Query: Advanced acoustic microimaging using sparse signal representation for the evaluation of microelectronic packages

title: Advanced acoustic microimaging using sparse signal representation for the evaluation of microelectronic packages
abstract: Acoustic microimaging (AMI) has been widely used to nondestructively evaluate microelectronic packages for the presence of internal defects. To detect defects in small devices such as /spl mu/BGA, flip chip, and chip scale packages, high acoustic frequencies are required for the conventional AMI systems. The acoustic frequency used in practice, however, is limited by its penetration through materials. In this paper, a novel acoustic microimaging technique, which utilizes nonlinear signal processing techniques to improve the resolution and robustness of conventional AMI, is proposed and investigated. The technique is based on the concept of sparse signal representations in overcomplete time frequency dictionaries. Simulation and experimental results show its super resolution and high robustness.

title: High resolution AMI technique for evaluation of microelectronic packages
abstract: An acoustic micro imaging (AMI) technique for the evaluation of microelectronic packages and populated circuit boards is proposed. The technique is based on the concepts of sparse signal representation in overcomplete time frequency dictionaries. The experimental results demonstrate its superior ability to conventional AMI techniques.

title: Resolution improvement of acoustic microimaging by continuous wavelet transform for semiconductor inspection
abstract: Acoustic microimaging (AMI) is used as an important non destructive tool in semiconductor reliability evaluation and failure analysis. As advanced microelectronic packages are being produced smaller and thinner, detection of the internal features and defects in the packages is approaching the resolution limits for conventional AMI. To meet this challenge, an acoustic time frequency domain imaging technique is proposed in this paper, which utilizes the excellent time frequency localization characteristics of the continuous wavelet transform (CWT) to improve the axial resolution of AMI, without increasing acoustic frequencies. The proposed technique is compared to time domain AMI, frequency domain AMI and sparse signal representation based AMI (SSRAMI) with respect to both axial resolution and robustness. Simulation results show that the proposed technique has superior performance compared to time domain and frequency domain AMI techniques, and has close performance to SSRAMI but with less computation load.

title: Ultrasonic flaw detection using sparse representation for failure analysis of next generation microelectronic packages
abstract: In this paper, a sparse Bayesian learning based ultrasonic signal processing method is designed to detect and characterize flaws in thin multilayer structures, where the reflectivity is sparse. Sparse Bayesian learning is first used to decompose an ultrasonic signal into sparse signal representations over an overcomplete dictionary that is learnt from a training data set in advance. Ultrasonic flaw detection is then carried out on the basis of the sparse signal representations. The performance of the proposed method is experimentally verified using ultrasonic traces acquired from microelectronic packages by a scanning acoustic microscope.

title: An improved acoustic microimaging technique with learning overcomplete representation
abstract: Advancements in integrated circuit (IC) package technology are increasingly leading to size shrinkage of modern microelectronic packages. This size reduction presents a challenge for the detection and location of the internal features/defects in the packages, which have approached the resolution limit of conventional acoustic microimaging, an important nondestructive inspection technique in the semiconductor industry. In this paper, to meet the challenge the learning overcomplete representation technique is pursued to decompose an ultrasonic A scan signal into overcomplete representations over a learned overcomplete dictionary. Ultrasonic echo separation and reflectivity function estimation are then performed by exploiting the sparse representability of ultrasonic pulses. An improved acoustic microimaging technique is proposed by integrating these operations into the conventional acoustic microimaging technique. Its performance is quantitatively evaluated by elaborated experiments on ultrasonic A scan sig.

title: Adaptive sparse representations of ultrasonic signals for acoustic microimaging
abstract: Acoustic microimaging (AMI) is a common nondestructive tool for failure analysis of microelectronic packages. Accurate estimation of the reflected ultrasonicechoes is essential for detection and location of defects inside the microelectronic packages. In this paper, an advanced AMI technique based on adaptive sparse representations is proposed to estimate the ultrasonicechoes and recover the reflectivity function. An adapted overcomplete dictionary capable of concise expression of ultrasonic signals is first learned by the focal underdetermined system solver based column normalized dictionary learning algorithm. The ultrasonic A scans generated by an AMI system are then decomposed into adaptive sparse representations over the learned dictionary using a sparse basis selection algorithm. Echo selection and echo estimation are further performed from the resulting adaptive sparse representations. The proposed technique offers a solution to the blind source separation problem for restoration of the reflectivity function and can separate closely spaced overlapping echoes beyond the resolution of the AMI system. Experimental verifications are carried out using both synthetic and measured data. The results show the proposed technique produces high resolution and accurate estimates for ultrasonicechoes.

title: Effect of sparse basis selection on ultrasonic signal representation.
abstract: Recently, adaptive sparse representations of ultrasonic signals have been utilized to improve the performance of scanning acoustic microscopy (SAM) a common nondestructive tool for failure analysis of microelectronic packages. The adaptive sparse representation of an ultrasonic signal is generated by decomposing it in a learned overcomplete dictionary using a sparse basis selection algorithm. Detection and location of ultrasonic echoes are then performed on the basis of the resulting redundant representation. This paper investigates the effect of sparse basis selection algorithms on ultrasonic signal representation. The overcomplete independent component analysis, focal underdetermined system solver (FOCUSS) and sparse Bayesian learning algorithms are examined. Numerical simulations are performed to quantitatively analyze the efficiency of ultrasonic signal representations. Experiments with ultrasonic A scans acquired from flip chip packages are also carried out in the study. The efficiency of ultrasonic signal representations are evaluated in terms of the different criteria that can be used to measure its performance for different SAM applications, such as waveform estimation, echo detection, echo location and C scan imaging. The results show that the FOCUSS algorithm performs best overall.

title: Microelectronic package characterisation using scanning acoustic microscopy
abstract: In a highly competitive market, reliable techniques for manufacturing quality control of electronic devices are demanded. Characterisation of modern microelectronic package integrity becomes more difficult due to the continued miniaturisation of electronic device and the complexity of advanced micro assembling technologies such as chip scale packages and 3D IC stacks. In this paper, sparse representations of acoustic signals are sought to improve the scanning acoustic microscopy (SAM) a common non destructive tool for failure analysis of microelectronic packages. Sparse representation of an ultrasonic signal is obtained by decomposing it in an overcomplete dictionary. Detection and location of ultrasonic echoes are then performed on the basis of the resulting redundant representation. The method offers a solution to the deconvolution problem for restoration of the ultrasonic reflectivity function. It can restore closely space overlapping echoes beyond the resolution of the conventional SAM system. It also produces high resolution and accurate estimates for ultrasonic echo parameters, i.e. time of flight, amplitude, centre frequency, and bandwidth. These merits of the proposed method are explored in various potential applications for microelectronic package characterisation.

title: An Advanced Acoustic Emission Sensor for Transient Elastic Wave Measurements
abstract: This paper reports the development of an advanced acoustic emission system for transient elastic wave measurements. The goal is to develop an inexpensive but effective acoustic emission system which can simultaneously obtain high frequency acoustic emission signals at multiple locations. The transient elastic signals can then be used for inversely deducing important characteristics of the acoustic emission event. To achieve this goal, a number of miniature PZT elements are first fabricated using an excimer laser micro machining technique. The PZT element is in conical shape and with an aperture size around 200 mm. Very similar to the conventional NBS conical transducer but much smaller in size, the miniature PZT element is an excellent acoustic emission transducer with much high bandwidth. A new packaging method of the mini conical element allows the transducer not only can measure conductive samples but also non conductive ones. This improvement greatly enhances the applicability of the acoustic emission system. A number of PZT mini conical transducers have been used to construct the acoustic emission measurement system. The transducer has been tested with standard glass capillary breaking methods. Finally, a mechanical fixture is designed to mount these acoustic emission transducers and to make the acoustic emission measurements much more convenient. To demonstrate the capability of this advanced acoustic emission system, a number of plate samples are used. Acoustic emission signal at multiple locations are obtained and compared with their theoretical counterparts. Good results are obtained. Further improvement and application of this system will be addressed.  Acoustic emission technique is a very powerful non destructive evaluation method  It has been widely used for detection and characterization of acoustic events, such as crack opening, crack propagation, fiber breaking, de lamination, impact damage, and so forth. It has also been used for manufacturing process monitoring such as cutting tool wear monitoring. Traditionally the acoustic emission measurements have been used in a more statistical way. Which means the obtained acoustic emission signals are process with some algorithm based on statistics such as setting a signal threshold, counting the number of events, and analyzing the histogram of the acoustic emission event, etc. However, acoustic emission can be also quantitative and analytic, which is known as quantitative acoustic emission. The basic approach of quantitative acoustic emission is to first obtain a number of transient wave signals, mostly on the surface of the sample under testing. The theoretical calculation of the acoustic emission signals is then established if the sample geometry, material properties, and acoustic source are given. The experimentally measured wave signals are then analyzed by the theoretical calculation in an inverse way to derive the characteristics of the acoustic events, such as the source location, the strength of the moment tensors, etc. Quantitative acoustic emission has been playing an important role in today's non destructive evaluation and a numerous number of engineering applications can be found [2 5] For a successful quantitative acoustic emission measurement, the transducer plays a vital role and needs to meet some requirements. First of all, it is usually required for the output signal (usually a voltage waveform) of , Vols. 270 273, pp 471 478 doi:10.4028/www.scientific.net/KEM.270 273.471 (c) 2004  No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org 13/03/20,17:37:47) Title of Publication (to be inserted by the publisher) the transducer to be directly related to a physical quantity, such as displacement, velocity, or acceleration, at the measured location so that the signal can be easily interpreted. If the signal is indirectly and/or mixedly related to several physical quantities, a complicated de convolution process will then be needed and will cause extra effort and even some error. Secondly, in most cases, multiple transducers are needed to simultaneously mount on the sample for the measurement to assure adequate amount of information is obtained for the inversion. Therefore, the transducer should be relatively inexpensive and can be readily available. Thirdly, it will be better off for data analysis if the measurement size of transducer to be small as compared to the shortest wavelength involved in the acoustic event. Also the frequency response of transducer should be wideband so that it can cover a wide range of signal frequency. Finally, the transducer should be easily mounted on the sample under investigation. There are several transducers which are frequently used for quantitative acoustic emission measurements. Air couple capacitive transducers  are very accurate dynamics displacement transducer with very good frequency response. But it is very delicate and not easily to implement. Pinducers [7] are piezoelectric transducers with small measurement size (around 1 mm) and can be easily mounted to a sample with epoxy gluing. However, its output signal cannot be directly related to a single physical quantity. Therefore, a complicated de convolution process is needed. Laser interferometer is very good for non contact dynamic displacement or velocity measurement with a very wide bandwidth, small measurement size, and high measurement accuracy. However, it is usually very expensive and not suitable for simultaneous multiple channel measurements. The NBS conical transducer was especially developed for quantitative acoustic emission in the earlier '80s at the National Bureau of Standards (NBS, now the National Institute for Standards and Technology, U.S.A.  The structure of the transducer is fairy simple and is depicted schematically in Fig.1. The transducer consists of a conical piezoelectric element and a heavy cylindrical brass backing. The piezoelectric element is polarized parallel along its axis and both end surfaces are coated with electrodes. The smaller end surface is in contact with the sample under measurement. The surface motion will excite the PZT element and produce an electrical output signal. It had been demonstrated that the output voltage signal of the NBS conical transducer is directly related to the normal surface displacement at the contact area with frequency up to few MHz [10] The transducer has been widely used for acoustic wave transducer calibration and quantitative acoustic emission measurements in laboratory. It is now commercially available (Industrial Quality Inc. Gaithersburg, MD) and some improvements and modifications have been reported [11] Although the NBS conical transducer is a good candidate for quantitative acoustic emission measurement, it does have several serious drawbacks. First of all, the aperture size of the transducer is around 1 to 2 mm in diameter. According to the analysis given by Greenspan  the aperture size will limit the transducer's Electrical signal out Conical piezoelectric element Cylindrical brass backing Vibrating surface Small contact area Electrodes Fig. 1. Schematic representation of a NBS conical transducer.

title: AE (Acoustic Emission) for Flip Chip CGA/FCBGA Defect Detection
abstract: C mode scanning acoustic microscopy (C SAM) is a nondestructive inspection technique that uses ultrasound to show the internal feature of a specimen. A very high or ultra high frequency ultrasound passes through a specimen to produce a visible acoustic microimage (AMI) of its inner features. As ultrasound travels into a specimen, the wave is absorbed, scattered or reflected. The response is highly sensitive to the elastic properties of the materials and is especially sensitive to air gaps. This specific characteristic makes AMI the preferred method for finding "air gaps" such as delamination, cracks, voids, and porosity. C SAM analysis, which is a type of AMI, was widely used in the past for evaluation of plastic microelectronic circuits, especially for detecting delamination of direct die bonding. With the introduction of the flip chip die attachment in a package; its use has been expanded to nondestructive characterization of the flip chip solder bumps and underfill. Figure 1.1 compares visual and C SAM inspection approaches for defect detection, especially for solder joint interconnections and hidden defects. C SAM is specifically useful for package features like internal cracks and delamination. C SAM not only allows for the visualization of the interior features, it has the ability to produce images on layer by layer basis. Visual inspection; however, is only superior to C SAM for the exposed features including solder dewetting, microcracks, and contamination. Ideally, a combination of various inspection techniques visual, optical and SEM microscopy, C SAM, and X ray need to be performed in order to assure quality at part, package, and system levels. This reports presents evaluations performed on various advanced packages/assemblies, especially the flip chip die version of ball grid array/column grid array (BGA/CGA) using C SAM equipment. Both external and internal equipment was used for evaluation. The outside facility provided images of the key features that could be detected using the most advanced C SAM equipment with a skilled operator. Investigation continued using in house equipment with its limitations. For comparison, representative X rays of the assemblies were also gathered to show key defect detection features of these non destructive techniques. Key images gathered and compared are: Compared the images of 2D X ray and C SAM for a plastic LGA assembly showing features that could be detected by either NDE technique. For this specific case, X ray was a clear winner. Evaluated flip chip CGA and FCBGA assemblies with and without heat sink by C SAM. Only the FCCGA package that had no heat sink could be fully analyzed for underfill and bump quality. Cross sectional microscopy did not revealed peripheral delamination features detected by C SAM. Analyzed a number of fine pitch PBGA assemblies by C SAM. Even though the internal features of the package assemblies could be detected, C SAM was unable to detect solder joint failure at either the package or board level. Twenty times touch ups by solder iron with 700degF tip temperature, each with about 5 second duration, did not induce defects to be detected by C SAM images. Other techniques need to be considered to induce known defects for characterization. Given NASA's emphasis on the use of microelectronic packages and assemblies and quality assurance on workmanship defect detection, understanding key features of various inspection systems that detect defects in the early stages of package and assembly is critical to developing approaches that will minimize future failures. Additional specific, tailored non destructive inspection approaches could enable low risk insertion of these advanced electronic packages having hidden and fine features.