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32 SMT Magazine • June 2015 Plastic packaged ICs, including BGAs, whether single or stacked, may experience warp- ing as a result of processing. Thermal stresses in a warped BGA package that has been mounted on a board can break solder ball connections. Often it is the corners of the package that curl upwards and cause the solder balls at the cor- ners to lose contact. Package-on-package assemblies (PoPs) are also subject to warping. The risk is greater if—as is usually the case—the bottom chip is a pro- cessing device and the top is a memory device, or even a stack of two memory chips wire-bond- ed to each other. Acoustic micro-imaging tools are used to im- age internal features, including anomalies and defects, in many types of components, but in plastic IC packages the surface topography may by Tom Adams SonoScAn inc. Acoustic Surface Flatness of Components and Boards be scanned simultaneously to produce surface flatness data and images. BGA packages are of- ten the subject of this imaging, although other package types are often imaged as well. Sonoscan has developed and patented a method that uses acoustic micro-imaging tools to measure and map the flatness of the top sur- face of a BGA package and other package types where warping may occur. The method is called acoustic surface flatness (ASF) and it is useful during the development of a BGA or PoP as a way to determine non-destructively whether in- ternal stresses are warping the surface. It is also used during production to remove BGA packag- es whose warping exceeds applicable standards, and after electrical failures in testing or in the field to isolate possible causes of a failure. The technology uses an ultrasonic trans- ducer that raster-scans the surface of the sample whose internal features are being imaged. While it is scanning, the transducer launches a pulse of ultrasound into the BGA package at each of mil- Feature

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