November_EDFA_Digital

edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 22 NO. 4 20 SAM THEORY AND CASE STUDIES IN RELIABILITY AND COUNTERFEIT DETECTION Eric Whitney NSWC Crane eric.p.whitney@navy.mil EDFAAO (2020) 4:20-25 1537-0755/$19.00 ©ASM International ® INTRODUCTION It can be argued that froman overall technical capabil- ity standpoint, the scanning acousticmicroscope (SAM) [1-5] is one of the most underutilized analytical tools present in most failure analysis labs today. It is important to emphasize that “underutilized” does not imply that the equipment sits idle. Rather, based on personal experi- ence, it denotes that the SAM is predominantly used to detect the presence of internal voids or delamination (plastic encapsulated components, compositemultilayer devices, or circuit boards, etc.) across a limited range of technologies. In part, the limited utilization of the SAM’s capabili- ties is not by intent but rather the result of organizational priorities. Despite monumental improvements in the automated control software, optimization of the imaging parameters and interpretation of the image results still remain as much of an art as a science. Consequently, optimal applicationof the SAM’s capabilities is ultimately a function of the operator’s level of experience and training. The objective of this article is to provide a brief over- viewof SAM fundamentals and thenpresent actual project case studies to illustrate SAM capabilities. SAM THEORY Through educational and work-related experience, most scientists andengineers havea fundamental working knowledge of the terminology and physical principles of optics associated with optical microscopes. Although the terminology may vary, the physical principles of optics, specifically geometric optics, applies equally to both electromagnetic and acoustic (vibration) waves and con- comitantly to both optical and non-optical microscopes. There are three primary scan modes available on confocal scanning acoustic microscope (CSAM) systems, A-scan, B-scan, and C-scan. A-scan refers to a point scan, which provides a 1D view of the interfaces along the path of the beam. B-scan refers to a vertical profile scan along the xz or yz plane, whichprovides a2Dcross-sectional view of the sample. C-scan, the most common mode, refers to a horizontal profile scan of the xy plane, which provides a 2D planar view of the sample. SAMoperates on theprincipleof ultrasonicwave reflec- tion and transmission at material boundaries. Ultrasonic typically refers to any acoustic wave whose frequency is 40 kHz or greater (above the audible frequency range). In most SAM applications, the acoustic wave frequencies range from 10 to 400 MHz. Research and development activities are currently underway which could extend the operating frequencies to theGHz range. Continuous-wave operation used in through sample imaging requires two ultrasonic transducers, one for transmission and one for receiving. In reflective mode operation, the transmission and receiving operations are performed by a single trans- ducer. Time of flight between the transmitted wave pulse and one of the multiple return signals is used to locate and characterize individual material interfaces located at different depths. Each transducer is tuned to oscillate at a specific frequency, denoted as its center or resonance frequency. However, due to a finite Q, each transducer transmits and receives over a narrowband of frequencies around the resonance frequency. Acoustic impedance, similar to index of refraction, denotes the amount of resistance an acousticwave experi- ences as it propagates through the material. Changes in the acoustic impedance results inpartial transmission and reflection of the incident wave. The acoustical impedance ( Z ) of any known material is found by multiplying the density (ρ) and velocity ( V ) of sound through thematerial. Z = ρ V (Eq 1) When an ultrasonic wave encounters an interface