Solder electromigration (EM) is recognized as a potential reliability problem in power modules. This study investigates the fundamental EM phenomena of a Cu/Ni-P/Sn-3.0Cu/Ni-P/Cu joint at 175°C and 50 A/mm² using a Sn-3.0Cu solder known as one of candidates for high-temperature solders. After reflow, a thicker Cu-rich (Cu,Ni)₆Sn₅ layer and a thinner P-rich layer formed at the solder-electrode interfaces. This structural feature suppresses both the disappearance of the (Cu,Ni)₆Sn₅ layer and increase of the P-rich layer caused by EM which results in an open failure at the cathode.

In the thermal design of electronic devices, it is necessary to estimate the temperature of the small electronic components used and examine the properness of the temperature. The temperature of a component is mostly influenced by two thermal resistances: that of the electronic component as determined by the thermal conductivity of the material, and that of the path between the mounting area and the air. Therefore, development of an evaluation apparatus for measuring these parameters is urgently required.

In this study, a test-head type thermal resistance measurement system for estimating the above-mentioned thermal resistances was developed. The measurement area of the system is 2 mm square, and the measurement range of thermal resistance is from 10.1 to 299.5 K/W. This paper discusses the validity of the measurement principle of the system and the verification results of the measurement accuracy.

The material of a printed circuit board corresponding to the high frequency band needs to have a low dielectric constant and a low dielectric loss tangent. Liquid crystal polymer (LCP) is one candidate material. However, LCP is a difficult material to plate on, and current pretreatment involves a long time in highly concentrated alkaline treatment to roughen the surface before plating. However, there is concern about transmission loss resulting from the roughened surface. In this study, in order to improve these problems, we investigated a pretreatment process that combined an alkaline treatment, a mild acidity permanganate treatment, and a silane coupling treatment. We report that this process makes it possible to form electroless copper-plated films with good adhesion on low-roughness LCP surfaces.

We have developed a new, thin, high-voltage transformer utilizing FPCs (Flexible Printed Circuits). In high-voltage power supplies, transformers are important components that convert low input voltages to high voltages. High-voltage transformers require a large number of turns, the capacitance formed between adjacent lines increases, the self-resonant frequency becomes low, and the switching frequency cannot be increased. Hence the switching frequency of the transformer cannot be increased. As a result, miniaturization is difficult. In recent years, FPCs have been used in various electronic devices by taking advantage of their thin and flexible performance. Focusing on the withstand voltage of this FPC and the ability to pattern with a fine pitch, we examined whether a high-voltage transformer could be developed. We also examined reducing the capacitance while maintaining the line withstand voltage. Furthermore, we devised a structure that folds the FPC while maintaining the withstand voltage, and developed a thin high-voltage transformer with a significantly smaller capacitance than conventional products.

To use millimeter-wave transmission efficiently, it is necessary to reduce transmission line losses. Thus, a dielectric substrate having a low dielectric constant and copper foil having a low roughness are used to create a copper-clad laminate (CCL). The physical properties of dielectrics and conductors for use in circuit design, such as dielectric properties and surface resistance, are measured independently. On the other hand, since an actual circuit is formed through various processes, the designed transmission characteristics are not always obtained. In this study, I propose a method to analyze the cause of the losses in millimeter-wave transmissions by separating the losses into dielectric losses and conductor losses for a microstrip line circuit actually fabricated from CCL. As a result, it was confirmed that the losses can be estimated based on the characteristics of the dielectrics and copper foils.

The newly developed lotus-type porous metal having a great number of small directional pores can be manufactured at low cost and its application as a heat sink has drawn a great deal of attention. Since the lotus-type metal consists of small pores extending in one direction only, the heat transmittance is higher and the pressure loss is smaller than that of a general isotropic porous body. In this study, a fast computing scheme for evaluation of the heat transfer characteristics of a heat sink made of a lotus-type porous metal was developed based on the finite element method for heat conduction analysis. The fin surface heat transfer coefficient was identified from the experimental results, and a general prediction equation for Nusselt number was derived. The evaluated heat transfer performance of a lotus-type porous metal heat sink structure was illustrated and agreed well with the experimental results.

We studied a method of evaluating joining materials for power semiconductor device packaging such as Ag sintering materials. For the fundamental properties, Young's modulus values were measured by three-point bending tests using joining materials deposited on thin Cu plates. Coefficients of thermal expansion were evaluated by warpage of the same samples using a Shadow Moiré measurement in an electrical furnace over a range of temperatures from 25°C to 250°C. Power cycling tests were performed using an Al plate contacted on the upper electrode of SiC diodes to avoid any effects from the other packaging materials. Electro-migration reliability tests were carried out by joining materials deposited on metalized Al₂O₃ substrates with high current densities.

Silicon carbide (SiC) devices exhibit several advantages such as high withstand voltage, high temperature operation, and low losses compared to conventional Si devices. However, in order to take full advantage of SiC devices, it is necessary to improve the characteristics of the power module package. In particular, the electrical insulation properties of the package depend on the insulating substrates. In this study, we focused on the difference in the electric field strength distribution when the front and back copper foils are moved horizontally relative to the aluminum nitride substrate.

We performed a dielectric breakdown voltage test and an electric field analysis corresponding to the test. It was clarified that the dielectric breakdown voltage has a correlation with the electric field strength of the triple junctions, and the dielectric breakdown points coincide with the triple junctions of the copper foil, ceramic and encapsulant.