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108 SMT Magazine • August 2016 100MHz captured with 4GHz digital sampling rate. At optimum transducer frequency and focus depth, the echo waveform composition can be understood by measuring arrival time provided that approximate sound speed and thickness of each material is known. In the SAT system we utilized in this study, the sound speed of most electronic materials are readily available, and the time of flight can be measured using cursors with a resolution of 0.5ns on time axis. There- fore, we focused the beam to the middle of the packages and opened 6 echo gates to image all layer interfaces simultaneously. Although the Figure 7(a): C-scan image from Gate #1 repre- senting bottom layer of HTCC1 at 100MHz probe frequency. All three units show very similar images, except the shadow of misplaced surface mount condenser is seen different at the center unit. Figure 7(c): C-scan image from Gate #3, UF1 and polymer 1 Interface at 100MHz probe frequency. The inspection algorithm is set so that red color paint to highlight whenever a delamination or void is detected. Serious flaws can be seen in the left middle of the left-most unit possibly from UF1. Figure 7(b): C-scan image from Gate #2, interface of HTCC1 top of UF1 at 100MHz probe frequen- cy. All three units show very similar images. The inspection algorithm is set so that red color paint to highlight whenever a delamination of void is detected. Figure 7(d): C-scan image from Gate #4, top of UF2 at 100MHz probe frequency. Both left and right units show significant delamination or less dense material regions as highlighted by red color. UF1 and some part of UF2 have flaws. NONDESTRUCTIVE INSPECTION OF UNDERFILL LAYERS