Ultrasonic C-scan imaging can provide high-speed, nondestructive flaw detection and material characterization
In any high-tech application, quality is of the utmost importance. The ability to detect even the smallest flaws and material imperfections is crucial to assessing performance, ensuring customer satisfaction and increasing sales. Because ceramic components can be tested over a broad ultrasonic frequency range, ultrasonic inspection is becoming widely accepted as a way to detect a variety of defects and measure both the density and homogeneity of samples. When paired with C-scan imaging, ultrasonic inspection provides a highly accurate, nondestructive means to ensure product quality.
Figure 1. High-frequency sound waves are sent into the material being tested, and the reflections are analyzed to identify defects and pinpoint their locations.
Advanced Ultrasonic Technology
Ultrasonic testing is a technique in which high-frequency sound waves (1 to 250 MHz) are sent into the material being tested, and the reflections are analyzed to identify defects (voids, pores, etc.) and pinpoint their locations (see Figure 1). The frequency of the reflected sound wave can indicate what type of defects or imperfections exist in the material, while the amplitude of the return sound waves provides information regarding the size of the defect or voids, as well as the inclusion and/or dispersion of defects within the material.
The time-of-flight of the reflected sound wave is also analyzed, and this information can indicate both the location of any defects and the material's sound velocity. For manufacturers that are looking for specific information about a material in addition to or instead of defects, the sound velocity can be used to characterize the material being analyzed, as well as provide a calibration and accurate measurement of the material's depth.
Conventional ultrasonic testing is generally a contact test and is subject to the operator's ability to efficiently hand-scan a part. It is limited to frequencies up to 25-30 MHz, and the results must be analyzed by reading a scope-type display or listening for an audio alarm.
Ultrasonic C-scan imaging, on the other hand, is generally a non-contact test and can automate the scan. It can operate at much higher frequencies (up to 250 MHz) and find smaller defects, down to 5-10 µm. Additionally, the results of the analyses are provided as a permanent image, as well as a digital automated analysis, which is more efficient than conventional methods. For this reason, many manufacturers that want the accuracy of an ultrasonic test without the hassles of conventional instruments are turning to ultrasonic C-scan technology.
An ultrasonic C-scan system typically consists of a computer, mechanical scanner, analog-to-digital converter, motion control subsystem, ultrasonic pulse receiver, ultrasonic transducer and system operating software. The transducer frequency is selected to be sensitive to a minimum defect size, which is usually determined empirically through a feasibility study. The materials engineer that sets the acceptance criteria for the component(s) being tested derives artificial defects from failure analysis testing. A reference standard with artificial defects is produced from the material, and this standard is used to qualify the ultrasonic C-scan inspection.
The size and shape of the parts/material to be ultrasonically tested will dictate the complexity of the mechanical scanner. A flat part, such as ceramic capacitors and many other electronic components, can be scanned simply with a three-axis scanner (X, Y and Z axes), while a cylindrical part would require four axes (X, Y, Z and turntable). Contour-shaped parts typically require six or more axes (X, Y, Z, turntable, gimble and swivel) for accurate analysis. The software is selected to match the motion control requirements that are related to the scan area to be inspected, while the scanner immersion tank and bridge can be customized to most part sizes and shapes. The inspection scan area, as well as the type and size of defects to be detected, are generally defined by the end user of the part.
Figure 2. An A-scan image of a ceramic component.
Identifying and Analyzing Defects
In an ultrasonic C-scan analysis, the instrument scans the part and initially displays the ultrasonic signal response on an A-scan-a scope-type display based on radar technology in which radio frequency (RF) waveforms show the ultrasonic signals and reflections from the ceramic component (see Figure 2). This display is used to set up the data collection gates, which are individual areas on the A-scan that correspond to specific depth ranges. The peak amplitude of the signal is then converted to a color or grayscale image called a C-scan, which shows the variations within the part. The data gates can either be placed along a single timeline (depth) to detect defects in the full volume of the material, or they can be located at various depths within the thickness of the material. Each data gate produces a separate C-scan image, which allows the part to be profiled through a top view (like an X-ray) or through individual layers.
Figure 3. In this C-scan image of a ceramic component, voids as small as 50 microns appear as small blue specs scattered throughout the material.
Figure 3 shows a C-scan image of a ceramic component in which small indentations, or voids, were detected. These reflections appear as small blue specs scattered throughout the part and are sized as small as 50 microns. (In some cases, depending on the application, it might be necessary to set the sensitivity of the instrument to detect flaws as small as 5-10 microns.)
Figure 4. A close-up of a random location on the full C-scan image shown in Figure 3.
Figure 4 is a close-up of a random location on the full C-scan image shown in Figure 3. By zooming in on the original C-scan, the operator can more easily observe and measure the size and shape of the voids to determine whether the part meets quality specifications. With some C-scan instruments, this qualification process can be automatically handled by the system.
Figure 5. An automatic analysis of the C-scan image determines that about 20% of the portion being analyzed contains voids.
The enlarged C-scan image can also be analyzed to determine the percentage of void indication compared to the area(s) with no voids. The operator simply selects an area of interest, and the system automatically calculates the percentage of voids in that area. If the void content exceeds the acceptable criteria, then the part is rejected. In Figure 5, the entire image is shaded, which denotes the area under analysis, and the analysis software has determined that about 20% of the shaded area contains voids.
The amount of time required for the inspection depends on the instrument used, the data detection density and the size of the defect. For the example shown in Figures 3-5, a 50 x 50 mm component was scanned at a data collection increment of 100 microns using a high-speed/high-resolution instrument,* and the inspection took approximately 60 seconds. Other systems and analyses might require more time to complete.
The system can be used in a separate failure analysis/quality control lab, or it can be customized to operate on-line during the production process. In either case, the information from the analysis can be fed back to the manufacturing process, where the necessary changes can be made to eliminate the cause of the defects/voids.
*Supplied by Matac Micro Electronics, Yardley, PA.
Improving Product Quality
Ultrasonic inspection can be a useful tool in detecting defects or voids in ceramic components. With C-scan imaging, defects can be automatically pinpointed and analyzed to determine their density, location (X, Y and Z) and shape without physically handling the component. Combining these two technologies provides a fast and reliable method for analyzing ceramic components and improving product quality.
For more information about ultrasonic C-scan imaging and analysis, contact Matec Micro Electronics, 301 Oxford Valley Rd., Suite 703B, Yardley, PA 19067; (215) 369-8077; fax (215) 369-9577; e-mail firstname.lastname@example.org
; or visit http://www.matec.com