Outsourced SAM testing provides a solution
Ultrasonic-based Scanning Acoustic Microscopy (SAM) has long been the method of choice for quality testing and failure analysis of silicon ingots, wafers, integrated circuits, and MEMS. 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 different layers, more advanced equipment is required to inspect each simultaneously.
Many are choosing to outsource SAM services rather than do this quality testing in-house. The company claim that outsourcing offers manufacturers key 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.
“Customers typically come to us for SAM testing to address one of three needs,” said Lisa Logan, SAM Applications Manager, at Sunnyvale. “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.”
“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 in bringing a SAM system in-house is unnecessary,” she added.
Contracting SAM services can also be a better match for one-time projects.
“The customer’s project may be finite,” said Logan. “They may only need to scan 10,000 parts, and then the project is done. It is much more cost-effective to outsource the work in this case.”
“Convenience is a big factor, too,” added Logan. “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.”
Newark uses SAM testing to validate and verify processes required by its customers and 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 defense customers, because of their unique requirements. These parts can be all sizes, anywhere from 1" by 1" to as large as 30 by 30".
He 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.”
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, reflected (scattered at 180°), or transmitted (scattered at 0°).
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.
At PVA, their 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.
Given the role it plays, the company say that in-depth knowledge of the advanced software features available is another considerable benefit.
“SAM testing houses that purchase equipment may only be familiar with some of the features of the software,” said Logan. “As the manufacturer, we have to understand all the features and can even request customisations if needed.”
A matter of interpretation
The quality of the equipment and knowledge of its use is only one factor in SAM testing. Operating a SAM system requires a trained technician and experience configuring the equipment and interpreting the scans. Setting up the scans and interpreting the images is similar to a radiologist reading MRI scans of a medical patient.
“Imagine if you got an MRI,” said Logan. “Wouldn’t you want your doctor to have the clearest image possible to make the best medical decision? When you realise that an undetected flaw can have a catastrophic impact on a part, it’s the same for quality testing too.”
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 valuable insight into material analysis, time-of-flight imaging, amplitude, and polarity. The A scan must be interpreted appropriately to produce an accurate B or C scan, which is shared with the customer.
“An operator needs to interpret, focus and change the volume of the A scan as well as adjust certain things for the image to be accurate,” said Logan. “It is important to ensure that the image does not convey something false.”
According to Logan, PVA TePla typically tests products ranging from the smallest electronic components to 50-pound aluminum parts.
“The smallest and thinnest parts increase the difficulty of interpretation because the echoes of the interface become really tiny and close together,” said Logan. “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.”
According to California Brazing’s Ager: “We have thought about bringing ultrasonic testing in-house, but to be able to do it well, we need someone who can read the images and help interpret them. It is not easy to find people who have that depth of experience.”
“We have a very good working relationship with PVA TePla,” said Ager. “As a result, there is very little interaction needed because she’s so familiar with our process and the types of products we manufacture. I think that’s the biggest advantage, a deep understanding of our process.”
“We typically get our results from PVA TePla in a few days, said Ager. “We might have a conversation about focusing further on a particular area or a certain level, after which Lisa would go back and focus on that.”
Finding your SAM partner
When selecting a SAM partner to outsource quality testing and failure analysis, 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. It is believed by the compant that the ideal situation is having factory-level experience in the manufacturing of acoustic microscopes and their software development.
With a strong SAM outsourced partner, manufacturers effectively add a powerful resource to resolve their quality testing and failure analysis challenges.