Outline of IEC62506, International Standard for Accelerated Reliability Testing and Key Points. Fumiaki Harada

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1 Outline of IEC62506, International Standard for Accelerated Reliability Testing and Key Points Abstract Fumiaki Harada Fuji Xerox Advanced Technology Co., Ltd. Accelerated testing is performed under severer conditions than specified condition and failure mechanism, therefore, sufficient attention is needed in result interpretation. IEC 62506, the international standard of accelerated reliability test was published in June 2013 due to necessity of common recognition of reliability measurement techniques and sufficient understanding of risks and limitations in accelerated testing. In recent years, accelerated testing is utilized not only to estimate the product life for short-term development but also as a technique as represented by highly accelerated limit test (HALT) to identify latent failure. Therefore, appropriate understanding of techniques is required to determine test conditions and judge the results, and systematic operation is also needed in the development phase. In this chapter, as a member involved in its establishment I will introduce the background of the establishment of this standard and its outline. The contents of this report is a modified version of the report at "the 23rd Spring Reliability Symposium of the Reliability Engineering Association of Japan" held in June Keywords: international standard, reliability, accelerated test, IEC 62506, HALT 1. Outline Reliability is a quality characteristic with time dependent as an element; it mean that the requirements need to meet during the expected period. This property is common for maintainability and safety; in recent years, products are required to perform the requirements throughout product lifecycle as a dependability requirements. Because of these background, techniques to identify latent defects and to verify design in a short time are increasingly needed. Accelerated reliability testing is a typical methodology, and its international standard IEC has been published in June The background of this standard, there was an international demand to guide the recognition of accelerated testing as a reliability technique for its understanding and application. To apply the accelerated testing to product development, limitations and risk of accelerated testing should recognized to avoid misunderstanding and over expectation. Also, high reliability and complexity of products make it difficult to detect failure, so new methodologies, which identify failure phenomena under the severer conditions, such as HALT, may be needed. Meanwhile, accelerating product life and failure detection may one of the design activities, and completely different from product guarantee. The technique is needed to be recognized as a technique to develop new products in an efficient manner. 2. Accelerated Testing Accelerated test is defined in IEC IEV as follows: Test in which the stress level, or rate of stress application, exceeds that occurring under specified operational conditions, to reduce the duration required to produce a stress response. In response, it will be defined as follows in JISZ8115 Glossary of terms used in dependability currently under revision: Test in which the stress level, or rate of stress application, exceeds that occurring under specified operational conditions, to reduce the duration required to produce a stress response. Note 1. Example: Test to determine heat-induced or mechanical fatigue life of parts Note 2. Basic failure mode, failure mechanism, or relative relationship of these must not be changed in the test. The significant point is that setting of stress level exceeding the specified conditions, basic fault mode, Test Navi Report No. 24 (96th Issue) 1

2 fault mechanism, and relative relationship of these shall be constant. In fact, the latter will be revealed in the analysis after the test, but all conditions require accumulation of information on failure and analysis techniques. Reliability is a time-dependent quality characteristic, so a long-term test is required to verify or validate. In reliability, classic improvement process is known as TAAF (Test, Analysis, and Fix), this means the procedures to identify latent failure with testing and estimate and improve the probability. This is still an effective measure today, as reliability prediction is emphasized in handling failures which are unknown events. 3. Roles and problem of accelerated test The methodologies to reduce test term and to promote reliability improvement in early stage, various types of reliability test, such as fixed number test and sequential testing, may be planned. These tests are focused on statistical properties of the number of failures and test the failure rate of the population, not association between failure and stress. Therefore, a fixed conditioned test may be used to judge with result of pass/failure, and confirm the reliability improvement efficiently. These tests may be performed under specific conditions which based clear association between known stress and failure or degradation in specific items. Reliability testing originally aims to identify a clue to reliability improvement from test results, so and primarily has the following roles: Appropriateness determination of item failure rate Estimation of item failure rate and life Confirmation of design allowance for specific stress Detection and revelation of latent failure As products become highly reliable, failure is difficult to appear under predetermined conditions, so interest in accelerated testing, which conducted under more strict stress conditions than standard or specified conditions is natural consideration. Accelerated testing is originally to estimate failure distribution based on the mechanism of failure occurrence; however, in recent years, a method to promote failure occurrence under very strict stress conditions such as HALT is also utilized. As a background, management demands development activities to proactively detect failure to improve reliability, as there is no room to wait for failure occurrence to secure reliability in a short time. In order to achieve reliability efficiently, not only failures identification but also quick verification is important to ensure the reliability. To estimate the failure rate and confirmation of design allowance, it is effective to use failure experience and knowledge based information of stress and failure mechanism in the company. Also, it is necessary to detect and improve failures in the early phase of development, and in the late phase design allowance verification and confirmation of recurrence prevention based on past failure mechanism may be required. Accelerated testing is positioned as a measure to collect information for such activity. As a technique, conducting accelerating tests systematically is important to achieve reliability in a short time, so, it is necessary to operate accelerating test with sufficient recognition of the test results and the risk and limitations. And it is also necessary to choose an appropriate model, analyze data according to the objective, data collection, and preparation of test sample and analytical devices. Since accelerated testing is conducted based on unique recognition and decision on each companies, but needs for common recognition of the technique of accelerated testing is increasing globally, and led to the establishment of this standard. 4. Study background NWI of IEC was proposed in 2011 from the United States in 2011, and a project team was founded with participation of experts from each country s National committee and started draft preparation and discussion. In the past, accelerating test has performed to estimate the failure distribution under normal use conditions in a short time by strict condition, which is not changing failure mechanism. It required accumulated information of stress and failure occurrence mechanism and a knowledge base which can determine design basis concerning reliability. Meanwhile, in addition to development period reduction, there is an increasing interest in cases where products are used under unexpected conditions and effect of failure occurrence. As a result, it is necessary to identify latent failure and alleviate its effect. Qualitative accelerated testing represented by HALT to be described later is a representative method Test Navi Report No. 24 (96th Issue) 2

3 for it and utilized in the development of many new products. Under such circumstances, accelerating test is used as a collective term of testing under strict stress conditions, but it is not standardized as a technique, which include applied phase, objectives and limitations. As a result, it is undeniable that the contents and results of accelerated testing are interpreted inappropriately. For the unification of the concept of accelerated testing, this standard describes classification of accelerating testing to guide the characteristics, objectives, models to be applied and their interpretation, timing of testing and limitations. Also, the concept of accelerated testing is expanded from the existing one in this standard, and accelerated testing is positioned to have a role to proactively extract failure possibility. Although there is a trend of covering HALT and highly accelerated stress test (HAST) in a disproportionate manner, a chapter is allocated for these qualitative accelerated tests as Type A accelerated tests, clarifying the role of accelerated tests in product development. Testing methods such as HALT are not fully penetrated yet, but many of clients request for testing using those methods and many suppliers use those tests as the basis of quality assurance, which enabled common understanding. Although the draft version stated that this standard may be applied to hardware and software, it is actually mainly applicable to hardware items and not inclusive of acceleration testing of software only. In the final stage, a correction was made to "items including these." The discussion of the committee draft for vote (CDV) was conducted in the Sydney meeting in Normally, around 100 comments are deliberated for CDV; however, there were over 400 comments for this standard, indicating high interest in accelerated test as well as actual state of operations based on the ideas of each country. The discussion was held by going back to the reasons for the necessity of accelerated testing, which made us re-recognize that there is little room to improve products while validating reliability after development completion in recent years. In addition, "discovery and mitigation" of failures is confirmed as a concept of accelerated tests to ensure reliability in development stage. Term of mitigation should be here included elimination of the cause of failure. Therefore, considering effective testing for "discovery & mitigation", in addition to measurement and analysis technology, the following technology will be needed: Coverage of wide-ranging and many types of stress Stress control with high accuracy Application of high level of stress in a stable manner Combination of multiple types of stress Real-time monitoring of the history leading to failure Highly accurate, highly robust testing equipment As many of these are already put into practical use, this standard provides guidance on the premise that these technologies are provided. 5. Outline of the standard and important points The structure of the standard is as follows. FOREWORD INTRODUCTION 1 Scope 2 Normative references 3 Terms and definitions 4 Symbols and abbreviated terms 5 Basis and general description of theaccelerated test methods 6 Accelerated test models 7 Acceleration testing strategy in product development Annex A F The reason for the necessity of this standard is summarized in the INTRODUCTION section. Many reliability test methods, such as fixed duration, sequential probability ratio, reliability growth etc, are usually lengthy, so accelerated testing is required to reduce product cost as well as development time. These explanations may be common issue for product development, however, short-term evaluation through failure mode detection and effect reduction and reliability growth are the background of accelerated testing is simple and easy to understand. As approaches to improvement by accelerated testing, the following 2 types are introduced: Verification of no failure possibility during a required period Estimation of the level and volume of failure during a certain period Test Navi Report No. 24 (96th Issue) 3

4 These are exactly the basic function of accelerated testing. In this standard, qualitative accelerated testing has been positioned as a technical approach which improve by identifying failure possibility and existing accelerated testing as quantitative accelerated testing to estimate failure distribution. Qualitative accelerated testing is conducted under strict stress conditions which products may encounter during product operational period to facilitate reliability improvement by taking measures against failures occurred, regardless of specifications. Existing accelerated testing is conducted on the premise that failure mechanisms are the same, whereas qualitative accelerated testing is focused on precipitation of a failure event itself. Therefore, determining types and levels of stress is important in qualitative accelerated testing, but its method is not mentioned in this standard. In the discussion stage, there was a description that temperature/humidity, vibration, and voltage account for 70 to 80% of stress, but it was deleted as a common belief. Also, although it is very important to analyze failure which occurred in qualitative accelerated test, there is no detailed explanation on failure analysis itself. These points may be considered to be issues in this standard. Description of quantitative accelerated testing has classified to 3 types; stress acceleration, time acceleration, and movement acceleration. Since the equivalency of failure mechanism is a precondition in these tests, failure analysis and know-how accumulation are required to understand the relationship between failure and stress, but that point is not particularly mentioned in the standard. Also, the standard introduces several physics of failure(pof) models as techniques to improve dependability, but does not guide the operations. As described in the Scope, this standard also covers product screening techniques, thus incorporates highly-accelerated stress screening (HASS). On this point, an objection comment was raised from Japan to the effect that accelerated tests and screening should be separated; however, it was remained unchanged due to similarity in test conditions and the objective to identify failure in a short time. Similarly, additional explanation of judgment acceleration using deterioration trend was proposed considering the effectiveness to accumulate knowledge on failure mechanism, however it was rejected for the reason that it is a prediction or judgment method, not a test method. 6. Standard structure The standard is comprised of general descriptions on accelerated tests, the aims and techniques of accelerated tests classified into types A to C, and the strategy of accelerated tests in product development; with specific examples consolidated in the Annex. As shown in Figure 1, the general descriptions provide explanation that accelerated tests are based on the principle that damage accumulates. This explanation gives an impression that only durability models are covered, but what it means is enough stress to cause failure, therefore, limit models are also covered. This standard explains the necessity that in order to assure reliability, accelerated testing is required to obtain information that there is sufficient allowance between product destruct limit and product design specifications. The idea is that enough strength needs to be secured to prevent failure within product life cycle. Figure 1 shows relative relationship of the design specification level by associating upper and lower limits of the destruct level, operating level, and reliability test level. Excerpt from IEC Figure 1. Virtual test conditions and use conditions Figure 1 is a little rough conceptual diagram where absolute values are simply indicated for all stress, operational limit, destruct level, thus respective failure models should be applied in actual application. The following are the 3 types of accelerated test methods and the characteristics of each type. Type A: Acceleration to detect phenomena and failure mode in qualitative accelerated testing Type B: Acceleration of product life in general in quantitative acceleration testing based on failure mechanism Test Navi Report No. 24 (96th Issue) 4

5 Type C: Time acceleration and movement acceleration in quantitative accelerated testing to predict failure distribution in use conditions As mentioned earlier, type A accelerated tests, which detect latent failure and improve reliability, aim to set strict conditions as described in the Scope section and perform discovery & mitigation. Although the explanation of HALT takes up a large space, pressure cooker test (PCT), some of the standard tests, and tests under specified stress conditions such as placement in a high temperature environment can be considered to be this type A accelerated tests. Types B and C accelerated tests aim to estimate failure distribution in short-term, normal use conditions. Type B tests are conducted on the premise of known failure mechanism such as physical failure model and generally used for life acceleration, whereas type C is movement acceleration or time acceleration test under known conditions, including failure rate acceleration. This description is the same as the traditional understanding of accelerated testing in Japan. However, considering the fact that these accelerated tests are conducted as kinds of conformance tests, it is useful to guide the characteristics by type and roles for reliability improvement. There are ideal development phases to which each type of accelerated tests (qualitative accelerated tests and quantitative accelerated tests) should be applied. (Table 1). This also indicates that ensuring reliability in a short time is the original aim of accelerated tests. Table 1. Test types and utilization phase Chapter 5 onward, which serves as a guide for the test methods of types A, B, and C, briefly summarizes the characteristics of each type of tests and each model and strong points. Technically, the described models are general models as physical failure models and nothing new, thus it is an easy-to-understand guide of methods. By using these, it is important to make good use of this standard in the upstream of development. 7. Significance of this standard and its use IEC 62506, which contains many descriptions about the meaning of accelerated testing and concept in details, also serves as a textbook in a way. Therefore, accelerated test methods can be chosen like choosing from a tool box, but implementation methods are not guided in a simple manner; thus we cannot say that sufficient explanation is provided on how to use models in Chapter 5 and how HASS, etc., should be incorporated. Also, the explanation has many inconsistencies; for example, stress is expressed with both level and magnitude. On the other hand, the strategy in accelerated testing in Chapter 6 describes what should be used as a reference in the planning stage of reliability management in an organized manner, briefly summarizing important points in each type and limitations in overall accelerated testing to make use of features and weaknesses of types A to C and plan, execution, and results of accelerated testing. Although it is difficult to judge the appropriateness of test conditions and result interpretation in accelerated testing without understanding and knowledge of specific technical background, the descriptions on overall precautions and limitations are considered to be very effective. Accelerated tests are often performed by experimental rule or unique recognition, and the procedures to confirm the appropriateness of models such as the Power low model for electronic devices are obscured in several cases. Considering this point, it is significant to have a common recognition of the role of accelerated testing and its limitations. Also, qualitative accelerated testing described in this standard is a relatively unfamiliar methodology in Japan. This methodology has an underlying idea that handling excessive stress-induced failure phenomena which do not occur in the actual market does not contribute to reliability improvement. It is true that activities with a focus on the physical relationship between stress and failure facilitated the advancement of failure mechanism analysis, contributing to the establishment of highly reliability technical strength. Also, in Japan, reliability testing itself is particularly viewed as conformance testing and people tend to be interested in its conformance or Test Navi Report No. 24 (96th Issue) 5

6 nonconformance, thus tests under unrealistically strict stress conditions tend to be alienated. Meanwhile, in complex and/or large-scale products and products whose high reliability is accomplished, the effect of failure may be significant, and in addition, compliance of the intended use in specified condition, and prevention of use exceeding product life or unexpected use are difficult; therefore, reduction of the failure effect is requested when failure is precipitated. Naturally, in products using new technology, identification of latent failure phenomena is very important role in reliability design and development. It is considered that qualitative accelerated tests not only precipitate weaknesses in early development stages but also are increasingly becoming useful in risk management. Accelerating testing is an important method to obtain information to achieve reliability. As this standard was organized, the position of accelerated testing as a method of reliability improvement was clarified. Since securing reliability is an issue in short-term development, it is desired that the standard will be utilized in various fields as a guide for understanding reliability accelerated tests and improvement methods in the future. References [1] IEC Methods for product accelerated testing 2013, 6 [2] IEC 61710, Power law model Goodness-of-fit tests and estimation methods [3] New version "Reliability Handbook" P.297, Accelerated test method [4] JISZ8115:2000 Reliability and Maintainability Terms. Japanese Standards Association [5] Harada 2011 "Trend of international standardization concerning dependability," Symposium of the Institute of Electronics, Information and Communication Engineers [6] Harada et al "An Issue and Foresight for Accelerated Test in Reliability Engineering," the Journal of Reliability Engineering Association of Japan (Fumiaki Harada, Fuji Xerox Advanced Technology Co., Ltd.) Test Navi Report No. 24 (96th Issue) 6