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102 SMT Magazine • August 2016 To extend these limitations to their ends, development of new transducers or probes with applicable frequency range from 50MHz to 300MHz are very crucial, along with other sig- nal handling advancements. Recently, Kitami, et al., reported develop- ment of a specially designed signal processing unit and high resolution probes that can im- age 1µm features engraved in silicon material [1] and an echo gating technique that intelligently tracks the surface plane so that it drastically re- duces invisible area due to rough exterior sur- faces of the package [2] . Encouraged by these new developments, we conducted a case study of SAT and X-ray CT im- aging for a multilayer package consisting of two ceramics with flip-chip packages on organics substrates in between. The SAT system available for this study is also equipped with high resolu- tion unit that generates well compressed pulses with excellent signal to noise ratio in a wide range of probe frequencies. In this report, we describe how flaws around flip-chip substrates embedded in a thick stack of ceramics mounted with surface components can be nondestruc- tively inspected layer by layer to pinpoint the manufacturing defects hidden in them. Experimental On the manufacturing floor, we noticed that some of the multilayer devices electrically failed but they were also unable to find out the root cause using existing analytical equipment. Cross sectioning the sample is the only option which is not only destructive but also time consuming just to find out about the flaws along one line out of entire surface. Therefore, we selected to study the most acoustically complex device to investigate the capability with state of the art X-ray and ultrasound imaging technologies. Sample Descriptions and Preview The sample consists of high temperature co-fired ceramics (HTCC) substrates as top and bottom layers embedded with chip packages on polymer substrates in between as illustrated in Figure 1. The dimensions are 18 mm width, 35 mm length, and 3.3 mm height. The regions of interest for possible delamination or voids are the joints to each interfaces between deep lay- ers. Of course, these HTCC layers themselves are multi-layered substrates as well. So far we have accumulated some knowl- edge on computed tomography (CT) inspection of various complex structures [3-4] , especially for solder joints with good and clear CT images. This ceramics package, however, was just quick- ly viewed with an X-ray CT system to find out any flaws in metal features and ceramics layers. 3D-CT inspection revealed that there are no ap- parent flaws in the material layers themselves; but no further attempts were made with CT in- spection because it is difficult to observe delam- ination type of flaws between materials given the nature of X-ray beam that can easily pen- etrate such a flaw without significant intensity reduction. Comprehensive study with X-ray CT maybe needed elsewhere. SAT Imaging During manufacturing process at the begin- ning, the samples were inspected with available NONDESTRUCTIVE INSPECTION OF UNDERFILL LAYERS Figure 1: Illustration of material structure of the sample and their thickness in micrometer. Outer layers are high-temperature, co-fired ceramics.