A simple method for diagnosing noise immunity of printed circuit boards (PCBs) by the bulk current injection (BCI) test was proposed, which can contribute to the PCB trace designs for common-mode noise. A grading index, which is defined as the ratio of the stray capacitances with and without critical IC of malfunction, was introduced to distinguish the PCB susceptible to the common-mode noise. This proposed method was validated experimentally using four PCBs with the same circuit but different trace design. It was observed that the noise immunity of PCBs had a good correlation with the values of these grading indices.

For communication relays, the contact reliability in the low electrical load circuit has been of great importance for the purpose of control of highly reliable electronic devices. With reference to the study of contact failure mechanism, the fact that the formation of black powder on the contact surface caused to increase contact resistance of relays in the organic gas atmosphere was well known. In this study, the electrical characteristics of black powder formed on the relay contacts was investigated in comparison with a sulfide. Cosequently, the interesting relationship between contact resistance and electrical breakdown was obtained. The inflection point was observed in voltage-current (V-I) characteristics for the black powder deposit which was more than 200 mΩ in contact resistance. And when the current increased, there occurred not only a decrease but also an increase in contact resistance. These results indicated that the accurate contact resistance should be evaluated using V-I characteristics. In addition to that, it was proposed that the allowable level of contact resistance for the contact failure was about 150 mΩ for the black powder deposit.

Contact contamination is verified by the measurement of static contact resistance so far. However, in case of the contamination of organic thin film, new evaluation method is needed in stead of static contact resistance. Then, quantitative analysis of the contact cleaning effect was conducted using contact activation characteristics, dynamic contact resistance pattern and so on when the ultrasonic cleaning, immersion cleaning, vapor cleaning and jet blow cleaning were performed for relay contacts. As a result ,it was clarified that characteristics of the dynamic contact resistance depends upon the nature of the organic film on the contact surface and the effect of each contact cleaning method was quantitatively compared.

Communication relays have been used for the control of low level circuits, and organic gases vaporized from relays or contamination of contact surface were studied as the factors lowering contact reliability so far. Conversely, contact materials of platinum group contacts like palladium and gold or silver alloy contacts were mainly evaluated as reliable contacts. Authors tried to clarify the contact failure mechanism of gold or silver alloy contacts at a extremely low electrical load which had minimal switching electrical energy. As a result, the oxides are detected from contact failure samples by the contact surface analysis, which suggests a small amount of impure substances in the gold alloy causes contact resistance increase. In addition, the influence of the ratio of oxygen and nitrogen on contact resistance was investigated. Then the contact failure has not occurred below 0.1% of oxygen concentration. As for some contact materials, the permissible contact force was also studied.