Holger Roth, Phoenix X-ray Systems + Services
Wilhelm Conrad Röntgen, discoverer of radiation and after whom X-rays in German (“Röntgenbilder”) were named, realized its benefits for non-destructive testing. In his publication “Über eine neue Art von Strahlen” (About a new kind of rays), he mentioned a photograph of a piece of metal, “the inhomogenity of which becomes noticeable through the X-rays”. The explanation of the radiation mechanism immediately led to the conclusion that the resolution or sharpness of the images could be improved by reducing the size of the X-ray source, which means by reducing the diameter of the focal spot of the electron beam on the anode. However, the electron-optical instruments such as lenses, etc., necessary to achieve this in an effective way, had still not been invented at this early time.
In 1951 V.E. Cosslett und W.C. Nixon (Cambridge) reported of an “X-ray shadow-microscope”, which they had constructed utilizing a magnetic electron-lens as proposed earlier by Manfred von Ardenne. Another two decades later, this technique found its first industrial applications when micrometer scale defects became an issue due to miniaturization of electronic devices and assemblies as well as increased reliability requirements.
In a microfocus X-ray inspection system, the specimen is irradiated by a cone-shaped beam. It generates a magnified image on a detector which , in most cases, is still an image intensifier. The detectabilty of structures or details through an X-ray image is ruled by three properties: magnification, resolution and contrast.
The resolution (image sharpness) of a system is mainly determined by the size of the X-ray source which is in the range of a few microns for microfocus X-ray tubes. The geometric magnification is given by the geometry of the beam (illustrated in figure 1), and can be as high as 1000x to 2500x at practicable distances. Apart from resolution and magnification, the recognition of an individual object feature depends on the contrast induced by this detail to the image. Physically, the contrast originates from different X-ray absorption properties of different object areas, and can be caused by varying thickness (e.g. shape of a solder joint), as well as by different materials such as copper conductors on a printed circuit board (see figure 2).
The detectabilty of a given contrast is technically limited by the detector properties. For the image intensifier, as a rule of thumb, we may say that the absorption difference must amount to at least 2% to be detected in the X-ray image. For a contrast solely caused by varying thickness, in first approximation a linear relation to the absorption difference can be assumed so that the difference in thickness must also exceed 2% of the total thickness to be visible in the image.
