December, 2020
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Echoes Up, Echoes Down: The Versatility of Acoustic Imaging
Continued from previous page
fairly sharp image displaying the acoustic shadows of features at all depths. Using the back-wall echo may increase the contrast of some features. It has other advantages, including the ability to image thin vertical cracks that might not be visible in a bulk scan image. Precise, high-resolution images are the goal
in much acoustic micro imaging of electronic com- ponents, but there are applications in which lower- resolution images may be more useful.
THRU-Scan Like the other two modes, the THRU-Scan
mode sends a pulse into the top of the target com- ponent. Unlike them, it does not receive any return echoes. The pulse, partly attenuated by interfaces and materials it may have encountered, instead exits the other side of the component, where it is collected by a second transducer. THRU-Scan images are typical-
ly not as high in resolution as reflec- tion images, because the pulses of ultrasound have penetrated the full thickness of the sample, including the upper and lower surfaces. But, this imaging mode is often used when it is desirable to find any structural defect at any depth. Where it is important to image large quantities of components and make a quick analysis of the results for imaging by a more detailed image, this is often the mode of choice. The images in Figure 2 were
made in a laboratory of Nordson SONOSCAN by a C-SAM® acoustic micro imaging tool. At left is the bulk scan acoustic image of a ceramic chip capacitor that contains a small void, visible here as small bright white cir- cle. It is bright because the interface between the air in the void and the ceramic reflects virtually all of the ultrasound that strikes it from above. All of the pulses elsewhere in
this bulk scan image were not reflect- ed from anything except the back wall or were heavily attenuated by their round trip and arrived at the transducer reporting low amplitude — and are therefore very dark. In the center of Figure 2 is the
LoBE image of the same ceramic chip capacitor. In LoBE imaging, pulses are launched by the transducer. But, after the pulse enters the component, echoes from material interfaces are ignored by the transducer. The trans- ducer collects only echoes from the back wall of the component. Each LoBE echo follows the
same path as the pulse, but in reverse. If the pulse was transmitted through an interface, the echo will pass through the interface in the same x/y location, but traveling upwards. The void that was bright in the
bulk scan image is dark there, because pulses that struck its solid- to-air interface were reflected. They never reached the back wall and were not reflected at the right time to be captured.
Directly above the dark void in
the LoBE image is a low contrast void. If the purpose of LoBE is to image everything from behind, how can this void be so bright? The void probably lies directly on the back wall, where no pulse can come at it from behind. The center of the void is actually about the same color as the color of the back wall surrounding it.
At right in Figure 2 is the THRU-Scan image.
Pulses traveling downward from the transducer above the sample to the second transducer below the sample present both voids as dark shadows. Resolution is low, but the voids are made visible as features that block transmission. Figure 3 shows, from left, the optical, THRU-
Scan, and LoBE images of a portion of an unpopulat- ed multilayer ceramic substrate. The purpose of imaging was to find possible defects before mounting components. The THRU-Scan image captures most features, but not the fine wires in the LoBE image. There are also a few anomalous white features in the THRU-Scan image, as well as three black rounded voids, visible in both acoustic images. To sum up: bulk scan receives echoes from the
full thickness of one layer of the component — typ- ically the layer where defects are known to occur. When the LoBE mode is launched, return echoes from interfaces above the back wall are ignored, but are then imaged as shadows by ultrasound reflected by the back wall. THRU-Scan simply insonifies the entire thickness of the component from below to create a less sharp image that shows defects at all depths. There are about a dozen addi- tional imaging modes, and when detailed analysis is needed, multiple modes may be used. Contact: Nordson SONOSCAN, 2149
E Pratt Boulevard, Elk Grove Village, IL 60007 % 847-437-6400 E-mail:
info@nordsonsonoscan.com Web:
www.sonoscan.com r
Page 49
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