ften used for quality testing and failure analysis of silicon ingots, wafers, integrated circuits, MEMS, and other electronic packages, manufacturers in the industrial, aerospace and medical sectors are increasingly turning
to Ultrasonic-based Scanning Acoustic Microscopy (SAM) technology to ensure good adhesion and mechanical integrity of devices by examining their internal structures, interfaces and surfaces. Ultrasound can locate voids and disbonds between material layers better than other non-destructive methods because sound waves can look inside the layers to detect the thinnest of air gaps and delaminations down to a hundredth of a micron. Many manufacturers, however, are choosing to outsource this task. Lisa
Logan, SAM applications manager at PVA TePla America, provides contract services and sales for both PVA TePla Analytical and OKOS, both of which design and manufacture advanced Scanning Acoustic Microscopes. “As part of product R&D, an engineering team may be evaluating welds, bonds or the effectiveness of an adhesive. When a product is in production, we may be asked to scan trays of parts for quality assurance testing. We may also use SAM testing to investigate why a particular part failed.” California Brazing uses SAM testing to validate and verify processes
required by its customers and to comply with American Welding Society specifications for brazing of materials such as aluminium, stainless steel and copper. Metals are joined together in the brazing process by melting and flowing a filler material into a joint without melting the workpieces. “Brazing specifications define accepted quality requirements for analysing
internal discontinuities using non-destructive testing,” said Jeff Ager, general manager of California Brazing. “In our experience, ultrasound [SAM]
gives us the highest resolution images, and they are also easier to interpret than with x-ray technology.” “We use the SAM
technology to validate and verify that our processes are always in spec. This testing happens initially on a new build of a part, particularly for our aerospace and defence customers, because of their unique requirements. These parts can be all sizes – anywhere from 1x1in to as large as 30x30in.” Ager added: “When we have potential brazing failures to investigate, we
first go to Lisa and her team at PVA TePla for testing to pinpoint exactly where it occurred. If necessary, we follow-up with destructive testing too, but only after we’ve identified the precise location of the failure using SAM.”
SAM uses the interaction of acoustic waves with the elastic properties of a specimen to image the interior of an opaque material. A transducer, the heart of a SAM system, directs focused sound at a small point on a target object. The sound hitting the object is either scattered, absorbed, reflected (scattered at 180 degrees) or transmitted (scattered at 0 degrees). By detecting the direction of scattered pulses and the ‘time of flight’, the presence of a boundary or object can be determined as well as its distance. To produce an image using SAM, samples are scanned point by point and
line by line. Scanning modes range from single-layer views to tray scans and cross-sections. Multi-layer scans can include up to 50 independent layers. The resolution of the microscopic image depends on the acoustic frequency, material properties, and aperture of the transducer. Transducers perform such a critical role that manufacturers like PVA TePla design and manufacture an extensive range of different transducers used in their contracted testing services. The frequency of the ultrasonic signals can be increased to the GHz range, which makes it possible to detect defects in the sub-micron range. PVA’s contract testing services are performed only on equipment the
company manufactures, namely the 300, 302 HD2, 500 and 501 HD2 scanners. These scanners provide scanning ranges from 200µm x 200µm to 500mm x 500mm using transducers up to 400MHz. PVA’s proprietary transducers deliver high image resolution to evaluate the integrity of each part. Operating a SAM system requires a trained technician and experience
configuring the equipment and interpreting the scans. SAM system experts know how to work with three different imaging modes: A, B, and C. The A mode is an X, Y or Z point and provides information on all the echoes
occurring inside of a part. These echoes provide insight into material analysis, time-of-flight imaging, amplitude and polarity. The A scan must be interpreted to produce an accurate B or C scan, which is shared with the customer. According to Logan, PVA TePla typically tests products ranging from the
smallest electronic components to 50-pound aluminium parts. She said: “The smallest and thinnest parts increase the difficulty of interpretation because the echoes of the interface become really tiny and close together. It causes an overlap of echoes, and you end up having to adjust the frequencies used. As you go higher up in frequency, it is more difficult to manage the transducers.”
When selecting a SAM partner, manufacturers should consider how well the SAM equipment matches their testing needs and the breadth and depth of experience of the testing team. In-depth knowledge of the SAM equipment is critical to optimise image resolution.
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