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August 2016 • SMT Magazine 105 can clearly observe circuit patterns of the inter- face. These initial results encouraged us to pro- ceed with this focal length probe, we can attain images of desired levels to find their flaws. When inspecting with SAT system, we pre- fer to use higher frequency probing for two rea- sons: improvements of image lateral resolution and better separation of layers or depth resolu- tion. Therefore, we gradually increase the probe frequencies to 75MHz, 90MHz, and 120MHz above in 1MHz step until the penetration depth cannot reach to the surface of HTCC2 layer. With similar focus conditions as in 50MHz imaging, 100MHz probe frequency is used to acquire the images as shown in Figure 4. With possible highest frequency, the SAT image qual- ity improved in contrast as well as better reso- lutions in all x, y, and z directions. As we ex- pect to gain clearer layer separation with higher frequency probing, we can now open up more echo gates for detail analysis which we will de- scribe in the next session. NONDESTRUCTIVE INSPECTION OF UNDERFILL LAYERS Image Inspection & Analysis In acoustic image analysis routine, one pop- ular method of finding flaws is to scan the sam- ple in the transmission mode. In this mode, a transducer is placed at one side of the sample to transmit ultrasound signals and another trans- ducer from opposite side listens the sound as it gets through all sample layers. If the lattices of the materials in the sample are mechani- cally connected and ultrasound energy is al- lowed to be transported, a portion of trans- mitted ultrasound gets through the sample, or otherwise the sound is blocked by the flaws such as voids and delamination. Therefore, the scanned image is quite straightforward to interpret: the sample under test is good when the pass–through sound intensity is high, or the image is bright, relative to flawed region that will appear dark. In Figures 5(a) and (b), transmission images of the sample inspected by using 15MHz and 25MHz transmission probes are shown. Figure 4(a): Image of underfill layer 1 with a 100MHz probe frequency. This layer is not in focus. Figure 5(a): Through transmission scan with 15MHz. Figure 4(b): Image of underfill layer 2 with a 100MHz probe frequency focused right at this layer. Features of UF2 surface patterns can be seen more clearly in better contrast. Figure 5(b): Through transmission scan with 25MHz.