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Chapter 10 Reliability Testing. Topics. Integrated Test Programs Reliability Testing Reliability Qualification Test/ Reliability Demonstration Test Reliability Growth Test Production Reliability Acceptance Test (PRAT) Accelerated Test. Big Picture of Reliability Engineering. 3.
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Topics • Integrated Test Programs • Reliability Testing • Reliability Qualification Test/ Reliability Demonstration Test • Reliability Growth Test • Production Reliability Acceptance Test (PRAT) • Accelerated Test
Integrated Test Program To confirm that the design meets the basic performance requirements Functional Testing To ensure that the design is capable of operating under the expected range of environments Environmental Testing Test Program To ensure that the product will operate without failure during its expected life Reliability Testing When appropriate Safety Testing
Test Strategy • Test strategy is the established strategic plan for cost effectively performing test measurements that add value to a particular product for its customers. • A test strategy is intended to verify the achievement of product goals, determine shortcomings needing corrective action, and identify opportunities for improvement in an efficient and cost effective manner.
A product-specific test strategy is needed to assure adequate confidence in reliability performance, and to avoid unnecessary expenses resulting from excessive scrap or non-value added tasks resulting from inappropriate tests. • Test strategy typically includes all testing done on a product, but this discussion will be limited to reliability test strategy.
Types of Reliability Testing • Reliability Qualification and Demonstration Test (RQT/RDT) • Reliability Growth Test (RGT) • Production Reliability Acceptance Test (PRAT) • Accelerated Test
RQT/RDT • 신뢰성 보증시험: Reliability Qualification and Demonstration Test (RQT/RDT) • Purpose: RDT/RQT is to reach statistically valid decisions regarding whether an item is in compliance with its specified reliability or not. • RQT is often used to designate testing imposed by a customer for verifying compliance with contractual objectives. • MIL–HDBK–781, Reliability Test Methods, Plans and Environment for Engineering Development, Qualification and Production
Benefits: Requiring the product to pass a RDT means that products which pass the test may be considered to have achieved their specified reliability with an acceptable risk of error. This permits the manufacturer to market the product, or the customer to begin using it, with confidence that projected performance reliability and estimated replacement or repair costs will be met.
Timing: RDT/RQT should be run on the first product prototypes just prior to the Production/Manufacturing product phase.
Demonstrations of statistical Reliability characteristics are based on statistical considerations. • By assuming a particular distribution for the Reliability characteristic of interest, a statistical test plan can be formulated. • This enables the accept/reject criteria for agreed values of decision risks to be pre-determined, and stated precisely before testing starts.
There are two main types of statistical test plan: • Fixed time/failure terminated test plans: testing is continued until a pre-determined test time has been exceeded or a pre-determined number of failures has occurred (reject) • Sequential test plans: both test time and failures are compared with established criteria to decide whether to accept, reject or continue testing the item.
RGT • 신뢰성 성장시험: Reliability Growth Test (RGT) • Purpose: Reliability growth test is a test conducted specifically to measure improvements in reliability by finding and fixing deficiencies, which is the basis of a Test, Analyze and Fix (TAAF) program. • Benefit: RGT/TAAF can be used to prevent reliability problems on new products, and to improve existing products with inadequate reliability. Dedicated reliability growth tests can prevent the delivery of unsatisfactory products to the customer, saving repair/replacement costs and customer dissatisfaction.
RGT is intended to identify failure modes and mitigate their root causes, thereby growing the inherent design reliability. • They differ from demonstration and qualification tests in that their purpose is to gather information rather than to provide confidence about reliability. • MI-HDBK-189, Reliability Growth Management
Timing: Growth tests require prototype samples to test and time to formulate and implement changes based on the test results, so they should be considered in the latter stages of Design/Development. They should precede any qualification tests, which, if performed, should serve to demonstrate that the growth program was satisfactory.
Reliability Growth Model: • AMSAA Model • Duane Model • Gompertz Model • Llyod-Lipow Model
AMSAA Model • US Army Material Systems Analysis Activity • A single repairable system is considered operating from age zero, fails at age is restored to satisfactory function by a corrective action, operates again fails at age gets restored again to function and the process gets repeated. • Within a test phase, failures are occurring according to a nonhomogeneous Poisson process.
Rate of occurrence of failure (ROCOF) • Instantaneous mean time between failure • The number of failures, , accumulated on all test items in a fixed test time is a random variable following a Poisson distribution.
Example • A prototype of a system is tested with the incorporation of design changes. A total of 23 failures occurs during a cumulative test time of 22,000 hr. The data are given below. Determine AMSAA reliability growth model that represents the data.
PRAT • 신뢰성 수락시험: A Production Reliability Acceptance Test (PRAT) • Purpose: PRAT is performed to measure any degradation in the reliability of a product over the course of production or to assure that products being delivered meet customers' reliability requirements.
Benefit: When a product is available in the marketplace, any delay in finding a solution to a reliability problem results in a proportionate number of dissatisfied customers. This is usually costly, and can often be disastrous. PRAT is intended to minimize the impact of production reliability problems by providing timely warning and the data needed for corrective action.
Timing: PRAT only takes place during the Production/Manufacturing phase of the product life cycle. • The simplest approach to PRAT, assuming that a RQT has been performed, is to repeat the RQT at intervals during the production run.
Accelerated Test • Purpose: Accelerated life tests are intended to measure the life of a component by testing it at high stress levels and extrapolating the results to normal operating conditions. • Benefit: Using accelerated testing, it is possible to establish the life of the component within a practical test program. • MIL-STD-883, Test Method and Procedures for Microelectronics • MIL-STD-202: Test Methods for Electronic and Electrical Component Parts. Applied to capacitor, resistor, inductor, switch, relay.
Assumption: Since higher stresses are used, accelerated testing must be approached with caution to avoid introducing failure modes that will not be encountered in normal use. The same failure mechanism will dominate the failures at normal and accelerated conditions.
Timing: Accelerated testing is best done on prototypes of a part or component early in the Design/Development product phase to establish its life characteristic before a significant number are produced, permitting any necessary changes to be made without a large amount of scrap. After a product enters the Production/Manufacturing phase, accelerated testing can be used for lot acceptance testing of parts and components.
Accelerating factors used • More frequent power cycling • Higher vibration levels • High humidity • More severe temperature cycling • Higher temperatures
Two categories of accelerated testing • Accelerated Life Testing: for life estimation • Accelerated Stress Testing: for weakness identification and correction • HALT • HASS
Accelerated Life Test • ALT uses a model relating the reliability measured under high stress conditions to that which is expected under normal operation to determine lifetimes. • Accelerated Life: Increasing the stress on the system shortens the expected life of the system.
가속수명시험에서 시험제품에 스트레스를 인가하는 방법
Accelerated Life Test Model: • Arrhenius Model • Inverse Power Model • Coffin-Manson Model • Norris-Landzberg Model • Eyring Model
Arrhenius Model • Increasing temperature is one of the most commonly used methods to accelerate a failure mechanism. • The Arrhenius relationship is a widely used model to describe the effect that temperature has on the rate of a simple chemical reaction.
The Arrhenius model for reliability applications is as follows: • : product life at use temperature • : thermal constant factor • : activation energy for the failure (eV) • : Boltzmann’s constant (8.623X10-5 eV/K°) • product temperature at use (K°)
The preceding form of the Arrhenius Model is seldom used as is. • It is usually written in terms of an acceleration factor, which compares the more stressful test environment at elevated temperature with the customer end-use environment. • Testing at constant elevated ambient temperature can be extrapolated to estimate the equivalent time in the end-use environmentusing the acceleration factor.
Acceleration factor • product life at use temperature • product life at accelerated temperature • maximum temperature at use (K°) • maximum temperature in accelerated • environment (K°)
Example A transistor used at 85°C in the normal condition is tested at an elevated ambient test temperature of 125°C for 3 hours. The targeted failure has an activation energy of 0.7eV. Calculate the Arrhenius acceleration factor and the equivalent time in the customer end-use environment.
Testing in the more stressful accelerated environment for 3 hours at 125°C ambient is equivalent to 30 hours in the customer end-use environment of a nominal 85°C.
Accelerated Stress Test • AST uses accelerated environmental stresses to precipitate latent defects or design weakness into actual failures to identify design, part, or manufacturing process problems which could cause subsequent failures in the field. • Highly accelerated life test (HALT) • Highly accelerated stress screen (HASS) • Environmental Stress Screening