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December, 2020


Echoes Up, Echoes Down: The Versatility of Acoustic Imaging


By Tom Adams, Consultant, Nordson SONOSCAN T


he ease with which acoustic micro imaging can display the interior of electronic components and other samples depends on the versatility


of the tool’s imaging modes. There are currently about 15 modes, some very specialized. Under standing three com- mon forms of imaging can help to out- line the overall process.


Acoustic Imaging When a pulse of ultrasound is


launched by the transducer of an acoustic micro imaging tool, it first travels through the water coupling the transducer to the target component. At each subsequent material interface that it encounters, the pulse will be partly transmitted deeper and partly reflected back to the transducer. When this first echo arrives at


the transducer, its arrival time becomes the reference point for meas- uring other arrival times and thus the distances from the top of the com- ponent to other material interfaces encountered by the pulse. The pulse loses energy with each subsequent echo it sends back to the transducer and with each solid-to-solid interface it passes through. Within a matter of nanoseconds, the transducer has sent back the echo from the deepest solid- to-solid interface it has encountered in the component. But a pulse that encounters a solid-to-air


Each second the scanning transducer exe-


cutes some tens of thousands of pulses, each at its own at x/y location. Each location’s echoes will


leaving the transducer may strike the mold com- pound-to-die interface of an integrated circuit pack- age, where it will be partly reflected and partly transmitted deeper. It will be reflected and transmitted again at the die-to-die attach interface, and at other inter- faces deeper in the component. It may also encounter unintend-


ed interfaces, where the echoes it sends back will provide the image of an unexpected feature. Most unexpect- ed features are air-filled voids, delam- inations and the like. The air-to-solid reflects essentially all of the ultra- sound to the transducer; none travels deeper. Ceramic chip capacitors, stacked


die and IGBT modules are among the items that are frequently imaged with bulk scan. The defects likely to be found by bulk scan include voids, delaminations and cracks.


LoBE Like bulk scan, loss of back echo Figure 1: three different modes of acoustic micro imaging.


interface (at the top of a void or delamination) can- not cross the interface to travel deeper, because virtually 100 percent of the pulse is returned to the


determine the color and brightness of one pixel in the completed acoustic image. The color or grayscale shade of the pixel may indicate the amplitude of the echo, its depth, its frequency, or its polarity. A single acoustic image may be assem- bled from thousands or even millions of pixels, each with its own x/y coordinates.


Bulk Scan In each of three common modes, the pulses


Figure 2: C-SAM images of a ceramic chip capacitor containing a small void.


transducer as a very high amplitude echo. Voids and the like are thus imaged in colors, such as white or red, that indicate high-amplitude echoes.


are launched into the sample from the top. When traveling through the sample, echoes are reflected upward from all material interfaces. The transduc- er’s receiver is programmed to accept only those echoes that arrive during a gate that may be only nanoseconds in length. Echoes arriving outside of the gated time are ignored. Bulk scan is capable of receiving echoes from nearly the entire depth of the component. A pulse


Figure 3: optical, THRU-Scan and LoBE images of an unpopulated multilayer ceramic substrate.


faces and have been reflected by it will be collected on their return to the transducer. The result is a


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(LoBE) mode images all or nearly all of the thickness of the component. Unlike bulk scan, LoBE displays only the acoustic shadows of the features. A pulse enters the top of a component and moves downward just as in bulk imaging. Echoes from interfaces above the back wall reach the trans- ducer, but the gate remains closed to


all of them and does not collect them. Those pulses that have reached the back wall without being entirely absorbed by material inter-


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