Sharpen x-ray images

Using the industry's biggest inspection machines to examine its smallest objects sounds like a perfect paradox, but for x-ray inspection done during development, quality assurance, or off-line defect analysis, that's the situation. In addition to requiring a lot of floor space, x-ray inspection machines are relatively expensive and require manual operation, but despite such apparent limitations, these machines deliver micron-level detail and the ultimate image quality. With them, you can view defects in objects such as flip-chip solder balls or IC bond wires with diameters as small as 25 microns (1 mil). Although the machines are unlikely to grow in size, object sizes will continue their downward trend. How to maintain and surpass today's performance is the challenge for designers in leading x-ray companies.

Figure 1. Focal spot size of an x-ray source determines the blurred perimeter of the image. A true point source theoretically eliminates this blur.

More than any other factor, resolution determines what you can ultimately discern in an x-ray image. Image resolution depends mainly upon two factors: resolution of the image detector and finite size of the x-ray source. For viewing micron-sized test objects, magnification must exceed 100 times, and can go as high as 1000 times. Even at such a high magnification, 576x768-pixel detectors or newer 1000x1000-pixel detectors easily resolve the resulting x-ray images. As a result, image resolution depends almost entirely upon the dimensions of the x-ray source, or what vendors call "focal spot," which has units in microns.

Focal spot refers to the point source of x-rays that radiate from the target that exists within all x-ray tubes. The tube's applied voltage—in the range of kilovolts—accelerates an electron beam onto this target, which, in turn, emits the x-rays. In theory, the focal spot would have a diameter close to zero. In practice, though, the focal spot has a finite, but small, diameter, and this diameter largely governs the resolution of the image. In operation, you place your test object very close—approximately 0.5 mm (0.02 in.)—to the x-ray source and capture the image some distance

Figure 2. X-ray images using a) microfocus and b) nanofocus show voids in x-ray-sensitive IC underfill material developed by Kester, Germany. Courtesy of IZM and Feinfocus.

Figure 3. Images (< 1-micron focal spot and magnification > 500) show defects in 25-micron (1 mil) diameter bond wires: (a) Bond wire destroyed by over-current. (b) Bond wire crack approximately 2 microns wide. Courtesy of Phoenix X-ray.

For the majority of x-ray machines currently in use in the electronics industry, focal spot size ranges upward from around 5 microns—loosely described as microfocus. In the last year, though, a few x-ray vendors (see the Survey) have introduced machines with focal spots of less than 5 microns. Some nanofocus x-ray systems offer focal spots as small as 1 micron—called nanofocus.

Most machine vendors specify focal spot size, for which calibration methods exist. Others use subjective expressions such as resolution, detail detectability, or feature recognition, which you cannot calibrate. In practice, a focal spot size of around 1 micron means you can clearly recognize features in a test object down to 500 nm. Of course, this area of performance depends on what a skilled operator can see. What an operator can discern also depends on contrast in an image, which, in turn, relates to the absorption of materials in the test object. So, while focal-spot size remains the most important and reliable guide to image quality, some people may observe submicron defects on machines with focal-spot specs above 1 micron. Figures 2 and 3 show a selection of images of defects you can expect to see using these micron-level machines.

To understand how designers achieve micron-level focal points, it's useful to review how today's x-ray tubes work. While early tubes used solid targets to emit x-rays, microfocus tubes used in electronics inspection have transmission targets and have in-line geometry (Figure 4). Electrons enter the back of the transmission target, and x-rays radiate from the front. In all tubes, the electron beam enters the target and collides with the target material's particles. With each collision, the electrons slow down, and their loss in kinetic energy translates into radiation energy. Around 1% of the energy appears as x-rays; the remainder becomes mostly heat. For this reason, the designers choose a durable material, usually tungsten, although copper and molybdenum find use as target materials, too.

Figure 4. Microfocus x-ray tubes focus the electron gun onto a laminated transmission target that consists of a few microns of high-density material supported by a low-density backing. Courtesy of Feinfocus.

Figure 5. A laminated transmission target confines the source of high-intensity x-rays to a spot at the junction of the two layers. The curve shows the relative output distribution of high-, medium-, and low-intensity x-rays.

Designers can choose the relative thickness of the hard and soft layers to inherently filter lower-intensity x-rays and, in doing so, reduce the focal spot that outputs high-intensity x-rays. One drawback of this technique is that the majority of electrons rush through the thin high-density layer and end up producing only low-intensity x-rays in the backing material without any benefit. As a consequence, this type of target produces a much lower x-ray dosage than solid-target design of similar overall dimensions. One plus point is that a laminated target allows the designer to maintain a 160-kV acceleration voltage, which is necessary to produce x-rays with enough intensity for electronics inspection.

As well as providing a convenient support for the sputtered-on tungsten layer, the backing material provides a convenient transition surface from the evacuated electron gun to the outside world. Open-tube designs, now commonly used, let you apply and release the vacuum and, in turn, let you remove targets for servicing. In summary, the thickness of the micron layer of tungsten controls the focal spot, while the requirement to support a vacuum determines the thickness of the backing material.

Microfocus tubes rely on a single electromagnetic objective lens to focus electrons on to the target, as Figure 3 shows. Nanofocus tubes add a further electromagnetic condenser lens close to the alignment unit to pre-focus the beam even before it reaches the objective lens. The designs also introduce a further aperture close to the target to finely trim the electron beam before it reaches the target.

Figure 6. Klasen’s method uses the density trace output of a scanning densitometer to evaluate unsharpness (U) from the edge of an image. Courtesy of Phoenix X-ray.

Figure 7. Using a calibrated zone plate with a grid of x-ray-absorbing gold concentric circles and non-absorbing spaces allows visual determination of image resolution. Courtesy of Feinfocus.

Note: A version of this article previously appeared in Test & Measurement Europe.

The following company information appeared in the original print version of this article. For up-to-date information about companies, visit the Inspection Equipment portion of our Buyer's Guide.