High-purity metals and alloys such as aluminium, zinc, cobalt, copper, magnesium, molybdenum, stainless steel, titanium, zinc and zirconium are used to produce components in many industries, from electronics to aerospace and medical device. Due to the critical nature of many of these components, said high-purity metals and alloys should be highly consistent and have extremely low levels of impurities and contaminations. This is driving the use of scanning acoustic microscopy (SAM), a non-invasive and non-destructive imaging technology, in the identification of small inclusions and other defects as small as 50 μm.
SAM technology
SAM works by directing focused sound from a transducer at a small point on a target object. The sound hitting the object is either scattered, absorbed, reflected or transmitted. By detecting the direction of scattered pulses as well as the time of flight, the presence of a boundary or object as well as its distance can be determined.
To produce an image, 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. Depth-specific information can be extracted and applied to create two- and three-dimensional images without the need for time-consuming tomographic scan procedures and more costly X-rays. The images are then analysed to detect and characterise flaws such as cracks, inclusions and voids.
SAM is widely relied upon in the semiconductor industry for 100 percent inspection of components, allowing for quality and failure analysis through the identification of defects such as cracks and voids as well as the delamination of layers in microelectronic devices. More recently, it is being increasingly applied to high-purity metals and alloys for the identification of subsurface cracks, dis-bonds, flaws and other irregularities. However, until recently, the challenge has been performing 100 percent inspection at sufficient throughput speeds. In the case of other inspection systems, increasing scanning speed has meant sacrificing scanning image resolution.
The OKOS VUE 250-P scanning acoustic microscopy (SAM) system.
Modern SAM systems
Fortunately, recent advancements in SAM systems have significantly improved throughput speeds and defect detectability. A conventional 5 MHz sensor can take up to 45 minutes to inspect an 8–10-inch square or disc alloy, but an advanced, phased array of 64–128 sensors and advanced software to render very high resolution images can reduce that time by up to 40 minutes as well as afford more granular detection of small impurities or defects.
This dramatic increase in SAM system inspection speeds means speciality metals companies are no longer limited to selective sample testing during in-process quality control (QC). SAM has long been utilised for this type of inspection but previously involved a handheld unit or a multi-point inspection. It is now more feasible to conduct 100 percent inspection of the entire surface/interface.
The latest SAM systems make it possible for speciality metals companies to move to a higher level of failure analysis because of the improved levels of detection and precision they offer. In the past, the requirement was to be able to detect a 500 μm defect, but today, it is to be able to detect a 50 μm defect. Also, in terms of high-purity metals, it is desirous to inspect every item, not just a few samples.
If 100 percent inspection is required at high throughput, ultra-fast single- or dual-gantry scanning systems are utilised alongside an advanced, phased array of 64–128 sensors. Multiple transducers can also be used to simultaneously scan for higher throughput.
A multinational, Fortune 500 company recently tested an OKOS SAM system on a sample of aluminum and a sample of steel, scanning took three minutes on each and it had previously taken 40 minutes.
Acoustic microscopy software
As important as the physical and mechanical aspects of conducting a scan, the software is critical to analysing the information and improving the resolution to produce detailed scans. OKOS therefore decided early on to develop a software-driven, ecosystem-based solution, the objective being to deliver high-quality inspection results at fast speeds as well as reduced costs.
The company’s ODIS WinSAM acoustic microscopy software supports a wide range of transducer frequencies, from 2.25 to 230 MHz. Other features include A-, B-, and C-scans, contour following, off-line analysis, and virtual rescanning, all helping to facilitate highly accurate internal and external inspection for defects and thickness measurement of composites, metals and alloys.
Summary
In the electronics, aerospace, medical device and other industries, there is an increasing demand for inspection equipment that can perform non-destructive imaging and materials analysis. The combination of an advanced, phased array of sensors and advanced software to render very high resolution images enables speciality metals companies to inspect 100 percent of their materials at a level one to two orders of magnitude higher to discover previously undetectable flaws.
The OKOS MACROVUE-P SAM system.
OKOS