Evaluation of AlGaInP LEDs reliability based on accelerated tests

High efficiency LEDs are replacing incandescent bulbs in many applications particularly those requiring durability and low power consumption. In some of these applications, as automotive and traffic signals, LEDs work in outdoor environment and in some cases in extreme temperature and humidity conditions. Main failure mechanisms have been identified for the different stress conditions (temperature/humidity accelerated tests). MTTF of 127,000 h have been evaluated for real working conditions by means of applying Arrhenius and Peck model values obtained in this paper.     

Comparative analysis of RF LDMOS capacitance reliability under accelerated ageing tests

This paper presents the results of comparative reliability study of C–V characteristics through three accelerated ageing tests for stress applied to an RF LDMOS: Thermal shock tests (TST, air–air test), thermal cycling tests (TCT, air–air test) and high temperature storage life (HTSL). The two first tests are carried out with a drain current flowing through the device during stress. The investigation findings of electrical parameter degradations after various ageing tests are discussed. Feedback capacitance (Crs) is reduced by 16% and gate–drain capacitance (Cgd) by 42%. This means that the tracking of these parameters enables to consider the hot carrier injection as the dominant degradation phenomenon. A physical simulation software has been used to confirm qualitatively degradation phenomena.     

Thermomechanical reliability characterization of a handheld product in accelerated tests and use environment

Thermomechanical reliability of electronics has commonly been studied by employing accelerated temperature cycling (ATC) tests. However, due to the localized heat dissipation in modern electronic devices, operational power cycling (OPC) is considered a more realistic testing alternative. In order to characterize the thermomechanical reliability of modern high-density electronics, the failure modes, mechanisms and lifetimes of a contemporary commercial handheld device were studied under the ATC and OPC conditions.The experimental measurements and finite element analysis (FEA) showed distinct differences in the thermomechanical response of the device component boards under the OPC and ATC conditions. The results from FEA showed that the interconnection deformations during the OPC test were mostly in the elastic region of the solder, whereas those during the ATC tests reached well into the plastic region. The inclusion of the product enclosure further emphasized this difference, as the enclosure restricted the thermal expansion of the component board during OPC testing. The experimental test results were consistent with the FEA results, as the device failed due to solder interconnection cracking under the ATC conditions within 18 days of testing, but those under the OPC conditions remained operational even after 460 days.Finally, FEA estimations suggest that even three times higher power dissipation levels compared to those found in contemporary handheld devices would result in many years of lifetime in OPC testing.     

Shock impact reliability characterization of a handheld product in accelerated tests and use environment

The effect of mechanical shock impacts is a key factor in the reliability of modern handheld products. Due to differences in product enclosures, impact orientations, strike surfaces and mountings of component boards, the loading conditions induced in a true product drop differ from those encountered in standardized board-level tests. In order to better understand the correlation between board-level drop testing and actual drops of a complete device, series of board and product-level drop tests were conducted using specialized test boards.The mechanical shock impact response of the commercial handheld device component board was characterized with the help of acoustic excitation laser vibrometry and finite element analysis. The results were used to design the mechanically compatible specialized test board for both 4-point supported board-level and unsupported product-level drop tests. Special care was taken to ensure that the vibration behavior of the test board accurately represented the vibration behavior of the commercial component board. Additional board-level drop tests were conducted using a JEDEC JESD22-B111 compliant component board for comparison.The drop test results showed that, even though the test board design and supporting method have a marked influence on the strain conditions and lifetime of solder interconnections, the primary failure mode and mechanism under the product-level drop tests is comparable to that typically encountered in the standard JEDEC JESD22-B111 board-level drop tests. More detailed analyses suggest that the comparability of the shock impact loading conditions affecting solder interconnections can be characterized using three metrics: (1) the maximum component board strain rate, (2) the maximum board strain amplitude and (3) the damping of the component board.     

Moisture absorption by molding compounds under extreme conditions: Impact on accelerated reliability tests

The Highly Accelerated Stress Test (HAST) [130 °C/85%RH or 110 °C/85%RH with applied bias voltage] or Autoclave test [121 °C/100%RH] are well-accepted tests during qualification and reliability testing of automotive microelectronics. These tests are often preferred since results can be obtained relatively fast due to the high acceleration. On the other hand, there is a risk of accelerating non-relevant failure modes e.g. due to changes in material behavior under test conditions, which will not occur during application. However, predicting what happens under test conditions is not straightforward since one needs to understand material behavior under such extreme conditions. Especially the amount of water absorption, which is highly relevant for test result interpretation, is challenging to obtain at temperatures above 100 °C, since here we deal with pressurized conditions.In this paper a novel, and straightforward, method is proposed to determine the moisture absorption under pressurized conditions. To verify the method, data obtained under pressurized conditions is compared to behavior under standard conditions. Under autoclave conditions it is shown that the moisture uptake at higher temperatures is much higher than predicted by extrapolation. The reported data can also be used to predict more reliably the moisture saturation level, and rate of moisture saturation during HAST.     

Optimal design for step-stress accelerated degradation tests

Today, many products are designed to function for a long period of time before they fail. For such highly-reliable products, collecting accelerated degradation test (ADT) data can provide useful reliability information. However, it usually requires a moderate sample size to implement an ADT. Hence, ADT is not applicable for assessing the lifetime distribution of a newly developed or very expensive product which only has a few available test units on hand. Recently, a step-stress ADT (SSADT) has been suggested in the literature to overcome the above difficulty. However, in designing an efficient SSADT experiment, the issue about how to choose the optimal settings of variables was not discussed, such as sample size, measurement frequency, and termination time. In this study, we first use a stochastic diffusion process to model a typical SSADT problem. Next, under the constraint that the total experimental cost does not exceed a predetermined budget, the optimal settings of these variables are obtained by minimizing the asymptotic variance of the estimated 100p/sup th/ percentile of the product's lifetime distribution. Finally, an example is used to illustrate the proposed method.     

Power cycling tests for accelerated ageing of ultracapacitors

In this paper, a test methodology is presented and evaluated in order to quantify the lifetime of ultracapacitors in case of active power cycling. The test profiles are specified for taking into account both the high charge-discharge levels and the periodicity of typical HEV applications. Before cycling, preliminary tests have been done with Maxwell BCap 2600F devices and have proved that pulsed current profiles can be predetermined in order to induce a given self-heating. Then, during power cycling, periodic characterization tests have been done in order to follow the evolution of cell parameters. The results at 25,000 cycles illustrate that impedance real part and capacitance are significant parameters for ageing quantification. Furthermore, the trend of these results shows that the self-heating acts as an accelerating factor for ageing.     

Test fixture for MESFET reliability life tests

DC tests may be the method on which to found an unitary methodology for setting and analysis of GaAs MESFET reliaiblity studies. Each failure mechanism must be stressed without introducing erroneous one, due to the artificial conditions of accelerated tests. The performance of high temperature life tests needs proper aging facility because these devices are prone to oscillate in a wide range of frequencies and can easily be damaged by electrical spikes. A test fixture for these tests must enable the highest control of aging conditions. This demand for ‘safe tests’ induced us to develop a specifically designed test fixture for a general use with devices of different powers and manufacturers.     

Evaluation of the reliability of high concentrator GaAs solar cells by means of temperature accelerated aging tests

Evaluating the reliability, warranty period, and power degradation of high concentration solar cells is crucial to introducing this new technology to the market. The reliability of high concentration GaAs solar cells, as measured in temperature accelerated life tests, is described in this paper. GaAs cells were tested under high thermal accelerated conditions that emulated operation under 700 or 1050 suns over a period exceeding 10 000 h. Progressive power degradation was observed, although no catastrophic failures occurred. An Arrhenius activation energy of 1.02 eV was determined from these tests. The solar cell reliability [R(t)] under working conditions of 65°C was evaluated for different failure limits (1–10% power loss). From this reliability function, the mean time to failure and the warranty time were evaluated. Solar cell temperature appeared to be the primary determinant of reliability and warranty period, with concentration being the secondary determinant. A 30‐year warranty for these 1 mm²‐sized GaAs cells (manufactured according to a light emitting diode‐like approach) may be offered for both cell concentrations (700 and 1050 suns) if the solar cell is operated at a working temperature of 65°C. Copyright © 2012 John Wiley & Sons, Ltd.     

A multiple objective framework for planning accelerated life tests

We consider a framework with two conflicting objectives in the planning of ALT; i.e., meeting a desired level of statistical precision for an estimate of interest, and meeting a cost target for conducting the test. This can easily be generalized to multiple objectives with, say, precision targets for various estimates, stress limits constraint, sample size constraint, etc. In the absence of actual cost, the cost of conducting the test is related to the total test time at different stress levels, and the total sample size needed for the test. The framework is applied to planning of constant stress ALT, in which the logarithm of the lifetime follows the extreme value distribution. The proposed framework consistently leads to more cost-effective test plans without compromising the precision of the estimate, and is equally robust to misspecification of initial estimates, compared with other test plans presented in the literature.     

Bayesian inference for partially accelerated life tests using Gibbs sampling

We consider a life testing situation in which several groups of items are put, at different instances, on the partially accelerated life test proposed by DeGroot and Goel [Naval Research Logistics Quarterly, 1979. 26, 223–235]. The combined failure time data are then used to derive empirical Bayes estimators for the failure of the exponential life length under normal conditions. The estimation which is implemented using the Gibbs sampler Monte-Carlo-based approach, illustrates once again the ease with which these new types of estimation problems often requiring sophisticated numerical or analytical expertise, can be handled using the sampling based approach.     

Design of accelerated corrosion tests for electronic components inautomotive applications

Two new accelerated laboratory corrosion tests for electronic components in automotive applications have been developed, based on the use of metallic copper as a meter for corrosivity. The accelerated tests are designed so that they reproduce the same kind of corrosion effects as observed with exposure of copper in real vehicle environments. The test cycle that best simulates the corrosion characteristics of an engine compartment is composed of the following steps: a) exposure to neutral salt spray for 2 h; b) drying at 23°C and 50% RH for 22 h; c) exposure to a test atmosphere of 1.5 ppm NO₂ and 0.5 ppm SO₂ at 25°C and 95% RH for five days; d) drying at 23°C and 50% RH for one day. The predominating corrosion products of copper, formed both during service exposure and in the accelerated test, are Cu₂O and Cu₂Cl(OH)₃, with a small amount of sulphur-containing corrosion products in the form of sulphates. In terms of corrosivity of copper, a test duration of six weeks corresponds to 7.5 years of exposure in the reference engine compartment selected for this study. The test cycle designed for the passenger compartment contains the steps: a) exposure for five days to a test atmosphere of 10 ppm NO₂ and 95% RH at 35°C, followed by b) drying at 23°C and 50% RH for two days. In this case, Cu₂O is the predominating corrosion product. In terms of corrosivity of copper, a test duration of four weeks corresponds to 7.5 years of exposure in the reference passenger compartment of this study     

Boundary walking test: an accelerated scan method for greatersystem reliability

Printed circuit board (PCB) interconnect test and reliability is addressed. The boundary scan test methodology proposed in the IEEE standard 1149.1 is reviewed, and its limitation is critically analyzed. Based on this, a technique is proposed to automate the interconnect wiring test, which is performed as part of the power-on self-test. Essential to the idea is the use of a walking sequence for test stimulus, and response compression by the multiple input signature registers (MISRs). The salient feature is its simplicity of operation. Unlike the existing boundary-scan methodology, interconnects are tested via on-site test generation; faults are detected by comparing the content of the MISR with the anticipated response. Formal analysis shows that the technique has a high test-coverage for the most common defects. As a result, PCB interconnect testing can be accomplished as part of the power-on self-test without external test fixtures     

基于加速寿命试验的SLD可靠性预计模型研究

超辐射发光二极管(SLD)已广泛应用于航空、航天等多个领域,但其预计模型的缺失使得SLD的可靠性分析工作缺乏有效指导。本文基于SLD主要失效模式、失效机理以及典型诱发应力,构建SLD可靠性预计模型,开展加速寿命试验,利用性能退化可靠性评估技术、图估计、最优线性无偏估计等方法对试验数据进行分析处理,确定管芯预计模型参数,应用国内外已有耦合、热阻及制冷器可靠性预计技术,确定管芯耦合及组件预计模型系数的表征,从而完成SLD可靠性预计模型的建立,为SLD工程应用过程中的可靠性分析工作提供技术参考。     

Reliability estimation of aeronautic component by accelerated tests

This article presents a reliability assessment of aeronautic electronic board using the accelerated test approach. The mission profile used is one defined in FIDES for electronic equipment in aircraft. The reliability function is defined taking into account the different phases of product life (Ground, Taxiing, Flying, Dormant). FIDES describes the environment (thermal and humidity, thermal cycling, mechanical, …) for each phase. The paper presents the different accelerated life models used to take into account all environment. A test plan is proposed to estimate the unknown parameters of accelerated life models. The thermal and humidity and thermal cycling tests have been performed and the data are analyzed. Finally, the reliability function is evaluated.     

Optimum simple step-stress accelerated life tests with censoring

The authors present the optimum simple time-step and failure-step stress accelerated life tests for the case where a prespecified censoring time is involved. An exponential life distribution with a mean that is a log-linear function of stress, and a cumulative exposure model are assumed. The authors obtain the optimum test plans to minimize the asymptotic variance of the maximum-likelihood estimator of the mean life at a design stress. Nomographs for the optimum time-step stress test are also given. Simple step stress test plans with censoring were studied. The optimum failure-step stress test plans are obtained in a closed form, whereas, for the time-step stress test, nomographs are given for finding the optimum plans on the basis of parameters. The parameters must be approximated from experience, similar data, or a preliminary test. For some selected values of the parameters, the effect of incorrect preestimates was studied in terms of the percentage of variance increase. The effect was found to be small     

Mean-time-to-failure evaluations of encapsulation materials for LED package in accelerated thermal tests

The mean-time-to-failure (MTTF) evaluation of encapsulation materials of LED package in accelerated thermal tests is presented. The Weibull and Arrhenius theory was used to characterize the thermal degradations of encapsulation materials for LED package in a quantitative investigation. The shape parameter, scale parameter, MTTF and activation energy (E_a) could be obtained to potentially provide clear values for comparison and understanding. The results showed that the glass as encapsulation material of LED modules exhibited better thermal stability than the silicone encapsulation by about 7 times in lumen loss, 2 times in chromaticity shift, and 3 times in correlated color temperature at 150 °C. Furthermore, an order magnitude MTTF improvement at room temperature was achieved in the glass encapsulation case.     

A novel accelerated test technique for assessment of mechanical reliability of solder interconnects

New generations of lead-free solder interconnects are widely used in consumer electronics. Reliability of the devices which are subjected to rough handling, depends on the fracture resistance of the solder interconnects to shock and mechanical loading. The conventional reliability testing procedures are reported to be expensive and time consuming. Thus alternative tests and evaluation methods for reliability assessment of solder joints are required. In this study a new method for quality assessment of solder interconnects under high strain vibrational shear loading is presented using an ultrasonic fatigue testing system in combination with a special experimental set-up. Using this technique lifetime curves for solder ball bonds of two different Sn–Ag–Cu lead-free alloys were obtained. Failure mechanisms of the solder ball bonds were studied using SEM methods and the reliability curves were discussed with regard to the failure modes and the composition of the lead-free alloys. The applicability of the proposed method is discussed with regard to the literature data.