Multi-Layer and 3-D Acoustic Imaging of Ceramics
Two refined imaging techniques are able to produce detailed and precisely oriented acoustic images for advanced ceramics and other materials.
Acoustic micro-imaging tools perform the non-destructive imaging and analysis of internal features (including anomalies and defects) in advanced ceramics and other materials by pulsing ultrasound into the sample and receiving the return echoes. The resulting acoustic image displays the material interfaces within the sample within the vertical limits defined by the operator.
Because a pulse of ultrasound may send back echoes from multiple depths within the sample, control of those echoes is generally important. “Control” in this case means defining the vertical limits that are imaged by collecting only those echoes that were returned from the depth of interest. In the imaging of multilayer ceramic capacitors (MLCCs), the standard approach is to use the echoes from all or nearly all depths, because the exact depth of a delamination or other defect in such a layered device is relatively unimportant; a capacitor having an anomaly at any depth that meets the user’s definition of a defect will be rejected.
In a more complex sample capable of having internal features at multiple levels, an experienced operator may be able to deduce the depth of a feature from their knowledge of the sample’s internal structure. But in many instances, it is desirable or necessary to go beyond single two-dimensional images in order to gather more precise data about internal features. Two refined techniques produce more detailed and precisely oriented acoustic images: simultaneous imaging of multiple defined depths within the sample; and three-dimensional imaging of critical portions of the sample.
Multi-Gate Acoustic Imaging
The acoustic images shown in this article were made using a tool with acoustic micro-imaging capabilities.* The tool’s transducer scans over the surface of the sample at high speed. While scanning, it pulses ultrasound into the sample at a given x-y location and receives the return echo, if any, a few microseconds later.
Despite the high lateral speed of the transducer, the tool carries out the pulse-echo function several thousand times per second. Echoes typically arrive at the transducer a few microseconds after pulsing. Only material interfaces produce echoes. Homogeneous materials have no internal interfaces to reflect ultrasound, although these materials will absorb some portion of the ultrasound.
The amplitude of a given echo is used to assign a color to the x-y location. At interfaces between bonded solid materials within the sample, the amplitude can be determined by the physical properties of the two materials. The highest amplitude echoes come from gap-type defects (e.g., voids and cracks) or, more precisely, from the interface between a solid material and the gas or vacuum in the gap.
To use gates in multi-layer imaging, the transducer collects information not from a single defined depth (which may be the whole thickness of the part), but from multiple thinner depths. At each x-y location, the transducer collects the echo from each selected depth, or gate. Many samples require only a few gates, but for other samples there may be as many as 200 gates, which may be of the same or different vertical extents, as well as adjacent, overlapping, or separated. When scanning is completed, software produces a separate acoustic image for each gate. When viewed in sequence, the images produce what is effectively a nondestructively obtained slideshow that moves downward through the sample.
Figure 1 is the acoustic image of Gate 3 out of 10 gates in a holed ceramic disk. The disk is 19.53 mm in diameter and 9.95 mm thick; each of the 10 adjacent gates is thus 0.995 mm in vertical extent. The center hole is 4.02 mm in diameter.
At the left of the image is the color bar used to make the image. At the top, bright red indicates the highest amplitude of positive echoes. In other words, at a given interface, the pulse of ultrasound crossed from a material having lower acoustic impedance (a function of a material’s density and acoustic velocity) to a material having a higher acoustic impedance. Moving down the color bar, regions in this gate are progressively colored gray, yellow, gray-black and white as amplitude diminishes. In the bottom half of the color bar, the same sequence is repeated in reverse to accommodate negative echoes (caused by crossing from a material having higher acoustic impedance to a material having lower acoustic impedance).
The lower left part of the disk in Figure 1 has an obvious defect. Within the vertical confines of Gate 3, the red portions are a void, or, more probably, part of a void that has a rough and irregular interface with the solid material around it. The irregular interface scatters some of the returning echoes, making the true nature of the defect harder to discern. The rest of Gate 3 is defect-free; the small edge anomalies at the top are probably the acoustic shadows of small chip-outs on the surface of the disk.
Eight of the 10 imaged gates are shown in Figure 2. Red areas indicating voiding are present in all gates except Gate 2. The largest areas of voiding are in Gates 4, 5 and 6, although again it must be recognized that the irregular nature of the defect may obscure voiding to some extent. Gate 7 has a separate small defect, probably a void, near the disk edge at about the 9:00 position. Overall, the individual gates give a picture of a complex defect that extends through much of the vertical extent of the disk.
*Sonoscan’s C-SAM® acoustic micro-imaging tool.
3-D Acoustic Imaging
To gather the data from which three-dimensional acoustic images are made, the transducer first scans the entire volume of the sample using the multi-layer technology described previously. It collects echoes from all material interfaces and identifies, by the absence of an echo, all locations from which no echo was received. To achieve maximum three-dimensional resolution, a large number of gates may be required, but the time required for scanning the part is the same as it would be if only one gate was needed.
Specialized software then processes the gate data to permit the manipulation needed to most usefully view the three-dimensional data. The three-dimensional acoustic images can show the sample from any perspective. The operator can also select various features to omit from a given three-dimensional image, which is often a necessity to present one feature from obscuring a more important feature. A variety of three-dimensional acoustic images can then be made from this data.
Figure 3 is the 3-D acoustic image of the same ceramic disk seen in Figures 1 and 2, but viewed from the side. The top surface of the disk is horizontal, so the top of the hole at the center of the disk is not seen. The parts of the image representing the bottom surface have been removed, along with the tubular hole in the center of the disk and exterior walls. The rest of the volume of the disk (the defect-free ceramic) is invisible acoustically because it is homogeneous and sends back no echoes to the transducer. This leaves only the three-dimensional images of the top surface (for reference) and of the voids inside the disk. The removed bottom surface lies just below the large collection of voids at the right.
Any number of three-dimensional images could be made by rotating the acoustic image in order to view the disk, and particularly the voids, from different locations around its circumference. This technique would be especially useful in a ceramic sample having multiple scattered internal features; it would permit seeing all of the features without grinding or otherwise destroying the part.
In Figure 4, the three-dimensional image of the disk has been tilted (but without horizontal rotation) to view a different perspective. First, the center hole is now visible at the top surface. Second, this perspective displays fine details of the large void complex at right, which is information that may be helpful in understanding how the voids were formed. By using the acoustic data to produce variously rotated and tilted images, the physically intact sample can be fully explored.
Multi-gate and 3-D acoustic imaging can be helpful quality control/assurance methods for manufacturers of advanced ceramics and other materials. The usefulness of these two methods lies in their ability—without damaging or destroying the sample—to manipulate acoustic data in ways that give the user more detailed views of the interior of a ceramic part.