It’s fun to look at the ads of bygone eras and reminisce (or maybe just laugh) before the centralization of the internet. When kids still read comics and print magazines, there would be ads hawking typical products that appeal to children: candy, toys, breakfast cereal, and more. Looking back, some of my favorites were the novelty products at the back of a comic book, with none being more famous than the “x-ray specs.” The glasses, of course, did not project x-rays, rather they placed a slit in two paper lenses with a thin material interspersed between them to diffract light and form two semi-overlapping images.
In reality, x-ray inspection has transformed our ability to view internals. While most of us are familiar with x-rays at the dentist or doctor’s office, they also have a relevant use in electronics manufacturing. Automated x-ray inspection, or AXI testing, is a crucial quality checkpoint that allows production to evaluate and refine processes while maximizing yield.
Demystifying X-ray Inspection Technology
PCBs are highly technical productions, marrying many different scientific disciplines. At its most basic level, a completed PCB is the culminating work of many fabrication steps and the addition of meticulously-built parts. Although shops and component manufacturers are vested in reducing the incidence of failure, it’s unrealistic to expect large quantity runs not to experience some failure. The task then becomes recognizing failure modes and ferreting them early as a cost-saving measure and, hopefully, a tool to further diagnose issues during manufacturing for future runs.
A major difficulty, however, is the inability to properly assuage completed work once it is bonded with other pieces. Solder joints, formed between components and the bare board, establish electrical and thermal continuity and allow for the typical functionality associated with electronics. Based on the complexity of the assembly, solder joints can be small, numerous, and densely distributed in such a way that makes standard optical inspection highly impractical.
AXI testing cuts to the heart of the matter by bombarding the PCB with high-intensity, short-wavelength light to assess the solder structure underneath leads. Different materials exhibit different rates of absorption/scattering due to their density. A detector opposite the PCB side facing the x-ray source will produce grayscale spectra depending on the amount of light passing through. This process is useful for all package types. Still, it becomes a necessity for BGAs, LGAs, and other similar devices where the leads are obscured by the package itself, and the low profile of the case prevents any possible viewing angle.
AXI testing can create images in two- or three-dimensions, depending on the technology utilized:
- 2D: Also known as transmission imaging. X-rays generated at a source pass through the board to a detector. Light passes easily through the substrate and with relative difficulty through solder metals, particularly in cases where leaded solder is still permissible.
- 3D: AXI machines take cross-sectional slices using a method known as laminography. The source and detector are displaced relatively to blur out the non-slice portions of the board selectively. Resolution is measured in the ability to take smaller slices of images. The number of slices for proper placement and solder detection can vary depending on the solder joint. BGAs require additional imaging at the center of the solder ball to detect its circumference. Still, more standard packages will only need to track the component’s orientation and the volume of solder deposited.
The Role and Value of AXI Testing
Where does the automation in AXI come into play? Computer systems can be trained on correct models to provide a heuristic for determining what constitutes a passing and failing solder joint. This machine learning is not instantaneous, instead taking numerous examples under training supervision to form its criteria. However, once trained, an AXI test can achieve high levels of defect discovery, even in HDI designs. Additionally, a massive increase in processing speed is realizable, further endearing the technology to high-volume lots.
Unsurprisingly, 3D imaging performance far exceeds that of 2D imaging, although the latter can still act as a valuable process indicator and comfortably detect at an overall high level. Whereas 3D imaging can identify several different factors of a solder joint, a 2D image can only determine a solder joint based on its area; area alone may be insufficient when determining the quality of a solder joint. However, this detection method is computationally lightweight and can be combined with other tests to diagnose the circuit’s potential failures accurately. From a more general production standpoint, AXI of any dimension can track the occurrence of failure modes throughout multiple revisions to ensure the minimization of yield loss due to manufacturing error.
Despite the sophistication of 3D AXI imaging, the procedure still has defects that may pass through undiagnosed. An x-ray image of a solder joint may show sufficient solder on a pad, yet the solder may have failed to wet and wick around the pin properly. Cases like these can be missed by x-ray inspection alone, but the takeaway should not be the false positive errors found in AXI testing. Instead, the focus should be on the defect coverage, and convenience AXI does provide. Examples include the ability to gauge otherwise unviewable solder joints accurately and the speed with which it can assess boards with bonds numbering in the tens of thousands.
Your Contract Manufacturer Offers Comprehensive Testing on Every Board
AXI testing is an invaluable diagnostic tool that, alongside other common inspection/evaluation methods such as AOI, ICT, and functional testing, bestows a high degree of manufacturing confidence in error detection. Established PCB manufacturers are well-suited not only to produce boards that minimize the occurrence of errors but also lend themselves to ease of detection with many common failure modes.
Whether you’re working on a board facing production-related revisions or entering uncharted waters with an NPI, VSE is here to help. At VSE, we’re a team of engineers who build electronics for our customers, and we take immense pride in our work, bringing customers’ products to fruition for over forty years. Operating with our valued manufacturing partners, we aim to deliver nothing less than an utterly exceptional PCBA.
