Ching-Hui Tsai

Electromigration-induced UBM consumption and the resulting failure mechanisms in flip-chip solder joints

Eutectic PbSn flip chip solder joint was subjected to 5×103 A/cm2 current stressing at 150°C and 3.5 × 104 A/cm2 current stressing at 30°C. The under bump metallurgy (UBM) on the chip was sputtered Ni/Cu, and the substrate side was a thick Cu trace. It was shown through in-situ observation that the local temperature near the entrance of electrons from the Al interconnect to the solder became higher than the rest of the joint. The accelerated local Ni UBM consumption near the entrance was also observed. Once the Ni was consumed at a location, a porous structure formed, and the flow of the electrons was blocked there. It was found that the formation of the void and the formation of the porous structure were competing with each other. If the porous structure formed first, then the void would not be able to nucleate there. On the other hand, if the void could nucleate before the UBM above lost its conductivity, then the joint would fail by the void formation-and-propagation mechanism.

Pronounced electromigration of Cu in molten Sn-based solders

The high local temperature in flip-chip solder joints of microprocessors has raised concerns that the solder, a low melting temperature alloy, might locally liquefy and consequently cause failure of the microprocessors. This article reports a highly interesting electromigration behavior when the solder is in the molten state. A 6.3 × 10 A/cm electron current was applied to molten Sn3.5Ag solder at 255 °C through two Cu electrodes. The high current density caused rapid dissolution of the Cu cathode. The dissolved Cu atoms were driven by electrons to the anode side and precipitated out as a thick, and sometimes continuous, layer of Cu6Sn5. The applied current caused the dissolution rate of the Cu cathode to increase by one order of magnitude. A major difference between the electromigration in the solid and molten state was identified to be the presence of different countering fluxes in response to electromigration. For electromigration in the molten state, the back-stress flux, which was operative for electromigration in the solid state, was missing, and instead a countering flux due to the chemical potential gradient was present. An equation for the chemical potential gradient, d /dx, required to balance the electromigration flux was derived to be d /dx N°z*e J, where N° is Avogadro’s number, z* is the effective charge of Cu, e is the charge of an electron, is the resistivity of the solder, and J is the electron current density.

Publisher’s noteThe Keelung Submarine Volcano in the near-shore area of northern Taiwan and its tectonic implication

Special Issue on Cenozoic Volcanism, Tectonics and Natural Resources in the East Asia. Due to a production error, the following article ‘‘The Keelung Submarine Volcano in the near-shore area of northern Taiwan and its tectonic implication” was wrongly included as part of the issue JAES_135C. This article is now replaced with this note and will be included in the special issue Cenozoic Volcanism, Tectonics and Natural Resources in the East Asia [SI: East Asia Tectonics & Resource].

Active mud volcanoes in the gas hydrate potential area of the upper Kaoping Slope, off southwest Taiwan

Thirteen mud volcanoes are identified from the multibeam bathymetric data in the gas hydrate potential area of the upper Kaoping Slope, off southwest Taiwan. The heights of the mud volcanoes range from 65 m to 345 m and the size of their bases range from 680 m to 4,100 m in diameter. The slopes of the mud volcanoes are very steep (from 5.3° to 13.6°), suggesting that the mud volcanoes are fed by high-viscosity flows. ROV observations reveal active eruptions of the mud volcanoes MV1, MV5 and MV12, and two modes of eruption (explosive and effusive eruptions) were identified. The MV1 and MV5 are characterized by explosive eruptions and the eruptive cycles are about 3~5 and 5~8 minutes, respectively. The MV12 shows an effusive eruption, characterized by the continuous outpouring of mud together with gas plume. The gas plumes were found on tops of mud volcanoes MV1, MV4, MV5, MV10 and MV12, as indicated by the high-frequency sonar (EK500, EK60 sonars and multibeam echo sounder) images. The mud flows on the flank of six mud volcanoes (MV1, MV3, MV5, MV6, MV9 and MV10), illustrated by the high backscatter intensity bands from the sidescan sonar images. As evidenced by the results of investigation, the mud volcanoes in the upper Kaoping Slope off SW Taiwan are very active.

Active normal faults and submarine landslides in the Keelung Shelf off NE Taiwan

The westernmost Okinawa Trough back-arc basin is located to the north of the Ryukyu islands and is situated above the northward dipping Ryukyu subducted slab. In the northern continental margin of the Okinawa Trough, the continental slope between the Keelung Valley and the Mein-Hua Submarine Canyon shows a steep angle and future slope failures are expected. The question is how slope failures will proceed? A sudden deep-seated slump or landslide would probably cause local tsunami and hit northern coast of Taiwan. To understand the probable submarine landslides, we conducted multi-channel seismic reflection, sub-bottom profilers, and multi-beam bathymetry surveys off NE Taiwan. Two general trends of shallow crustal faults are observed. The NE-SW trending faults generally follow the main structural trend of the Taiwan mountain belt. These faults are products of inversion tectonics of reverse faults from the former collisional thrust faults to post-collisional normal faults. Another trend of roughly E-W faults is consistent with the current N-S extension of the southern Okinawa Trough. The fault offsets in the eastern portion of the study area are more pronounced. No obvious basal surface of sliding is found beneath the continental margin. We conclude that the movement of the submarine landslides in the Keelung Shelf off northeastern Taiwan could be in a spread type. The submarine landslides mainly occur in the continental slope area and it is more obvious in the east than in the west of the Keelung Shelf. Article history: Received 1 October 2016 Revised 21 March 2017 Accepted 2 July 2017