Masateru Suwa

Reliability of thick Al wire bonds in IGBT modules for traction motor drives

Reliability enhancement of thick Al wire bonds during thermal fatigue test has been investigated from a metallurgical viewpoint. Al wire bonds degrade with the increase of crack length during thermal fatigue tests with high /spl Delta/T/sub j/ due to the tensile stress generated by the thermal expansion coefficient mismatch between Al wires and Si. It is also found that cracks propagate along the small grain boundaries of Al wires at the bonding interface. It is predicted that the Al wire bonds may not degrade due to thermal fatigue if /spl Delta/T/sub j/ is controlled below 40 K, i.e., keeping it within the actual temperature fluctuation range in IGBT modules for traction motor drives. The reliability of Al wire bonds can be enhanced by increasing the grain size of the Al wire at the bonding interface. The high temperature bonding is considered to be a good candidate for enhancing the reliability of Al wire bonds.

A Void Free Soldering Process in Large-Area, High Power Insulated Gate Bipolar Transistor Modules

We have developed a new void free process for making the solder joint between the chip mounted AlN substrate and the metal substrate in large-area, high power insulated gate bipolar transistor (IGBT) modules. This new process consists of two steps. First, Ar+ were used to clean the surfaces of Ni plated film on a metal and AlN substrates which were then coated with 0.5-µm-thick Ag film. Second, 50 wt% Pb–Sn solder was sandwiched between the two substrates and heated to 503 K in a vacuum for 5 min before being cooled in a N2 atmosphere. By using this process, the area percentage of voids in a soldering area up to 130×190 mm2 can be reduced to less than 0.1%. IGBT modules made by this process were also found to exhibit satisfactory current-voltage characteristics.

Formation of W underlayer by switching bias sputtering to plug 0.25 μm contact holes

Physical vapor deposion by a newly developed switching bias sputtering method for the formation of both adhesion and barrier metal underlayers on 0.25 μm contact/via holes with sufficient step coverage has been investigated as a hole filling process. The method features alternating operation of sputtering to deposit and resputtering of the film to enhance step coverages of the side walls by resputtering of the bottom. It was found that W films formed for holes with aspect ratios of 3.0–3.5 gave step coverages of 17%–23% for the side wall and 30%–33% for the bottom. The resistivity of W films deposited on planar surfaces was 10.5 μΩ cm, similar to values for W films deposited by conventional direct current (dc) sputtering. The deposition rate of the sputtering was 50%–80% of the rate of conventional dc sputtering.

Ball Formation in Aluminum Ball Bonding

The effects of various factors, i.e., shield gas, current density, wire polarity, and wire material, which can influence oxidation of ball surfaces were investigated. Good quality aluminum balls were found to be formed if optimum electric and shield gas conditions were employed; for example, current density geq 2.5 GA/m2wire polarity: cathode; shield gas: Ar+H 2 . Good quality aluminum balls of 1.7 to 2.5 times the wire diameter could be formed by varying discharge time and/or current density. It was found that they could be obtained independently of the wire material. The relation between morphologies of aluminum balls and the degree Of oxidation was also investigated in order to examine the mechanism for obtaining a good quality bail. A close correlation was found between ball morphologies, i.e., eccentricity, sphericity, and constriction, and the degree of oxidation. The morphologies were significantly improved as the oxide film thickness was reduced.

Microstructure of Rapidly Solidified Nb3Al-X Ribbons

Nb-Al and Nb-Al-X(X is Cr,Ti or Zr) ternary alloys ribbons, with compositions around the A15 (Nb 3 Al) structure, a candidate intermetallic as an advanced high temperature material, were rapidly solidified by an arc melt spin process. The rapid solidification structure of these tri-niobium aluminide alloys and the relationship between formed phases and compositions of ribbons are investigated.

Effect of chemical composition on corrosion resistance of zircaloy fuel cladding tube for BWR.

Effects of Fe and Ni contents on nodular corrosion susceptibility and hydrogen pick-up of Zircaloy were investigated. Total number of 31 Zr alloys having different chemical compositions; five Zr-Sn-Fe-Cr alloys, eight Zr-Sn-Fe-Ni alloys and eighteen Zr-Sn-Fe-Ni-Cr alloys, were melted and processed to thin plates for the corrosion tests in the environments of a high temperature (510°C) steam and a high temperature (288°C) water.In addition, four 450kg ingots of Zr-Sn-Fe-Ni-Cr alloys were industrially melted and BWR fuel cladding tubes were manufactured through a current material processing sequence to study their producibility, tensile properties and corrosion resistance.Nodular corrosion susceptibility decreased with increasing Fe and Ni contents of Zircaloys. It was seen that the improved Zircaloys having Fe and Ni contents in the range of 0.30[Ni]+0.15[Fe]≥0.045 (w/0) showed no susceptibility to nodular corrosion.An increase of Fe content resulted in a decrease of hydrogen pick-up fraction in both steam and water environments. An increase of Fe and Ni content of Zircaloys in the range of Fe≤0.25w/0 and Ni≤0.1w/0 did not cause the changes in tensile properties and fabricabilities of fuel cladding tube. The fuel cladding tube of improved Zircaloy, containing more amount of Fe and Ni than the upper limit of Zircaloy-2 specification showed no susceptibility to nodular corrosion even in the 530°C steam test.