Masahiro Inoue

Bonding materials with high thermal and electrical conductivity and reliable resistance to thermal stress are required. The authors have been conducting fundamental research on sintering-type bonding, in which metal micro-fillers are low-temperature sintered in the resin-bonded type electrically conductive adhesives (ECAs), as a new bonding technology, with the aim of easing thermal stress through the resin binder. This study investigated the influence of the kind of additive diluent in epoxy-based ECAs containing silver (Ag) micro-flakes on the microstructure development in the adhesives and the connection properties to metal electrodes. As a result, the sintering of Ag micro-flakes was observed to proceed in the adhesive once cured at 150 °C and by post-annealing at 250 °C. Furthermore, the sintering behavior varied greatly depending on the kind and composition of the binder additive diluent, with corresponding changes in electrical conductivity and connection characteristics with metal electrodes. Additionally, electrode surface conditions affected the connection performance. These findings are valuable for designing sintering-type bonding using resin-bonded ECAs, optimizing interfacial interactions between binder chemicals and metals.

The effects of binder chemistry on electrical-conductivity development, i.e., dynamic percolation, in carbon nanotube (CNT)-filled epoxy-based pastes during curing were investigated. The electrical resistivities of the pastes are influenced not only by the dispersivity of the CNT filler but also by the binder chemistry of the pastes. The electrical-conductivity-development kinetics are accelerated by introducing a reactive diluent, namely phenyl glycidyl ether, to the binder. As shown by alternating current impedance spectroscopy, the electrical-conductivity development is governed by the decrease in interfacial electrical resistance between the filler units. Electrical-conductivity development does not occur synchronously with the curing shrinkage of the binder; thus, the chemical state of the interface adjacent to the fillers is a key parameter for enhancing electrical conductivity. Thus, control of this interfacial chemistry is an essential concept for developing electrically conductive CNT-filled pastes.

A phenolic-based electrically conductive adhesive containing copper fillers was prepared. The copper fillers were treated using tetraethylenepentamine（TEA）before mixing into the binder to suppress oxidation of the fillers. Although the adhesive exhibited lower electrical resistivity（50-100 μΩcm）after curing in air, variation in the resistivity during environmental tests was different depending on the curing temperature. When the adhesive was cured at 100℃, inter-filler conductive contacts and cross-linked polymer structure in the binder were not fully formed during curing. The electrical resistivity of the adhesive cured at 100℃ was varied during exposure to 85℃/85 %RH environment due to annealing effect and oxidation of the fillers. By contrast, the interfiller conductive contacts and cross-linked structure were sufficiently developed during curing at 150-200℃ to improve the electrical reliability. Therefore, curing condition is one of the important factors in determining the reliability of the adhesive.

Interfacial interaction between binder and fillers in an epoxy-based electrically conductive adhesive containing silver flakes was investigated using TD（time-domain）-NMR spectroscopy. Two different types of diluents were used for the adhesive. Heterogeneity in the epoxy-based binder was clearly characterized by TD-NMR. In addition, interfacial interaction between filler surface and some domains formed in the binder was successfully analyzed by TD-NMR. When a reactive diluent（phenyl glycidyl ether, PGE）was used, a PGE-rich domain was clarified to preferentially interact with the fillers. The interfacial interaction caused by the addition of PGE was suggested to enhance interaction between fillers by a rheological analysis of the adhesive paste. Electrical conductivity development in the adhesives during curing was analyzed by a free-damped oscillation method concurrently with electrical resistance measurement. The analysis clarified formation of filler network was enhanced by the addition of PGE during curing. As a result of this work, TD-NMR can provide useful information for material design of electrically conductive adhesives.

The Holter monitor is used to measure an electrocardiogram (ECG) signal while a subject moves. However, the Holter monitor is uncomfortable for the subject. Another method of measuring the ECG signal uses a smart shirt. We developed a smart shirt that has six electrodes on the chest and can measure a detailed ECG, obtained with unipolar precordial leads. The electrodes and wires of the shirt are made of conductive ink that is flexible and stretchable. The smart shirt is stretchable and fits the body well. However, because of the gap between the smart shirt and the body, electrodes V1 and V2 do not touch the body consistently. We developed a conductive foam block that fills this gap. We investigated the characteristics of the conductive foam block, and measured ECG signals using the smart shirt. The electrical resistance of the conductive foam block was reduced by pressure. This characteristic could be utilized to measure the ECG signal because the block was pressed by the body and smart shirt. We could measure the ECG signal using the smart shirt and blocks while the subject walked and could detect peaks of the ECG signal while the subject jogged slowly.