In this study, effect of Ag addition on microstructure of In-48 wt%Sn solder was investigated to improve mechanical properties. The microstructure of In-Sn-Ag solder was formed by β phase, γ phase and two different intermetallic compounds (IMCs) which are Ag2In and AgIn2. Ag2In in In-Sn-Ag solder improves the mechanical strength but deteriorates the ductility. However, AgIn2 enhances the ductility but decreases the mechanical strength. To improve the both mechanical strength and ductility, control of the amount of Ag2In and AgIn2 was found to be important, and 3 wt% Ag addition was found to improve both mechanical strength and ductility of In-48 wt%Sn solder.
The energy regeneration from small area heat sources which is essential for energy harvesting etc. requires the device which is small and high power generation efficiency per unit area. It is necessary for that purpose to form small size and low height thermoelectric elements, and mount them with high density. Bi-Te material, which has been used widely as thermoelectric elements of a low-temperature region cleaves easily, therefore it is difficult to form a small element by conventional cutting method. In our developed process, small size and low height element can be realized in short cycle time by crystal growth of the thermoelectric material in a small tube, while a large cost cut can be achieved by reduction of cycle time and a material loss at cutting.
Demands of the raising operation temperature of power modules have been increasing in recent years. However, the power cycle capability is insufficient when used in a high temperature environment to apply the conventional Sn-based solder. In this study, we have developed a highly reliable bonding technology that improves the characteristics of the Sn phase by adding additional elements Bi, In, Sb to the Sn-7Cu solder. Power cycling test (Tjmax 175°C) was carried out to evaluate the reliability. Power cycling reliability of Sn7Cu3Bi, Sn7Cu10Sb is approximately 3 times, 6 times higher than Sn7Cu.
A process condition and reliability of die-attach technique using Ni-nanoparticles for high temperature electronics was investigated. Use of Ni for die-attach material decrease thermal stress induced by the difference of coefficient of thermal expansion and improve the bonding strength of the joints, because the coefficient of thermal expansion of Ni is between that of Si or SiC device and Cu substrate. In the case of the die-attach technique,‘two-step sintering’, the temperature of second-step sintering must be higher than a reduction temperature of NiO which was formed on surface of Ni-nanoparticle before sintering. In order to optimize the process conditions of the two-step sintering, in particular, the temperature of second-step sintering, the behavior of NiO reduction has been investigated by bonding strength, thermal analysis, and TEM observation. As a result, the joints using two-step sintering technique has superior reliability on thermal cycle test with -40/+250°C.
Recently, power modules have become widespread because the need for energy-saving technology is increasing. Using wiring technology in place of Al wire for power semiconductor chips has attracted attention along with the progress made in miniaturizing the modules. We investigated a wiring technology for direct ultrasonic bonding Al/Cu lead terminals on semiconductor chips. As shown in the results, chip tilt after die-bonding led to reducing the bonding quality. Regardless of the tilt, a large bonding region was obtained by using Al/Cu lead terminals with bumps. In addition, a thermal cycle test was conducted for 1000 cycles between -45°C and 150°C. It was found that tne Al/Cu lead terminals with bumps maintained ample bonding area even after rigorous reliability test.
Silver nanowires of 20-60 nm in width and 10-50 μm in length have been examined as alternative materials to ITO (Indium Tin Oxide) due to their unique conductivity and transparency. Here we report the synthesis of silver nanowires and direct patterning of silver nanowires ink using screen printing and treatment methods for generation of conductivity. Fine patterning (line/space = 110 μm/70 μm, narrow space = 180 μm) and lower resistance by photonic curing than mechanical pressing, in addition the development of flexible silver nanowires films were achieved.
In the estimation of the fatigue life of lead-free solder joints under cyclic temperature load, it is important to use FE analysis technology using a constitutive model which can exhibit mechanical properties of the lead-free solder joints. In previous study, we proposed a decoupled viscoplastic-creep constitutive model which can describe stress strain curves, stress strain hysteresis loops and stress relaxation curves, with these temperature dependencies and these rate dependencies. In this study, the algorithm for stress calculation is built by implicitly-time-integration scheme to our proposed model, aiming at improvement of stability. Next, the algorithm was implemented into finite element analysis code ABAQUS via its user material subroutine UMAT. Then, tensile tests, tension-compression tests and stress relaxation tests were analyzed, using this program. It was confirmed that the stability of the analysis was improved.
Two kinds of new Fe-Cr system brazing filler metal, Fe-20Cr-43Ni-10P and Fe-20Cr-20Ni-8.0P-5.0Si-2.0Mo, were developed as a substitute for an expensive Ni-based brazing filler metal in brazing of SUS304 stainless steel. Corrosion resistance of the joints brazed with new filler metals were investigated and compared with the joint brazed with the conventional Ni-based brazing filler metal. Microstructures of fillet areas of the joints brazed with the Fe-20Cr-43Ni-10P filler metal and the Fe-20Cr-20Ni-8.0P-5.0Si-2.0Mo filler metal are made up of primary crystal and the final solidified region. The dark gray phase and the bright gray phase were found in the final solidified region on the basis of the back-scattered electron image observation. Corrosion attacks occurred only in the final solidified regions for both filler metals. The bright gray phase in the final solidified region is preferentially corroded for the Fe-20Cr-43Ni-10P filler metal due to the formation of Cr-poor phase. In contract, for the Fe-20Cr-20Ni-8.0P-5.0Si-2.0Mo filler metal, the dark gray phase is preferentially corroded because the bright gray phase is of the high level of Cr content. Compared with the Ni-based filler metal, the corrosion resistance of the Fe-20Cr-20Ni-8.0P-5.0Si-2.0Mo filler metal is slightly weaker. On the other hand, the Fe-20Cr-43Ni-10P filler metal does not nearly provide corrosion resistance as a result of the formation of Cr-poor phase. Furthermore, it is confirmed that the formation of Cr-poor phase in the Fe-20Cr-43Ni-10P filler metal is related to volume fraction of the phase.