FEATURE Machine Vision
Product quality testing with Scanning Acoustic Microscopy
Stephen Armstrong, technical writer based in the US, discusses the options avaiable to manufacturers when it comes to non-destructive testing of products
U
ltrasonic-based Scanning Acoustic Microscopy (SAM) has long been the method of choice for quality testing
and failure analysis of silicon ingots, wafers, integrated circuits, MEMS and other electronic packages. 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. Increasingly, however, manufacturers in the industrial, aerospace and medical sectors are turning to SAM technology to ensure good adhesion and mechanical integrity of devices by examining their internal structures, interfaces and surfaces. Because potential defects can occur in diff erent layers, more advanced equipment is required to inspect each simultaneously.
In-house testing or outsourcing? Many are choosing to outsource SAM services rather than do this quality testing in-house. Outsourcing off ers manufacturers advantages, including capital cost savings on SAM systems, access to specialised expertise in image management and analysis and the ability to diagnose and isolate material defects. “Often, when customers come to us, they don’t know what problem they have. If it is relatively easy to solve, investing the time and expense
24 March 2022 | Automation
in bringing a SAM system in-house is unnecessary,” said Lisa Logan, SAM Applications Manager, at Sunnyvale, California-based PVA TePla America. “As part of product R&D, an engineering team may be evaluating welds, bonds or the eff ectiveness 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.”
Contracting SAM services can also be a better match for one-time projects. “The customer’s project may be fi nite. They may only need to scan 10,000 parts, and then the project is done. It is much more cost-eff ective to outsource the work in this case,” said Logan. “Convenience is a big factor, too. It is just simpler to give parts to a trusted partner to evaluate. Our customers often give us a part to analyse when they can’t isolate the issue and ask us to use our expertise to determine the problem for them.”
Problem solving
Newark, California-based California Brazing uses SAM testing to validate and verify processes required by its customers and to comply with American Welding Society specifi cations for brazing materials such as aluminum, stainless steel and copper. Metals are joined together in the brazing process by melting and fl owing a fi ller material into a joint without melting the workpieces. “Brazing specifi cations defi ne 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.”
The testing is initially done on a new part, particularly for aerospace and defence projects, which require very specifi c and unique specs. Parts can be all sizes – anywhere from 1x1 inch to 30 x 30 inch.
Ager said: “If necessary, we follow-up with destructive testing too, but only after we’ve identifi ed the precise location of the failure using SAM.”
Superior non-destructive testing 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, refl ected (scattered at 180 degrees), or transmitted (scattered at 0 degrees). By detecting the direction of scattered pulses and the “time of fl ight”, 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
automationmagazine.co.uk
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