Closing the gap between AXI and semi-automated systems
Boris Mathiszik and Holger Roth, phoenix|x-ray Systems + Services -- Test & Measurement World, 8/1/2005
Manufacturers of industrial x-ray equipment judge the quality of an x-ray system by the images it produces and the information derived from those images. While the hardware is certainly a key to system quality, it is software that differentiates the many low-cost manual systems from the mid-priced semiautomated and higher-priced automated systems.
The line that separates these inspection classes, however, is beginning to blur. Semi-automated machines now include sophisticated image-processing software once found exclusively on automated x-ray inspection (AXI) systems, and they can replace AXI systems in many applications.
Obtaining quality images
In electronics imaging, magnification and focal spots are often misunderstood. While many manufacturers quote the software magnification (or pixel magnification), it is a system's geometric magnification that places limits on image quality (Figure 1). Most x-ray machines achieve high magnification by moving the target, the detector, or both, which takes time. Thus, AXI machines often sacrifice magnification to minimize test time.
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| Fig. 1 The geometric magnification of an x-ray shadow microscope equals the focus-to-detector distance (FDD) divided by the focus-to-object distance (FOD). |
At higher magnifications, the focal spot size of the x-ray source limits resolution and causes image blurring or a "penumbra" effect. The image on the left in Figure 2 represents an image from a large-spot size x-ray tube architecture that can resolve images down to a few microns, a technology that was introduced about 20 years ago. Newer designs improve on that performance by an order of magnitude. By manipulating additional lenses and apertures to produce a converging beam hitting the output tungsten target, the resulting tube provides an extremely small focal spot that reduces the penumbra effect (right side of Figure 2). Incorporating this capability into semi-automated x-ray systems allows those systems to meet customer demands at a lower cost.
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| Fig. 2 Because of the penumbra effect, the size of the source limits the resolution of the image. |
Achieving a high-quality image also requires a good detector. The quality of the detector—even for the latest digital types—primarily affects image contrast. Other techniques that aid in capturing clear images include tilting the target and a technique that we at phoenix|x-ray call oblique view at highest magnification (OVHM). Although effective, tilting takes time for switching to the oblique angle and realigning the often shifting field of view. Our OVHM technique allows full off-axis detection while keeping the target close to the focal spot. We find that the technique delivers high-quality images in a reasonable time with no loss of magnification.Software
Software and image-analysis tools are becoming the differentiating features that separate less than $100,000 x-ray systems from semi-automated and AXI machines. Fast, effective image-analysis software has more impact on speed, accuracy, and repeatability than any other aspect of the system. Software can also blur performance differences between AXI and semi-automated tools, making automated handling the primary difference.
Whereas most low-cost semi-automatic x-ray machines include limited software packages, many of the tools used on AXI and automated optical inspection (AOI) machines are migrating down to the mid-priced semi-automatic systems and are changing the way these machines are being used. Suppliers of semi-automated x-ray systems have begun to add BGA defect coverage ranging from ball presence/absence and bridging detection to void analysis, diameter measurements, roundness checks, and automated wetting analysis. For IC packaging applications, the tools perform automated void detection for conductive and nonconductive die attach, solder-bump inspection, wire-bond integrity checks, and wire-sweep measurements.
In the latest software packages, standard BGAs, fine-pitch BGAs (FBGAs), and chip-scale packaging (CSP) device libraries allow users to teach inspection systems to recognize component layout, proper solder diameter, and acceptable gray levels with minimal setup time. Many packages even offer default threshold values, reducing setup times even further.
Like AXI and AOI systems, semi-automated machines are using image-processing software packages to import PCB CAD data, reducing setup time. The software integrates automated inspections, automated pass-fail decisions, and data links to rework or paperless repair stations.
Defect coverageManufacturers typically use semi-automatic x-ray inspection equipment in two ways—either as manual real-time failure-analysis tools in laboratories or as quality-assurance tools in production. Some semi-automatic x-ray systems now incorporate AXI tools for production applications.
For BGA, CSP, and flip-chip applications, automated inspection using oblique views has increased defect coverage. AXI based on oblique viewing provides two advantages. First, angled views and sample rotation allow programmers to maximize signal-to-noise ratios on double-sided boards where signals from the other side of the PCB blur or deform the signal of the primary inspection target. Second, oblique viewing enables automated wetting inspection.
Customers of semi-automatic x-ray equipment continue to ask vendors to extend their automated defect coverage beyond standard BGAs. Typical applications include inspection of gull-wing devices such as quad-flat packs (QFPs), plated-through holes (PTHs), and connector pins. Adding such measuring techniques enables new customers to increase the automated defect coverage without buying a fully automated machine. Existing users can also take advantage of the new features at little extra cost. Unavoidable extra costs include acquiring the appropriate software modules, minimizing programming time, and potentially increasing inspection cycle time. The cost savings result from less rework and scrap in the field and from transferring test coverage from a later manufacturing and test process step to an earlier step—typically after reflow in the SMT process.
True submicron x-ray inspection systems have been the "holy grail" of the x-ray industry for many years. Yet, submicron x-ray tubes are unlikely to replace large-spot tubes for all inspection applications in all industries. Lower resolution systems will still be used to inspect large, dense objects, because their ability to create a large x-ray flux cannot be duplicated with submicron sources.
| Author Information |
| Boris Mathiszik is VP of phoenix|x-ray's East Coast division. He has been involved in product management for AOI and x-ray equipment for the past 10 years. bmathiszik@phoenix-xray.com. |
| Dr. rer. nat. Holger Roth is applications manager at phoenix|x-ray Systems + Services in Wunstorf, Germany, and is in charge of the company's inspection service laboratory in Stuttgart. hroth@phoenix-xray.com. |
