Toshinori Kawamura

An Embrittlement Mechanism of Impact Fracture of Sn–Ag–Cu Solder Joints on BGA Using Electroplated Ni/Au Surface Finishes

The impact toughness evaluation and fracture mechanism analysis in board level of Sn-3 mass%Ag-0.5 mass%Cu solder joints of ball grid arrays (BGAs) using electrolysis Ni/Au plating were performed. The cause of impact toughness degradation of BGA solder ball joints is the segregation of impurities to the (Cu, Ni)6Sn5 intermetallic compound grain boundary formed in the solder joints. The impurities, consisting of Cl and organic matters, are taken in the Ni plating film at the time of Ni plating. The organic matter impurities come primarily from the solder mask of the BGA interposer substrates. To improve the impact toughness of the Sn-3 mass%Ag-0.5 mass%Cu solder joint of the BGA, it is necessary to lower the concentration of these impurities. This, in turn, places importance on solder mask material selection (to minimize Ni plating bath contamination) as well as contamination prevention and plating bath sanitization.

Studies on CO/sub 2/ laser drilling: formation mechanism of residual thin materials at the bottom of laser via

Formation mechanism of residual thin materials at the bottom of a microvia processed with CO/sub 2/ laser is studied with a bromine (Br)-containing epoxy film designed for sequential build-up printed wiring boards. By observing Br content distribution in a cross section of the epoxy film after laser processing, it is estimated that the film around the via is heated up to 1600 K and that the residual thin film at the bottom of the via is heated up to 2000 K upon laser irradiation. Based on heat transfer simulations for epoxy and copper layer structures, the epoxy film within a 0.1-/spl mu/m distance from the copper surface is unable to be removed by laser irradiation because the temperature of this region cannot be heated above the decomposition temperature of /spl ap/2500 K due to a large heat flow from the epoxy layer to the copper.

The Embrittlement Mechanism and Improvement of Impact Strength for Lead-Free Solder Joints in BGA Packages Using Electrolytic Ni/Au Plating

Impact strength evaluation and fracture mechanism analysis in board level of Sn–3mass%Ag–0.5mass%Cu solder joints of ball grid arrays (BGA) using electrolytic Ni/Au plating were performed. The cause of impact strength degradation of BGA solder ball joints is the existence of low density defects, which contain organic materials, in the (Cu,Ni)6Sn5 intermetallic compound grain boundary formed in the solder joints. These organic materials are taken in by the nickel plating film at the time of nickel plating. To improve the impact strength of the Sn–3mass%Ag–0.5mass%Cu solder joint of the BGA, it is necessary to lower the concentration of these organic materials. The contamination prevention and nickel plating bath sanitization, solder mask material selection (to minimize nickel plating bath contamination) and higher current density of nickel plating are effective to keep a lower concentration of organic materials in nickel plating film.

Development of Inherently Safe Technologies for Large Scale BWRs: (3) Infinite-Time Air-Cooling System

To address long-term station black outs, which occurred at the Fukushima Nuclear Power Station, we have been developing the infinite-time air-cooling system which operates without electricity by a natural circulation loop. The air-cooling heat exchanger, which is located outside the primary containment vessel of a reactor, transfers the decay heat to the atmosphere by natural circulation resulting from the density difference of the air. Improvement in the heat-transfer performance of air-cooling is a key technology in the development of the infinite-time air-cooling system. In this paper, we developed the air-cooling enhancing technology for the infinite-time air-cooling system by using a micro-fabrication surface, turbulence-enhancing structures, and heat-transfer fins. To evaluate the performance of this air-cooling enhancing technology, we conducted a heat exchange test using an element test apparatus. A single tube of the air-cooling heat exchanger, which includes a sheath heater and thermo-couples, was used. The air flow outside the tube and the heat quantity were respectively controlled using an air-compressor and the sheath heater. The heat-transfer performance was calculated from the heat-quantity and temperature difference measured using thermo-couples. The developed air-cooling enhancing technology demonstrated superior heat-transfer performance in this test. The heat-transfer performance increased approximately 100 % with this technology compared with a bare pipe. From these experimental results, we confirmed good feasibility for implementing the infinite-time air-cooling system.Copyright

Improving Impact Toughness of BGA Lead-Free Solder Joints by Using Higher Current Density in Electrolytic Ni Plating

Improving the toughness against impact of an electrolytically plated (Ni/Au) Sn-3 mass%Ag-0.5 mass%Cu solder joint for a ball grid array package was examined. The concentration of impurities (C, O, S, and Cl) that reduce the toughness against impact of Ni plating is shown to be inversely proportional to the rate of Ni deposition. The constant of proportionality becomes the rate of adsorption of impurities and depends on how clean the plating bath is. These results indicate that the concentration of impurities in Ni plating can be controlled by the rate of deposition (or current density). Creating the plating at a higher current density from a depth of about 1 μm from the finished surface is shown to improve the toughness against impact of solder joints formed with the combination of Ni plating and Sn-3 mass%Ag-0.5 mass%Cu solder. This method improves toughness against impact while suppressing variation in thickness of the Ni plating.

Numerical study by large-eddy simulation on effects and mechanism of air-cooling enhancing technologies

ABSTRACT Learning from the lessons of the Fukushima Daiichi Nuclear Power Station incident in which a long-term station blackout occurred, we have been developing an air-cooling system that can operate without electricity for a virtually indefinite time. We developed air-cooling enhancing technologies by using heat transfer fins, turbulence-enhancing ribs and a micro-fabrication surface. To achieve further improvement of the heat transfer performance, it is important to understand the mechanism of the air-cooling enhancing technologies. In this study, we used numerical analysis to investigate the effects and the mechanism of the developed air-cooling enhancing technologies. We confirmed that the Nusselt number was increased 75% by the heat transfer fins. In the heat transfer enhancement by the turbulence- enhancing ribs, the Nusselt number was increased 43% by the turbulence-enhancing ribs. The enhancement ratio of the Nusselt number by the micro-fabrication surface can be explained by the apparent thermal conductivity. The Nusselt number was increased 4%–8% by adding the micro-fabrication to the surface of the pipe with the turbulence-enhancing ribs. For the combination of the micro-fabrication surface and the turbulence-enhancing ribs, the interaction between the better heat transport in the thermally conductive layer and the mixing effect by the large-scale vortex is the heat transfer enhancement mechanism.