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Imaging BGA package at multiple depths

Posted: 29 Oct 2012 ?? ?Print Version ?Bookmark and Share

Keywords:acoustic microscope? delamination? transducer?

The user of an acoustic microscope usually has a clear idea of the depth he wants to image in a particular sample. If the possibility of a die face delamination in a plastic-packaged IC is his concern, he will image a shallow depth from just above the die face/mold compound interface to just below that interface.

He does this by using only echoes from with the defined depth to make the acoustic image, and ignoring echoes from other depths. The echoes are said to be gated on the depth of interest. Any die face delamination will lie within the depth he has gated, and he does not need to include other depths from which additional features might clutter the image. If he wants to look specifically at other depthsthe bulk of the molding compound above the die face, for examplehe can gate on those depths separately, and this is sometimes how the microscope operator examines a particular sample.

Some samples, though, can benefit from being imaged at several or many depths. Sonoscan recently developed an automated tool for use with its C-SAM�systems that can image a sample at as many as 200 depths. The depths are defined before scanning; the operator selects a beginning depth and an ending depth on the waveform of the sample, and specifies the number of gates into which the desired thickness of the sample will be sliced. The ultrasonic transducer scans the sample only once to make a separate acoustic image for each depth selected. The resulting acoustic images, whether 5 or 200, present what amounts to a slide show of acoustically visible features in the sample at increasing depths.

Developers of the tool predicted correctly that it would be very useful for samples having complex internal structures or defects. Ceramic samples with vertically extensive voids have been imaged to produce a 20- to 50-image slide show, which helps significantly in learning the origin of the voids. (Alternately, single-material samples such as ceramics are sometimes imaged with a single wide gate covering the whole vertical dimension in order to identifyalthough without depth informationvoids or other gap-type defects anywhere in the sample.)

Figure 1: Side-view diagram showing the 20 gates at which the plastic BGA package was imaged acoustically.

Early work with IC packages suggested that a multigate image series was frequently useful when imaging plastic-packaged ICs, which are relatively thin and which have well-defined internal layers. Since then more IC packages have benefitted from the multigate approach. Figure 1 is a side-view diagram showing the 20 gates into which a plastic BGA package (described in detail below) was divided in order to produce 20 sequential images.

Figure 2: Gate #2's acoustic image shows details of the mold compound and a few voids (red).

Figure 2 is the acoustic image of Gate #2 of a sequence of 20 gates from a plastic-packaged BGA. Gate #2, like the other 19 gates, is 50 microns thick. In total, the 20 images cover the top 1mm of the BGA package whose total thickness, including the 0.6mm substrate, is 1.7mm.

Most of the area of the acoustic image of Gate #2 shows tiny features of the mold compound itself. The white and black pattern does not depict individual filler particles, but instead is a result of the texture of microscopic inhomogeneities within this 50 micron gate. There may also be microporosity within the mold compound.

The red features in figure 2 are voids in the mold compound. (The image of Gate #1, which is not shown here, was very similar but showed only a few small voids.) The ultrasound is actually being reflected by the air-to-mold compound interface at the top of each void. In this image, red represents echoes of the highest amplitude; any air-to-solid interface reflects >99.99% of the ultrasound. These voids are scattered, and with the exception of the one at extreme right indicated by the arrow, are not very large. Some voids extend vertically through more than one gate; looking at other gated depths can reveal what the vertical extent of a void is.

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