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106 SMT Magazine • August 2016 As seen in the Figures 5, a through scan im- age quality is normally inferior to reflected echo image for several reasons. To penetrate a sample composed of organic compounds, lower fre- quency ultrasound must be used because more than 35MHz sound wave will be very difficult to penetrate most materials used in electronic packaging. A lower frequency (15MHz) unfo- cused probe will only offer low resolution im- ages because their beam spot cannot be made small enough. Furthermore, through scan im- ages do not offer layer specific layer information because transmitted sound wave have poor time resolution compared to the waveforms avail- able in reflection mode. Therefore, this method is not appropriate for analyzing current sample that has several material layer interfaces, where we would like to find out about their flaws. As we sweep probe frequencies from 50MHz to 120MHz for C-scan, best highest frequency for the sample is found to be around 100MHz at source. Therefore, we decided to employ this particular frequency in order to split layer in- terfaces and image them in a multi-gate setup. Figure 6 is the sample structure illustration and its associated actual echo waveform (A-scan) at Figure 6: Echo pulse-train of the multilayer sample impinged with 100MHz acoustic wave packet. The arrows indicate the timed locations of each interface. Waveform sampling rate is 4GHz, and time reso- lution is 0.5 ns. Also note that HTCC and Polymers are multilayer stacked-ups. NONDESTRUCTIVE INSPECTION OF UNDERFILL LAYERS