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76 SMT Magazine • November 2014 curately see and analyze anomalies in compo- nents. Suppose there is a delamination at the substrate level in a plastic BGA package. Stan- dard amplitude mode imaging will scan the component and assign a pixel value, based on signal amplitude, to each of the x-y coordinates into which it pulses ultrasound. The pixel value will correspond to the single highest amplitude found among the many echoes received from each x-y coordinate at the level of the delami- nation. This is standard imaging practice and in most situations gives excellent results. But some components, and particularly those going into critical roles such as medical applications, will respond better to an improved and more subtle approach using the integral mode. Integral mode uses the sum (rather than the average) of the amplitude of all the echoes received from the depth of interest to assign the pixel value. A delamination imaged first in standard amplitude mode may show the same bright white delamination, but integral mode may show a zone of irregularities—varying shades of gray, for example. Figure 3 shows, at left, the standard ampli- tude mode image of a plastic BGA package, gat- ed to include depths from mold compound-die interface to the die attach-substrate interface. Within the square area of the die at center are seen the dark acoustic shadows of a few voids in the mold compound above the die, along with the rather bright image of the mold compound- die interface. Because of the physical properties of these two materials, most of the ultrasound is reflected, and only perhaps 25% travels deeper. There are some slight irregularities in the gray areas here, but they are too faint to interpret. At right in Figure 3 is the integral mode image of the same depth. By using the sum of the echoes received at each pulse location, this method shows a dark region that indicates a de- lamination somewhere below the die face. By making an A-scan (i.e., single pulse) waveform of a few locations within the apparent delami- nation, it was possible to tentatively identify the delamination as lying within the die attach rather than within the substrate below. smt arTiCle aDvances in acoUstic imaging oF meDicaL eLectronics continues Figure 3: Standard amplitude mode imaging shows the square die region as bright gray (left), but integral mode imaging reveals the dark blue delamination below the die (right). Tom adams is a consultant for Sonoscan.