Laurent Bechou

Detection and location of defects in electronic devices by means of scanning ultrasonic microscopy and the wavelet transform

In a highly competitive market, reliable techniques for manufacturing quality control of electronic devices are more and more demanded. In particular, scanning ultrasonic microscopy is nowadays showing itself a suitable, non-destructive tool for detecting and locating defects in a die-attach assembly of a wide range of components. Due to multiple reflections and scattering of the ultrasonic beam in multilayer die-attach assemblies, ultrasonic images can often appear very confused to be correctly interpreted, and the nature of defects very difficult to be pointed out. In the paper, the use of a digital signal-processing method, based on the Continuous Wavelet Transform, is suggested for automatically detecting and measuring the time-of-flight between ultrasonic echoes thus improving ultrasonic image understanding of complex structures. Methods performance is verified by means of numerical tests on several ultrasonic signals simulating different operating conditions; its reliability and efficacy come out from experiments on the die-attach assembly of actual electronic devices.

Effects of silicone coating degradation on GaN MQW LEDs performances using physical and chemical analyses

This work presents a physics of failure (POF) methodology coupling failure signatures with physico-chemical analyses. The aim is to work out electro-optical failure signatures located in packaged InGaN/GaN Multiple Quantum Wells Light Emitting Diodes (MQW LEDs). Electrical and optical characteristics performed after accelerated ageing tests (30 mA/85 °C/1500 h), confirm a 65% drop of optical power and an increase of one decade of leakage current spreading at the silicone oil/chip interfaces. Through measurements of silicone coating fluorescence emission spectra, we demonstrate that the polymer enlarges the LED emission spectrum and shifts central wavelength. This shift is related to silicone oil spectral instability and the central wavelength of packaged LED appears to be temperature insensitive. In this paper, we discriminate the degradation of bulk silicone oil responsible for optical losses from the polymer/chip interface inducing larger leakage current.

Improved performances of polymer-based dielectric by using inorganic/organic core-shell nanoparticles

BaTiO3/hyperbranched polyester/methacrylate core-shell nanoparticles were studied by varying the shell thickness and the methacrylate ratio. We demonstrated that coalescence typically observed in traditional composites employing polymer matrices is significantly reduced. By modifying the shell thickness, the equivalent filler fraction was tuned from 7 wt. % to 41 wt. %. Obtained permittivities were compared with reported models for two-phase mixtures. The nonlinear behavior of the dielectric constant as a function of the equivalent filler fraction has been fitted with the Bruggeman equation. Methacrylate groups reduce by a decade the loss factor by improving nanoparticles adhesion. The permittivity reaching 85 at 1 kHz makes core-shell nanoparticles a promising material for embedded capacitors.

Early failure signatures of 1310 nm laser modules using electrical, optical and spectral measurements

Abstract In this paper, we propose a methodology to discriminate electro-optical failure signatures related to an optical alignment drift in the laser module in comparison of those related to the gradual change of electro-optical parameters of the laser diode. A specific test bench with temperature dependence has been developed to monitor P ( I ), I ( V ) and L ( λ ) of 15 DCBH InGaAsP/InP 1310-nm Fabry-Perot laser modules and has allowed to extract three main failure signatures after 300 thermal cycles. A study of experimental I ( V ) curves from reference laser modules has resulted in the development of a DC electrical circuit with a non-linear element tunnel diode D tunnel contributing to a better understanding of current distribution parallel paths inside the laser diode. After accelerated ageing tests, two signatures are related to an gradual change of both electrical and optical parameters conducting to identify failure mechanisms in the laser diode and discrimination of failures zone was improved by a reverse bias I ( V ) measurement. The third signature has shown an optical gradual change without any modification of the threshold current or ideality factor. We also demonstrate the strong interest of L ( λ ) measurements complementary to I ( V ) and P ( I ) characteristics.

Probabilistic design-for-reliability concept and novel approach to qualification testing of aerospace electronic products

Qualification testing (QT) is the major means to make a viable device into a reliable product. The short-term goal of a particular electronic device manufacturer is to conduct and pass the established QT, without questioning if they are adequate. The ultimate long-term goal of electronic industries, whether aerospace, military, or commercial, regardless of a particular manufacturer or a product, is to make their deliverables reliable in the actual operations. It is well known, however, that todays electronic devices that passed the existing QT often fail in the field (in operation conditions). Are the existing QT specifications adequate? Do electronic industries need new approaches to qualify their devices into products? Could the existing QT specifications and practices be improved to an extent that if the device passed the QT, there is a quantifiable way to assure that its performance will be satisfactory? At the same time, there is a perception, perhaps, a substantiated one, that some electronic products “never fail”. It is likely that such a perception exists because these products are superfluously durable, are more robust than is needed for a particular application and, as the consequence of that, are more costly than necessary. To prove that it is indeed the case, one has to find a consistent way to quantify the level of the product robustness in the field. Then one could establish if a possible and controlled reduction in the reliability level could be translated into a significant cost reduction. We suggest a concept that enables one to provide affirmative answers to the above questions. One effective way to improve the existing QT and specs is to use the probabilistic design for reliability (PDfR) approach, and based on such an approach 1) conduct the appropriate accelerated life testing (ALT), i.e., failure oriented accelerated testing (FOAT) at both the design and the manufacturing stages, and, since ALT/FOAT cannot do without predictive modeling (PM), 2) carry out PM to understand the physics of failure; 3) predict, using the results of the carried out ALT/FOAT and PM, and the PDfR modeling, the probability of failure in the field (operation, mission); 4) carry out sensitivity analyses to establish the acceptable probability of failure; 5) revisit, review and revise the existing QT practices, procedures, and specifications; and 6) develop and widely implement the probabilistic design for reliability (PDfR) concept, methodologies and algorithms, considering that the probability of failure is never zero, but could be predicted and, if necessary, minimized, controlled, specified and even maintained at an acceptable level. Prognostication and health monitoring (PHM) approaches and techniques could be very helpful at all the stages of the design, manufacturing and operation of the electronic or photonic system, with or without considering the role of the human factor.

An improved method for automatic detection and location of defects in electronic components using scanning ultrasonic microscopy

The paper deals with a method for the automatic analysis and characterization of defects due to encapsulation or/and surface mount processes of microelectronic devices. This method is based on digital signal processing of ultrasonic signals in the 10-100 MHz frequency range and in particular used for automatic evaluation of time-of-flight between echoes received by the acoustic transducer. The signals are firstly preprocessed by a new algorithm, based on the Wiener filtering, and then by a numeric algorithm, based on the wavelet transform, already applied successfully to this problem. The preprocessing phase increases the sensitivity of successive numeric algorithm. The theory underlying the preprocessor and the chosen procedure to implement it are described in detail. Furthermore, experimental results obtained applying the proposed method on acoustic signals from an electronic structure acquired through an ultrasonic scanning system are given and discussed.

Localization of defects in die-attach assembly by continuous wavelet transform using scanning acoustic microscopy

Abstract The main goal of this paper is to describe a new method based on Continuous Wavelet Transform (CWT) algorithm and successfully implemented in a Scanning Acoustic Microscope, developed at IXL Laboratory, for the measurement of Time Of Flight (TOF) between ultrasonic echoes and contribute to the help of defect diagnosis and failure analysis of a die-attach assembly. The paper is divided into three main parts. After a brief description of the different methods for the TOF measurement, we give the strong interest to use Wavelet Transform due to the nature of ultrasonic signals. The principle of the CWT algorithm and the TOF measurement method associated are explained. The robustness of the CWT algorithm is evaluated to achieve information concerning its performances in presence of different signal parameters and SNR with a statistical treatment. 1n final, we propose an application of this algorithm to the help of acoustic images interpretation. This application concerns the detection and localization of defects into a die-attach assembly, specially the detection of fine crack.

Non-destructive detection and localization of defects in multilayer ceramic chip capacitors using electromechanical resonances

High reliability multilayer ceramic chip capacitors are necessary for use in surface mounting processes which are more mechanically and thermally severe than the traditional through-hole processes. Moreover, in specific environments, even a small defect can be considered as catastrophic for the working of the electronic circuit or even of the entire system. In order to look for the failures—intrinsic latent defects and those caused by SMT soldering processes—appearing in these components, many techniques of analysis can be used. With this present work, we focus on one technique based on the principle of electromechanical resonances existing in piezoelectric materials under a d.c. bias field. The free correlation between the impedance measurement of the chip under a sufficient voltage allows us to highlight some conclusions concerning the behaviour, the nature of the defects and the long-term reliability of ceramic chip capacitors. This method has the advantage of being non-destructive, rapid, efficient and low-cost.

New methods for scanning ultrasonic microscopy applications for failure analysis of microassembling technologies

Scanning acoustic microscopy (SAM) is now a common detection method which produces high resolution images with focused ultrasonic waves ranging from 10 to 500 MHz. In this paper, first we propose improved methodologies in order to measure time-of-flight (TOF) with high accuracy and so localize defects in depth by digital signal processing used for the study of nonstationary signals as acoustic echoes. Secondly, we compare imaging mode capabilities associated with conventional acoustic focused probe propagation for SAM. Then, we apply these methods for localization of defects and failure analysis of ceramic capacitors, die-attach assembly and solder joint evaluation in a CBGA technology by C-SCAN analysis.

Three-dimensional FEM simulations of thermomechanical stresses in 1.55 μm Laser modules

Abstract The purpose of this study is to present three-dimensional simulations using finite element method (FEM) of thermomechanical stresses and strains in 1550 nm Laser modules induced by Nd:YAG crystal Laser welds and thermal cycles on two main sub-assemblies: Laser submount and pigtail. Non-linear FEM computations, taking into account of experimental σ(e) measured curves, show that Laser welding process can induce high level of strains in columns of the Laser platform, bearing the Laser diode, responsible of an optical axis shift and a gradual drop of the optical power in relation with relaxation of accumulated stresses in the sub-assembly. In the case of thermal cycles, stresses can occur on elements sensitive to coefficient of thermal expansion mismatches such as solder joint between the Laser platform and thermoelectric cooler and as fiber glued into the pigtail leading to crack propagation with sudden drop of optical power. The main objective of the paper is to evaluate thermomechanical sensitivity and critical zones of the Laser module in order to improve mechanical stability after Laser weld and reach qualification standards requirements without failures. Experimental analyses were also conducted to correlate simulation results and monitor the output optical power of Laser modules after 500 thermal cycles (−40 °C/+85 °C VRT).