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104 SMT Magazine • August 2016 CSAM system without much success, pointing at that complicated sample structure; eventu- ally requiring destructive cross sectioning. For nondestructive inspection, it is challenging to construct acoustic image of multilayer struc- tures, because the echoes usually involve multi- ple reflections as well as their interferences. The difficulty increases when the sample is thicker as it requires lower frequency acoustic beam with smallest possible beam spot at longer focal lengths to reach deep layers. To overcome these hurdles, we need to get optimized transducers [5] suitable to this particular sample. All SAT results presented in this paper are from a new SAT system with high resolution unit option. This new SAT system has better ca- pabilities covering transducer frequencies from 5MHz to 300MHz at prescribed frequencies, and especially with high resolution hardware op- tion the user can virtually set any desired ultra- sound frequencies that should fit to a particular sample conditions. As preliminary inspection, we used lower frequency probes to image different layer inter- faces and try to identify them with their pattern appearances. A 50MHz, 7 mm focal length probe was used to image the underfill layers UF1 and UF2 simultaneously with two echo gates. Figure 2 shows these images acquired by focusing up- per underfill layer UF1. After confirming solder balls of UF1, the fo- cus target layer is shifted to lower level of sec- ond underfill layer UF2. The resultant images are shown in Figures 3 (a) and (b) in which we NONDESTRUCTIVE INSPECTION OF UNDERFILL LAYERS Figure 2(a): Image of underfill layer 1 focused with a 50MHz probe frequency. This layer in- cludes solder ball connections between adjacent layers. The big square shadow is laser marking and smaller rectangular shadows are surface mounted components. Figure 3(a): Image of underfill layer 1 with a 50MHz probe frequency. This layer is not in fo- cus. Figure 2(b): Image of underfill layer 2 acquired by focusing upper UF1 layer. Figure 3(b): Image of underfill layer 2 with a 50MHz probe frequency focused right at this lay- er. Features of UF2 surface patterns can be seen more clearly at this focus condition.