Circuits Assembly - August 2008 - (Page 30)

Cover Story Simultaneous Acoustic Imaging and Surface Mapping By Tom Adams Combining the methods reveals not only the anomaly, but the extent of 3-D distortion. lastic-encapsulated microcircuit (PEM) warping is of interest to manufacturers for several reasons. First, component warping may create internal stresses that, if eventually released, may result in electrical failure. For example, warping eventually may cause the silicon die to crack. Second, warping may cause internal delaminations or cracks within the PEM. In this case, the stresses already have been at least partly relieved by the formation of a delamination or crack. Electrical failure might be immediate if the anomaly breaks a wire or a solder bump. But delaminations or cracks that slowly expand or promote corrosion may cause electrical failure in the future. In some components, surface topography irregularities are not the result of warping, but are caused by upward pressure from internal anomalies such as voids. All these scenarios make it worthwhile to collect data on both the surface topography of the component and its internal features, including possible anomalies. A new (patent pending) development in acoustic micro imaging makes it possible to map compo30 Circuits Assembly AUGUST 2008 P nent surface topography at the same time that internal features are imaged. Ultrasound pulsed from the scanning transducer of an acoustic micro imaging system is reflected from material interfaces, but not from homogeneous bulk materials. The first reflection is from the interface between the water couplant and top surface of the PEM. The time-of-flight of this reflection is used only to measure distance, and as the transducer scans thousands or millions of x-y points, the data for mapping the surface topography are collected. A portion of the ultrasonic pulse trav- Figure 1. Acoustic image of a plastic BGA’s interior, specifically at the interface between the mold compound and silicon die. Near the center of the die is a circular void (arrow); there are other voids at the die perimeter. circuitsassembly.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - August 2008

Circuits Assembly - August 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing Auditing a Fabricatior Cutting Machine Programming Time Simultaneous Acoustic Imaging and Surface Mapping Tech Tips Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Component Advances Product Spotlight Ad Index Assembly Insider Techincal Abstracts

Circuits Assembly - August 2008

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