A system such as missiles and spare parts of aircrafts is in storage for a long time from the delivery to the usage and its reliability goes down with time. It should be inspected and maintained at periodic times to hold a high reliability. This paper suggests a periodic inspection of a storage system with two kinds of units where unit 1 is inspected and maintained at each inspection and unit 2 is degraded with time. However, a system can not be used at finite numbers of inspections, because it involves some parts which have to be replaced when the total operating times have exceeded a threshold level. A system is replaced at next inspection when it has failed or at N-th inspection, whichever occurs first. Then, the expected cost per unit of time is derived and an optimal time which minimizes it in case of periodic inspections is discussed. Numerical examples are given when failure time distributions are exponential and Weibull ones.
第35回国際信頼性物理シンポジウム(1997'IRPS/35th International Reliability Physics Symposium)で報告された論文の概要を述べ、最近の半導体関連の信頼性研究の動向をまとめた。信頼性研究の動向としては、スクリーニング技術やパッケージ技術等にあるいはデバイス技術関連に代表される実用上の問題から提起される課題への対応や、ホットキャリア劣化やESD等の信頼性シミュレーション、ゲート酸化膜破壊などの故障物理への学術的な追求などの方向に分極化の傾向が見られる。