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70 SMT Magazine • November 2014 arTiCle When a medical electronics system fails in service, the cause of failure is often a bro- ken connection within a plastic-encapsulated component package, and the breaking of the connection is often caused by a structural de- fect in the package material. Typically, a struc- tural anomaly in the plastic package itself has grown or changed until it caused the connec- tion break. What the user of the electronics sys- tem observes is a monitoring device that begins to give erratic readings, a blood pressure sensor that fails, or an implanted electronic device that stops working. For this reason the individual components are often screened before surface-mounting to remove the ones with internal structural anom- alies. The anomalies include delaminations, voids, non-bonds, cracks and other gap-type features. The screening is typically done by an acoustic micro imaging tool such as those in So- noscan's C-SAM ® series. The key to acoustic imaging is that an in- terface between two different materials will re- flect a predictable portion of an ultrasonic pulse beamed into the component package. The per- centage of the pulses reflected is determined by the density and acoustic velocity of the two materials. At an interface where the protective plastic mold compound is well bonded to the die face about 48% of the pulse to about 70% will be reflected in amplitude mode imaging, the most frequently used mode. The remainder of the pulse will go deeper and be reflected by the next interface. The reflections—the return echoes—are used to give a pixel color value to each of the thousands or millions of x-y coor- dinates that are scanned by the system's ultra- sonic transducer. Well-bonded interfaces between two solid materials are typically some shade of gray. A technician looking for a questionable compo- nent will look for areas of bright white, the sig- nature of gap-type defects that occur when the interface is between a solid and air or another gas. The properties of, say, mold compound and air are so different that essentially all (>99.99%) of the ultrasound is reflected. The imaging pro- cess is nondestructive, and the technician view- ing the acoustic images can remove the compo- nents that meet his company's definition for re- jection. Alternately, especially when the lot size is large, the "viewing" and decision-making can be performed by software analyzing the acous- tic imaging data. The components that can be imaged acous- tically include IC packages (plastic, ceramic and polyimide), ceramic chip capacitors, some types of resistors, and most recently IGBT mod- by tom adams SoNoSCaN iNC. Advances in Acoustic Imaging of Medical Electronics

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