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94 SMT Magazine • September 2016 The ultrasonic frequencies pulsed into sam- ples by the transducers of acoustic microscopes range from a low of 10 MHz (rarely, 5 MHz) to a high of 400 MHz or more. Across this spec- trum of frequencies there is a trade-off of pen- etration and resolution. Ultrasound at low fre- quencies (such as 10 MHz) penetrates deeper into materials than ultrasound at higher fre- quencies (see Figure 4), but the spatial resolu- tion of the acoustic image is less. On the other hand, ultrasound at very high frequencies does not penetrate deeply, but provides acoustic im- ages having very high resolution. The frequen- cy chosen to image a particular sample will de- pend on the geometry of the part and on the types of materials. Figure 4 schematically compares a few fea- tures of flip chip CGA/BGA detectable by AMI and X-ray. X-ray uses high-energy electromag- netic radiation with shorter wavelengths than ultraviolet light to detect inner features. They are highly penetrable depending on the X-ray's energy, which increases with frequency. As fre- quency and thus penetration increase, the type of X-ray moves from "soft" to "hard." The reflec- tive nature of AMI allows for detection of delam- ination, whereas the penetration of X-ray allows detection of both short and large voids. These two inspection approaches are complementary techniques that should be used to reveal differ- ent features. The X-ray technique relies on the differential attenuation of X-ray energy, where- as the AMI technique relies on material change. The practical result is that AMI is orders of mag- nitude more sensitive for detecting air space type defects such as voids, delaminations and cracks. AMI for Microelectronics Inspection Applications In a previous comprehensive study, Sandor and Agarwal 7 utilized the C-SAM nondestruc- tive technique to evaluate commercial-off-the- shelf (COTS) plastic encapsulate microcircuit (PEM). Samples from different commercial ven- dors were used for detecting internal defects due to various environmental exposures. PEM failure modes reported in industry due to de- lamination are summarized as: • Stress-induced passivation damage over the die surface • Wire-bond degradation due to shear displacement • Accelerated metal corrosion • Die-attach adhesion • Intermittent electrical signals at high temperature • Popcorn cracking • Die cracking • Device latch-up DEFECT FEATURES DETECTED BY ACOUSTIC EMISSION Figure 3: Selection of appropriate transducers is key in optimizing penetration and resolution for the flip-chip BGA and CGA. Figure 4: Key features of defect detectability by X-ray and C-SAM for flip-chip BGA/CGA.