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August 2016 • SMT Magazine 109 flaws can be detected by using one sample only, we present images of 3 samples together to make the discussion easier to visualize. Figures 7(a), (b), (c), (d), (e), and (f) show 6 SAT images acquired by opening six different gates responsible for each layer interface to be inspected for three packages. The package at the middle is known to be an internally good unit while there is a misplaced surface mount device on it. The unit at left side has known electrical failure at left bottom area, and the unit at right side has electrical failure with no further infor- mation available. While acquiring the images of every echo in a gate, the intensity level is simultaneously ana- lyzed by an inspection algorithm to highlight red color over any area with abnormal incre- ment of echo intensity caused by delamination or marginally low material density. As expected, the good part at the center exhibits very small red area for any layers imaged by the gates num- bers 1 through 6. Inspecting the images of left part revealed that middle to bottom area at the lower level underfill UF2 has serious delamina- tion, the finding generally agreed with the open pin area as indicated by independent electrical test report. Recalling the images on Figures 2–5, the right side sample is the one we emphasize in this capability study with electrical fails at unknown area. The SAT inspection of this sample indicates that while upper underfill layer is free from flaws, top left corner of low- er underfill layer (UF2) has delamination as shown in Figure 7(d). This flaw area is also appeared in Figure 7(e) at the attachment to the surface of HTCC2. All these flaw locations can also be confirmed by taking multiple syn- chronized A-scan waveforms of interested locations on the samples and by comparing their peak heights. Cross Sectional Confirmation We have selected the left most package of Figure 7 which showed delamination flaws from the top to the lower polymer layer to cross-section if we can confirm the flaws high- lighted by red color. As expected the upper lay- er flaws at the center of the device are clearly observed under 150x optical microscope as shown in Figure 8. The delamination at lower level are some- what difficult to distinguish optically at 150x, and required higher magnification and image comparison. We carefully have inspected lower level joints at higher magnification up to 500x as well as by comparing with images of similar location of a different part. We noticed stronger contrasts at the boundary indicating a signature of thin delamination at the stack-up joint. Figure 7(e): C-scan image from Gate #5, UF2, organics interfaces at 100MHz probe frequency. Both left and right units show significant delami- nation or less material regions. Figure 7(f): C-scan image from Gate #6, top of HTCC2 at 100MHz probe frequency. NONDESTRUCTIVE INSPECTION OF UNDERFILL LAYERS