By Stuart Wright, X-Tek, London, England
X-rays are produced when free electrons (usually from a heated filament) are accelerated in a vacuum into a target material. In operation, incident electrons impart their energy into the orbiting electrons of the atomic structure of the target. This produces an unstable state in the target, and when the electrons return to their stable state, the energy from the incident electrons is released as x-rays. X-Ray Systems Because of the dangers associated with x-ray equipment, most people stay away from the technology unless there is no alternative. This results in a relatively high level of misunderstanding and confusion regarding the difference between the types of x-ray systems in use around the world. A few of the most common applications are medical, security (airport baggage), site radiography (welds in oil pipelines), closed-cabinet casting inspection (alloy car wheels and aircraft components) and food inspection (bone fragments in meat products and metal or glass debris in cans and jars). The applications listed above produce a 1:1 or reduced size image of something quite large on a small screen. What makes the applications in electronics different from the others is that the sample is very small and of a complex nature. This requires a highly magnified and high-resolution image, often to see features of just a few microns. The resolution of the x-ray detector is limited by current technology so that electronic magnification of the captured image does not reveal further detail. The only practical way to see features of a few microns is to geometrically magnify the x-ray pattern before converting to a visible or video image. Microfocus Source Geometric magnification of the x-ray image with high resolution is made possible by the use of a microfocus x-ray source (Figure 1), which is the key to systems designed for the electronics market.
In a microfocus source, the electrons, after acceleration inside the vacuum envelope, are focused to a fine point using some form of electrostatic or electromagnetic lens onto the target. X-rays are only produced from the target at the point where the high-energy electrons strike. If the electrons are all focussed to the same point, this becomes the x-ray emission point or "focal spot" of the x-ray source. This spot is normally less than 10 microns, compared to conventional x-ray tubes, which often feature a focal spot of 500 microns. Geometric Magnification Magnification is achieved by positioning the sample between the x-ray source and an imaging system. This positioning casts an x-ray shadow of the sample onto the x-ray-sensitive video system in the same way that a magnified shadow of a person's hand can be projected onto a screen using a flashlight in the dark. Open Versus Sealed Tubes For highest magnification, it is advantageous to reduce the distance from the sample to the x-ray focal spot. This, however, is usually limited by the x-ray tube's design. There are two tube families: sealed and open (or vacuum de-mountable) tubes. Sealed tubes are manufactured using a glass or ceramic housing with the cathode, anode, target and focussing apparatus inside. The air is pumped out, and the housing has a permanent seal, rendering the whole assembly useless if any of the components inside fail. This category produces tubes that are easy to use, have a limited performance, are low cost and exhibit a reasonable but unpredictable life span. Open tubes contain the same components as the sealed type. The housing, however, is machined from metal, and the vacuum is created by a sophisticated pumping system that runs continuously while the system is switched on. If any part fails, the tube can be opened and the parts replaced on site. Open tubes offer the highest performance, require some preventative maintenance, are more expensive to buy but have no end of life. Magnification Stages There are three magnification stages in an x-ray system. The first and most important is the geometric magnification, which can be increased in two ways. One way is by making the distance from the sample to the focal spot as small as possible. This is the preferred method and is the sign of a good system. The second way is to increase the distance between the sample and the imaging system, which results in a low-contrast, noisier image. The second magnification stage is produced when the x-ray image is transferred from the imaging system to the display screen. If the imaging device is very small and the display screen is very big, this will produce a large magnification. The input window of the imaging device should always be bigger than the maximum field of view required by the application, however. For example, for inspecting a BGA package, a 4-inch image intensifier is usually the smallest recommended. Display screens are not normally larger than 14-inches in diameter because screen resolution is optimized at about this size. The third stage of magnification is not normally used, but can be provided by electronic pixel magnification in the image processor. A system with a magnification of 200X is adequate for standard BGA balls of around 1 mm in diameter. However, with CSP and flip-chip solder connections becoming smaller, when inspecting for cracks or voids in the smallest joints, it is important to be able to resolve down to 1 or 2 microns. Reaching to this level requires a magnification of more than 1000X.
Image Sharpness The factors that affect the image sharpness are easy to define. They are the resolution of the detector system, which affects the sharpness of a low-magnification image, and the size of the focal spot, which determines the resolution of a high-magnification image. The effect that focal spot size produces on image sharpness can be shown by Figure 2. As the focal spot increases, so does the unsharpness of the x-ray shadow. X-Ray Penetration The thickness of any material that an x-ray can penetrate depends on the energy of the x-ray. This energy is measured in kV. As a rule of thumb, 30 kV is enough to penetrate most plastics found on a populated PWBs. However, energy on the order of 100 kV will be needed to penetrate a BGA's 1 mm solder ball. If an assembly is to be viewed at an angle (or contains any thick metal layers), then the energy needed will be higher, 160 kV being standard with most systems. Intensity The intensity of X-rays at any particular energy can be controlled by changing the milliamps (mA). This will alter the effective brightness, and a high mA will result in better image contrast. There are two detrimental side effects to increasing the mA, however. First, space charge effects in the electron beam expand the focal spot, reducing the resolution. Second, a high mA causes high power dissipation in the target, which can melt, resulting in a breakdown of the tube.
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