Acoustic micro imaging (AMI)
07 October 2003
The use of Acoustic micro imaging (AMI) in the failure analysis laboratory is relatively new and has become an integral part of the modern analysis tool kit. The technique is nondestructive in nature and gives important information about the samples that is not easily obtained by any other techniques including X-ray equipment. For example, AMI allows examination of the bonding interface between internal layers of an electronic component or the bonding of components to a substrate. In an electronic device for example, bond quality has a significant impact on reliability, especially when a mismatch of thermal expansion coefficients between materials causes disbanded areas to grow larger over time.
In the failure analysis laboratory, AMI is useful at various stages of analysis and especially prior to performing any type of destructive analysis such as cross sectioning or package opening. AMI assists in the study of internal construction, and can precisely locate the position of physical changes within components during accelerated life tests. AMI is not always a substitute for destructive physical analysis; rather, it is useful tool for making destructive analysis more efficient by revealing and locating suspicious areas on which to perform a cross section.
In the production environment, AMI technology is becoming necessary for screening many, even thousands, of samples. The basic principles of the analytical and screening modes of AMI are the same but, in screening mode, computer analysis and automation of parts handling greatly speed up the inspection process. Parts belonging to a suspicious batch or a batch with an unacceptable reject rate can be segregated into accept/reject categories with little or no operator intervention. Under fill and screening and joint quality of BGA's is a prime application for mass inspection by AMI.
The types of defects that AMI is particularly sensitive to are usually associated with unintentional air gaps between materials. Disbonds and delamination are simple examples. Voids, cracks, porosity, and so forth exhibit similar high-contrast detail when imaged with AMI. Since high frequency ultrasound does not propagate well in air or vacuum, maximum image contrast is obtained when such interfaces are encountered. Ultra sound does, however, propagate well through most solid materials used in electronic components. To produce acoustic images, the ultrasound must be effectively delivered or coupled to the sample. Since air is not a good ultrasonic energy conductor, this is accomplished through a fluid such as de-ionised water in which the sample is immersed. The lens portion of the ultrasonic transducer is also immersed. Equipments employing AMI are generally referred to as acoustic microscopes. There are several systems but the two most popular systems are scanning laser acoustic (SLAM) and C- scanning acoustic microscope C-SAM.
Acoustic microscopes.
The most important types of acoustic microscopes, which are in use today, are the scanning laser acoustic microscope (SLAM) and the C-mode scanning acoustic microscope (SAM). Both instruments utilise high-frequency ultrasound to non-destructively detect internal material interface discontinuities in materials and components. Ultrasound frequencies employed range from 10 to 230 MHz. The high frequencies are associated with higher resolution because of the shorter wavelength, and the lower frequencies are needed to penetrate thicker or lossier materials. The frequency chosen for analyzing a sample is a compromise between resolution and penetration to the depth of interest. The penetration depends strongly on specific material. For example, at any specific frequency, crystalline materials typically might be the most transparent to ultrasound, followed by ceramics, glasses, metals, and with polymers being the most absorbing.
Conclusion.
Acoustic micro imaging was not developed to compete with X-ray technology, but to complement it as a tool to allow further study of a component internal structure. X-ray tools are ideal to study the construction of a component using the variation in X-ray transmission absorption of the differing materials used to construct the component. It is difficult to locate voids etc using X-ray imaging as the component alloy lead frame may mask the void. Some visibility of the void may be possible with careful control of the X-ray energy and orientation of the component in the X-ray field but images are often poor and lack resolution.
Acoustic micro imaging has the obvious draw back requiring the component to be immersed in fluid. Although most volume AMI screening systems have a drying system attached to the equipment it is still not totally desirable to immerse particularly large low profile plastic IC packages in water because of the danger of epoxy moisture absorption.
Waterfall acoustic transducer is now becoming available which solves this problem and dramatically reduces the component immersion time to a few seconds instead of minutes. Water flows over and under the component and transducer only during the scan period. This feature will greatly increase the use of AMI in the manufacturing environment with respect to component contamination and basic convenience of use of the equipment.
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