Improving the Reliability of Si Die Attachment with Zn-Sn-Based High-Temperature Pb-Free Solder Using a TiN Diffusion Barrier

The thermal fatigue reliability of Si die-attached joints with Zn-30wt.%Sn, high-temperature, Pb-free solder was investigated, focusing on the interfacial microstructure and joining strength of a Cu/solder/Cu joint during thermal cycling. A sound die attachment on an aluminum nitride (AlN) direct-bonded copper (DBC) substrate was achieved by forming Cu-Zn intermetallic compound (IMC) layers at the interface with the Cu of the substrate. During the thermal cycling test performed between −40°C and 125°C, thermal fatigue cracks were induced by the growth of Cu-Zn IMCs at the interface with the Cu. A␣thin titanium nitride (TiN) film was applied to suppress the formation of Cu-Zn IMCs. Adequate joint formation was accomplished by using an Au/TiN-coated DBC substrate, and only the TiN layer was observed at both interfaces. In conjunction with the TiN diffusion barrier, the Si die-attached joint created with Zn-30wt.%Sn solder exhibited a stable interfacial microstructure during thermal cycling. No microstructural changes, such as IMC formation, grain growth or formation of fatigue cracks, were observed, and the joining strength was maintained even after 2000 cycles.     

Interfacial Reactions of Si Die Attachment with Zn-Sn and Au-20Sn High Temperature Lead-Free Solders on Cu Substrates

The interfacial reaction of Si die attachment with a high temperature lead-free solder of Zn-xSn (x = 20 wt.%, 30 wt.% and 40 wt.%) was investigated, and the currently used high temperature lead-free solder of Au-20Sn was compared. A sound die attachment to a Cu substrate can be achieved with Zn-Sn solder. No intermetallic compound (IMC) phase was observed in the solder layer, and only primary α-Zn and Sn-Zn eutectic phases were observed. At the interface with the Si die, with a metallization of Au/Ag/Ni, an AgAuZn₂, IMC layer was formed along the interface, and the Ni coating layer did not react with the solder. At the interface with the Cu substrate, CuZn₅ and Cu₅Zn₈ IMC layers were confirmed, and their thicknesses can be controlled by soldering conditions. During multiple reflows, the growth of these IMC layers was observed, but no additional voids or cracks were observed. For more reliable die attachment, a titanium nitride (TiN) coating layer was applied to suppress the formation of Cu-Zn IMCs. The Si die attached joint on the TiN-coated Cu was quite stable during the multiple reflows, and no visible IMC phase was confirmed in the interfacial microstructure.     

Variations in the composition of various acyl lipids,tocopherols and lignans in sesame seed oils roasted in a microwave oven

Seeds from various strains of cultivated Sesamum indicum Linn (colour of seeds: black, brown and white) were exposed to microwave roasting for 16 and 30min at a frequency of 2450 MHz and were studied not only for different acyl lipids and their fatty acid compositions, but also for the contribution of anti-oxidants to the oxidative stability of the oils. Lipids from all-seeded strains were comparable in their total fatty acid composition, with linoleic, oleic, stearic and palmitic acids as the major acids. The total lipids were isolated by thin-layer chromatography into the following five fractions: triacylglycerols (TAG), diacylglycerols, free fatty acids, polar lipids and steryl esters. The TAG were slightly and randomly hydrolysed by microwaves, but was still representing 900 g kg^?1 of the total lipids at 30min of roasting. Although burning and bitter tastes occurred at the time, the tocopherols and lignans still amounted to over 80% of the original level. The results suggested that the oxidative stability of the oils would probably be due to the synergism between endogenous antioxidants and browning substances produced during microwave roasting.     

cis-Vaccenic acid in mango pulp lipids

A peak corresponding to a methyl octadecenoate other than oleate has been detected on the capillary gas chromatogram of the methyl esters of mango pulp fatty acids. This octadecenoate was isolated by silica gel and argentation column chromatography, high performance liquid chromatography and argentation thin layer chromatography, and then analyzed by infrared, nuclear magnetic resonance and gas chromatography-mass spectrometry, chromatographic separations and oxidative degradation. These analytical data proved that the octadecenoic acid wascis-vaccenic acid (cis-11-octadecenoic acid). The concentration of this acid in total octadecenoic acids ranged from 35% to 50% in the pulp of mangoes from Fiji, Mexico, the Philippines and Taiwan.cis-Vaccenic acid was revealed to be one of the major component fatty acids of non-polar lipids (mainly triacylglycerols), glycolipids and phospholipids in mango pulp. The glycolipids containedcis-vaccenic acid (ca. 20%) in higher concentration than oleic acid (ca. 15%). A trace amount ofcis-vaccenic acid (0.5%) was detected in the total lipids of mango seeds. Profile of fatty acid composition of mango pulp lipids (0.2–0.3 wt% of wet pulp) was characterized by the presence of n−7 acid isomers,cis-vaccenic acid and palmitoleic acid, and unusual mono- and dienoic positional isomers.     

Effects of microwave energy on the relative stability of vitamin E in animal fats

The relative stabilities of individual tocopherols during microwave heating were evaluated in purified animal fats. Each tocopherol (α, β, γ and δ) was added to the purified beef tallow and lard at 0.25 μmol g^?1 fat immediately before use. During microwave heating the highest rate of loss was shown by α-tocopherol, followed by γ-, β- and δ-tocopherols. This order did not depend on the types of fats present. The indices such as peroxide value, carbonyl value and anisidine value for assessing quality of fats showed a significant difference (P < 0.05) in the final stage (12–20 min) of heating. It is therefore necessary that particular attention be paid to the loss of vitamin E when cooking pork or beef in a microwave oven.     

Preliminary test of an ultrasonic liquid film sensor for high-temperature steam–water two-phase flow experiments

A prototype liquid film sensor for high-temperature steam–water experiments has been developed. The sensor shape simulates a boiling water reactor (BWR) fuel rod. The pulse-echo method can be utilized to measure the thickness of the liquid film covering the sensor surface. A piezoelectric element is soldered onto the inside of the sensor casing which consists of two curved casing pieces. After the piezoelectric element is attached, the two casing pieces are laser welded together. It is confirmed that the temperature rise at the time of the laser welding does not influence soldering of the piezoelectric element. The pressure proof test shows that the sensor can be used at a high-pressure condition of 7 MPa. Simple air–water experiments are done at atmospheric pressure to confirm the liquid film thickness can be measured with the sensor. The fluctuation of the liquid film thickness is satisfactorily captured with the sensor. The minimum and maximum thicknesses are 0.084 and 0.180 mm, respectively. The amplitude of the waveform at 286 °C is predicted by the calculation based on the acoustic impedance. It is expected that the sensor is able to measure the liquid film thickness even at BWR operating conditions.     

Viscosity Effects on One-Dimensional Consolidation of Clay

A constitutive relationship for one-dimensional consolidation of clays is presented. It recognizes the importance of structural viscosity and yielding in controlling many of the phenomena associated with the consolidation of clays. The governing equation for the large strain consolidation of clays, incorporating the new constitutive relationship, has been solved using a finite difference technique. The computer program has been verified using laboratory experiments including those performed on an interconnected consolidometer. The laboratory tests and numerical results are used to examine many of the current hypotheses used in predicting field consolidation. The results show that the deformation of thick clays in the field are different from those predicted from a thin laboratory specimen using the square of the drainage length or the uniqueness of the end of primary consolidation concepts. The dominance of the structural viscosity during the primary consolidation stage has been shown to be the main source of the discrepancy.