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Warranties: what are they? What do they really cost?
World Abstracts on Microelectronics and Reliability and MICHAEL G. PECHT. Proc. ,4. Reliab. Maintainab. Symp., 451 (1991). The goal of automating PWB solder joint fatigue life calculations into a design environment is to make reliability assessment information available to the designer as early as possible, and in an easily understood and implemented manner. This paper discusses the assumptions and details of the fatigue life calculations under a combined thermal and vibrational load. A methodology is presented 2. R E L I A B I L I T Y
OF
COMPONENTS,
Incorporating the effects of time estimation into humanreliability analysis for high-risk situations. JOESEPH SHARIT and DAVID M. MALON. IEEE Trans. Reliab. 40(2), 247 (1991). This paper provides a modeling framework for predicting the effects of discrepancies between subjective and objective measures of elapsed time on performance of human-machine interaction tasks in high-risk situations. Following a review of the major theories and experimental research findings in time estimation, the importance of time estimation in terms of its potential for affecting human performance in critical human-machine system operations is discussed. The formulations are primarily motivated by limitations in existing methods of human-reliability analysis that adjust human-error probabilities, often severely, on the basis of stress inferred from the ratio of estimated time available for task completion to time required. Our approach for dealing with time-related task demands is to describe the ongoing dynamic processes during task performance that are associated with temporal estimation, and to assess their effects on human reliability. This is accomplished through a conceptual framework that is depicted and discussed in terms of several task features and information-processing mechanisms, and that implicitly recognizes the experimental evidence concerning human time-estimation performance. This concept is illustrated through special cases that are formulated and analyzed in terms of their potential for contributing to human error as a function of time-estimation considerations. New international standards on reliability growth. LARRYH. CROW. Proc. A. Reliab. Maintainab. Syrup. 478 (1991). This paper reviews two new international standards on reliability growth developed by the International Electrotechnical Commission (IEC) for world trade. The first IEC standard discussed is IEC 1014, "Programs for reliability growth", and the second IEC standard discussed is IEC 56(Central Office)150 "reliability growth models and estimation methods." IEC 1014 was issued as an international standard in 1989 and IEC 56(Central Ot~ce)150 is an improved draft international standard. This document has recently been reviewed and approved as an international standard and will be published in the near future. These standards cover the management and modeling aspects, respectively, of a reliability growth program. An overview of these documents is given together with a more detailed discussion of various reliability growth terminology, procedures, and models represented in these international standards.
Some thought experiments In reliability. PAUL GOTTFRmD. IEEE Trans. Reliab. 40(2) 131 (1991). Some basic misconceptions about the relationships between life distributions and physical processes keep recurring in the reliability literature. Unfortunately, it is not practical to conduct laboratory experiments that would discriminate decisively among competing concepts. However, it is possible to devise some thought experiments that can be helpful: they cannot tell us what is right, but they can tell us what cannot be right. Warranties: what are they? What do they really cost?. TOBY M. BERKE and NICHOLAS A. ZAINO JR. Proc. A. Reliab. MR 32/4--J
585
to evaluate the combined effects of simultaneous vibration and thermal cycling of solder joints. This combined loading situation is simulated by superposing the effects of the vibrational and thermal loads. The damage due to each load-type acting individually is determined and then superposed to assess the overall effective fatigue life of the joint. As a first order approximation, Palmgren-Miner's linear superposition rule is utilized. TUBES,
TRANSISTORS
AND
ICs
Maintainab. Symp., 326 (1991). The statement of a warranty requires knowledge of the life distribution of the product under consideration and a cost model based on a stochastic process, e.g., a renewal process. This process allows one to look at the cost over a time period from both the buyer's and manufacturer's viewpoints for a particular warranty statement. For a manufacturer, the models can be used to make decisions such as warranty duration and length of a free replacement period. For a buyer, the models can be used to make decisions based on life cycle cost of an item or a fleet. Based on their generality and/or current usage, two types of warranties were examined in detail. The first was the combination policy which has an initial free replacement period followed by a pro rata period during which the replacement item's cost is calculated on a sliding scale. This warranty is used for items which may have early field failures and then gradually wear out. The other scenario is the fleet warranty which guarantees a purchaser of a large quantity of like items an average field performance. The fleet warranty has been given little discussion in the literature. To our knowledge, some of the fleet warranty results appear here for the first time. The combination policy and fleet warranty were evaluated for a non-repairable module within a commercial product that was being sold directly to the marketplace and to a system integrator. In this particular example, the fleet warranty had significant advantages to the manufacturer over the combination policy. The fleet warranty concept merits additional study.
Four decades of reliability experience. RALPH E. KUEHr~. Proc. A. Reliab. Maintainab. Symp., 76 (1991). Reliability experiences affecting progress in the reliability art over the past four decades are discussed. Specific progress has been demonstrated in design, parts, testing, contracting, and management attitude.
Progress in international standardization of verification and evaluation procedures. JAN L. RxSE. Proc. A. Reliab. Maintainab. Symp., 468 (1991). The paper presents a structure of procedures for estimation, compliance and comparison of reliability characteristics. Repaired and non-repaired items are treated separately. For non-repaired items, the procedures address the Weibull, normal, lognormal and exponential (constant failure rate) distributions including goodness-of-fit tests. For repaired items, the procedures cover tests for trends (constant failure intensity) and trend analysis modelling based on the power law model. Related to the above structure, an overview of existing international standards and those in progress is given. A number of IEC publications are currently being revised to cover both failure rate (non-repaired items) and failure intensity (repaired items). A powerful tool-box for the demonstration and testing of reliability characteristics is currently being devel. oped. The tools and the corresponding application guides are being published as the generic International Standard IEC 300.
OF
COMPONENTS,
Incorporating the effects of time estimation into humanreliability analysis for high-risk situations. JOESEPH SHARIT and DAVID M. MALON. IEEE Trans. Reliab. 40(2), 247 (1991). This paper provides a modeling framework for predicting the effects of discrepancies between subjective and objective measures of elapsed time on performance of human-machine interaction tasks in high-risk situations. Following a review of the major theories and experimental research findings in time estimation, the importance of time estimation in terms of its potential for affecting human performance in critical human-machine system operations is discussed. The formulations are primarily motivated by limitations in existing methods of human-reliability analysis that adjust human-error probabilities, often severely, on the basis of stress inferred from the ratio of estimated time available for task completion to time required. Our approach for dealing with time-related task demands is to describe the ongoing dynamic processes during task performance that are associated with temporal estimation, and to assess their effects on human reliability. This is accomplished through a conceptual framework that is depicted and discussed in terms of several task features and information-processing mechanisms, and that implicitly recognizes the experimental evidence concerning human time-estimation performance. This concept is illustrated through special cases that are formulated and analyzed in terms of their potential for contributing to human error as a function of time-estimation considerations. New international standards on reliability growth. LARRYH. CROW. Proc. A. Reliab. Maintainab. Syrup. 478 (1991). This paper reviews two new international standards on reliability growth developed by the International Electrotechnical Commission (IEC) for world trade. The first IEC standard discussed is IEC 1014, "Programs for reliability growth", and the second IEC standard discussed is IEC 56(Central Office)150 "reliability growth models and estimation methods." IEC 1014 was issued as an international standard in 1989 and IEC 56(Central Ot~ce)150 is an improved draft international standard. This document has recently been reviewed and approved as an international standard and will be published in the near future. These standards cover the management and modeling aspects, respectively, of a reliability growth program. An overview of these documents is given together with a more detailed discussion of various reliability growth terminology, procedures, and models represented in these international standards.
Some thought experiments In reliability. PAUL GOTTFRmD. IEEE Trans. Reliab. 40(2) 131 (1991). Some basic misconceptions about the relationships between life distributions and physical processes keep recurring in the reliability literature. Unfortunately, it is not practical to conduct laboratory experiments that would discriminate decisively among competing concepts. However, it is possible to devise some thought experiments that can be helpful: they cannot tell us what is right, but they can tell us what cannot be right. Warranties: what are they? What do they really cost?. TOBY M. BERKE and NICHOLAS A. ZAINO JR. Proc. A. Reliab. MR 32/4--J
585
to evaluate the combined effects of simultaneous vibration and thermal cycling of solder joints. This combined loading situation is simulated by superposing the effects of the vibrational and thermal loads. The damage due to each load-type acting individually is determined and then superposed to assess the overall effective fatigue life of the joint. As a first order approximation, Palmgren-Miner's linear superposition rule is utilized. TUBES,
TRANSISTORS
AND
ICs
Maintainab. Symp., 326 (1991). The statement of a warranty requires knowledge of the life distribution of the product under consideration and a cost model based on a stochastic process, e.g., a renewal process. This process allows one to look at the cost over a time period from both the buyer's and manufacturer's viewpoints for a particular warranty statement. For a manufacturer, the models can be used to make decisions such as warranty duration and length of a free replacement period. For a buyer, the models can be used to make decisions based on life cycle cost of an item or a fleet. Based on their generality and/or current usage, two types of warranties were examined in detail. The first was the combination policy which has an initial free replacement period followed by a pro rata period during which the replacement item's cost is calculated on a sliding scale. This warranty is used for items which may have early field failures and then gradually wear out. The other scenario is the fleet warranty which guarantees a purchaser of a large quantity of like items an average field performance. The fleet warranty has been given little discussion in the literature. To our knowledge, some of the fleet warranty results appear here for the first time. The combination policy and fleet warranty were evaluated for a non-repairable module within a commercial product that was being sold directly to the marketplace and to a system integrator. In this particular example, the fleet warranty had significant advantages to the manufacturer over the combination policy. The fleet warranty concept merits additional study.
Four decades of reliability experience. RALPH E. KUEHr~. Proc. A. Reliab. Maintainab. Symp., 76 (1991). Reliability experiences affecting progress in the reliability art over the past four decades are discussed. Specific progress has been demonstrated in design, parts, testing, contracting, and management attitude.
Progress in international standardization of verification and evaluation procedures. JAN L. RxSE. Proc. A. Reliab. Maintainab. Symp., 468 (1991). The paper presents a structure of procedures for estimation, compliance and comparison of reliability characteristics. Repaired and non-repaired items are treated separately. For non-repaired items, the procedures address the Weibull, normal, lognormal and exponential (constant failure rate) distributions including goodness-of-fit tests. For repaired items, the procedures cover tests for trends (constant failure intensity) and trend analysis modelling based on the power law model. Related to the above structure, an overview of existing international standards and those in progress is given. A number of IEC publications are currently being revised to cover both failure rate (non-repaired items) and failure intensity (repaired items). A powerful tool-box for the demonstration and testing of reliability characteristics is currently being devel. oped. The tools and the corresponding application guides are being published as the generic International Standard IEC 300.