sn-Pb solder interface, no matter for the source or the drain. Usually, the crack would initiate at the cathode side due to the sole effect of electromigration. In this work, the crack initiated at the cathode side for the drain, but at the anode side for the source. According to the working condition of the MOSFET device, this failure crack can be attributed to the combined effects of electromigration and thermo-migration on multi-element interfacial diffusion and vacancy migration. In detail, for the source of MOSFET device, the direction of Cu thermo-migration flux is opposite to the electro-migration flux, while the thermomigration flux is large so that it can compensate the effect of electro-migration flux, thus leading to the crack at the anode. For the drain of device, the direction of Cu thermo-migration flux is the same as that of electro-migration, which leads to a severe cracking at the cathode. Moreover, in order to reveal the cracking process, SEM analysis was conducted on the fracture surface where plenty of fine Fe-rich granules and some Cu-Sn IMCs were observed. Furthermore, the electron probe micro analysis (EPMA) demonstrated that these fine Fe-rich granules form a continuous layer adjacent to the crack. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were also conducted in order to characterize the interfacial microstructure with Fe-rich granules/layer. At the C194/IMC interface, the Fe granules with body-centered cubic (bcc) structure accumulated as a continuous layer with an obvious grain growth compared to the uniformly dispersing Fe granules within the C194 alloy matrix. The existence of segregated Fe layer and Kirkendall voids would weaken the original bonding between C194 and IMC, so the crack always initiated along the Fe layer or Kirkendall voids when the thermal stress or mechanical stress applied. Above all, this coupled open-pin failure mode and its mechanism would provide a theoretical guidance for the optimization of product process and the improvement of service life.
Gao Liyin,Li Caifu,Cao Lihua,et al. The open-pin failure of power device under the combined effect of thermo-migration and electro-migration[J]. Chinese Science Bulletin,2020,65(20):2169-2177.