Circuits Assembly - August 2008 - (Page 31)

Cover Story els deeper into the PEM, where it is reflected from deeper material interfaces; these reflections are used to image internal features. From each of the x-y points, signal amplitude data are collected. The amplitude determines the brightness of a pixel in the acoustic image. In operation, this method collects both surface topography and internal feature data several thousand times a second. The result is two images: one showing the internal images, and the other showing the surface topography. The user is therefore immediately able to relate these two sets of data. A PEM might, for example, have no internal anomalies, but might be so warped – as measured by the distance between its highest and lowest elevations – that it will be rejected because of the high likelihood of eventual electrical failure caused by the internal stress load. Or a PEM might have no significant warping, but may show internal anomalies such as cracks, voids or delaminations. Other possible scenarios are useful. For example, the surface may show distortion, but the internal acoustic image may reveal no anomalies. In this case, the irregular surface topography might mean anomalies are present in the PEM, but at a depth lower than the depth from which return echoes were selected to make the acoustic image. Simultaneous surface mapping and internal imaging is useful with many component types. Some are obvious candidates for this method: BGAs may be bowed; flip chips may be domed. But larger items such as multilayer printed wiring boards can also display surface warping and internal damage. Figure 1 is an at-depth acoustic image of the interior of a plastic BGA. To make this image, return echo signals from the depth of interest – the interface between the mold compound and the top of the silicon die – were used. The high-amplitude white circle at the die center is a void (arrow), a small bubble of air trapped in the cured epoxy. Other voids are present at the die edges. Voids and delaminations between the die surface and mold compound are unacceptable because they may expand as a result of normal thermal cycling and break a wire bond. Figure 2 is the topographic map of the surface of the same BGA, made by using the time-of-flight information (but not the amplitude or polarity) of the echoes from the top surface of the part. The color bar at left indicates changes in elevation: The lowest points are white, magenta or blue, and the highest points are green. No points on the surface of this BGA reach the yellow-red-brown region at the top of the color bar. Away from the periphery, the lowest region on the surface of the BGA is the dark blue region near the part center. The conspicuous pale blue dot near the center is a higher point on the surface where the epoxy has been pushed up by the void between the die and epoxy. There are additional voids at the die edges, but they are too thin to have pushed up the surface, or – more likely – are indistinct in this image because they are at a distance from the center where the BGA is beginning to bow upward. Once the map of surface topography has been made, differences in elevation can be measured. The pale blue surface “spike” caused by the delamination at the center of the die Figure 4. Optical photograph of a small PWB that may be warped. Figure 2. Surface mapping of the same BGA. The bright spot at center is the upward deformation of the surface caused by the void. circuitsassembly.com Figure 3. 3-D mapping of the BGA surface. The spike at center is void-related deformation. Figure 5. Surface mapping of the PWB in Figure 4. Highest points are red-to-black, lowest points are magenta. 31 Circuits Assembly AUGUST 2008 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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