November_EDFA_Digital

edfas.org 21 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 22 NO. 4 between materials of different acoustic impedance, a certain percentage of the incident wave energy will be transmitted with the remaining energy in the reflected wave. This conditionwill repeat at each subsequent inter- face, as illustrated in Fig. 1. Consequently, the ability to detect a thin material layer depends on the ability to dis- criminate between the reflected wave from the top inter- face with the reflected wave from the bottom interface. The transmission and reflection coefficients are based on the ratio metric changes between the acoustic imped- ance of the adjacent materials ( Z 1 and Z 2 ). R A is the reflec- tion coefficient and T A is the transmission coefficient of the incident wave. (Eq 2) These calculations are often needed during analysis to determine the relative energy of the reflected wave, which determines the ability to detect discontinuities or resolve features. Given an incident wave in the water region, the following calculation illustrates the percentage of the incident wave that would be reflected back into the water at the water-to-air boundary based on the acoustic impedances of air ( Z A ) and water ( Z W ). (Eq 3) Given the high acoustic impedance of air (gas) relative to the acoustic impedance of liquid or solids, virtually 99% of the incident wave will be reflected. The high reflection coefficient is what enables the SAM to detect small voids ineither throughor reflectedmode operation. In reflected- mode operation, the minimumdetection limit is approxi- mately one percent of the amplitude of the incident wave. The most common question during SAM discussion is “What is the resolution of the SAM?” At this point, it is important to distinguish the difference between resolu- tion and detection. Detection is a binary threshold for determining presence, while resolution provides feature information about the object. The first step inCSAManaly- sis is detecting the region of interest (ROI). Only after the ROI is detected does resolution become important. In Fig. 2, the detected waveform on the left is neces- sary for resolution while the composite waveform on the right is all that is necessary for detection. Resolution of the right composite signal can be increasedwith frequency at the cost of penetration depth. While both resolution and Fig. 1 Reflectionandtransmissionatboundariesof different materials. Fig. 2 Fully resolved waveform, left, composite of two or more unresolved waveforms, right. Fig. 3 Common color maps used to produce a CSAM micro- graph, polarity inwaveform is depicted as correspon- ding color in pictograph.