X-ray Inspection: Evolution from Microfocus to Nanofocus
12/01/2003
MANUFACTURING TECHNOLOGIES CREATE CHALLENGES IN THE INSPECTION OF COMPONENTS AND PACKAGES
By Norbert Daneke and Bianca Schanklies
Today's trend of increased miniaturization of components and packages is continuing in the electronics and micromechanics industries. Design and manufacturing technologies such as optoelectronics, microelectromechanical systems (MEMS) and micro-opto-electromechanical systems (MOEMS) are creating new application requirements and inspection needs. In certain cases, inspection can be achieved only by using X-ray tubes with focal spot sizes of 1 µm and below. This has led to the development of nanofocus and multifocus X-ray sources. Multifocus tubes allow an operator to switch from either high power, microfocus or nanofocus imaging, depending on the requirements of particular applications. While the selection of microfocus or nanofocus depends on the required spot size, the high-power mode in a multifocus tube is for imaging and inspecting dense parts. The tube is designed to offer a particular advantage by delivering the highest intensity possible in a 160-kV transmission tube.
How X-ray Works
X-rays are electromagnetic radiation with high energy — typical photon energies range from 100 eV to 1,000 keV. X-rays have shorter wavelengths than ultraviolet light, but longer wavelengths than gamma rays. Their wavelengths fall into the range of 10-8 to 10-12 m.
Since X-rays are electromagnetic radiation, their velocity in a vacuum is the same as visible light (186,000 miles per second). Highly penetrating, hard X-rays have a high frequency and short wavelength, while soft X-rays have a much lower frequency and less energy.
X-ray tubes, betatrons or linear accelerators are typically used to generate X-rays. Penetration strength depends on the energy of the electromagnetic wave, but is also influenced by both the density and nuclear charge of the material being penetrated. Silicon, for example, is a low-density, easily penetrable material. Lead is more opaque, and therefore not as easy to penetrate.
A visual image of a material's internal structure is produced when X-rays pass through the material and strike a photographic plate or fluorescent screen. Shadows that appear on the plate or screen depend on the relative opacity of different parts of the sample. A crack in a solder ball is easily visible, for example, because the ball itself is more opaque than the void created by the crack.
Microfocus X-ray Inspection Systems
X-ray systems basically consist of a sealed or open X-ray source, a fixture for holding and manipulating the sample to be inspected, and a radiation detector (Figure 1). X-ray tubes are available in various configurations and differing performance capabilities. Open microfocus tubes (a stainless steel tube that can be opened at any time for cleaning and maintenance, and is always evacuated prior to each use) are used in high-resolution applications of electronics assembly and packaging. Such tubes can provide a spatial resolution of less than 1 µm, with geometrical magnifications of as much as several thousand times.
High-voltage generator. A high-voltage generator provides the required power for the electronics emitting from an electron gun within the X-ray tube, controls, computers and image-processing software. The basic components are arranged in an X-ray room or installed in a radiation-shielded cabinet.
Manipulator. An X-ray system's "manipulator" is used for high-precision x-y-z positioning and rotating/tilting of samples. The manipulator should be capable of directional and rotational speeds for requirements ranging from quick overview searches at low magnification to very low speeds at high magnification. For example, x-y speeds may range from a few micrometers per second to several hundred millimeters per second.
 Figure 1. Diagram of a typical X-ray system used for industrial inspection. |
null
 Figure 2. Microfocus and directional transmission tubes. |
null
Detector. The function of a detector is to process data of X-ray waves in real-time into an image of light visible to the human eye or to electronic vision systems. While the most common detector is an image intensifier/video camera that converts X-rays into visible light, other recently developed detectors include high-dynamic cameras and flat-panel direct digital detectors.
High-power tubes are likely to use solid targets to emit X-rays. Microfocus tubes for electronics inspection usually have transmission targets that consist of two different layers of material (Figure 2). Most of these transmission targets are made from a thick layer of backing material, such as beryllium or aluminum, with a low density and a low atomic number and weight. The purpose of the backing material is to close the tube and maintain a vacuum. It also forms the backing, which provides mechanical strength for the target layer in which X-rays are generated. Target layers usually consist of a thin, 5-µm layer of metal such as tungsten, molybdenum or copper, with a high density and high atomic number and weight. The target layer is sputtered onto the backing layer.
With transmission tubes, as opposed to directional tubes from which the X-ray beam is issued at a 30-, 60- or 90-degree angle, the design features forward-beam geometry. Electrons enter the back of the transmission target, and X-rays radiate from the front (Figure 3).
In all tubes, the electron beam emitting from the cathode enters the target and collides with particles of the target material. When an electron beam hits the target surface, the electrons enter the target material (interaction layer) and collide with target material particles, and are slowed and deflected in various directions. They then collide again and again with target material particles until the kinetic energy drops to practically zero.
With each collision, electrons are slowed and their loss in kinetic energy translates into radiation energy. Less than 2 percent of the energy appears in the form of X-rays. The remainder is mostly heat. Tungsten is the material used in most tubes, because of the need for a material with a high melting point.
Focal spot is a key factor in determining image resolution and the quality of an X-ray image (Figure 4). As the focal spot decreases in size, resolution and the ability to detect detail are improved — enabling geometric or projection magnification without peripheral shadowing (Figure 5). Ideally, a focal spot has a diameter close to zero. In practice, depending on tube design, a focal spot can be as small as 1 µm or less in diameter.
The spatial resolution of an X-ray tube is approximately half the focal spot size. Feature recognition for the tube is half of the spatial resolution. For features in the 125-nm range, an X-ray tube must have a focal spot size of about 500 nm.
Nanofocus X-ray Inspection
The electronics industry's trend toward smaller and more densely populated components, and the emergence of MEMS and MOEMS, led to the development of nanofocus X-ray technology. Nanofocus technology is defined as having a focal spot of less than 1 µm in diameter, which enables the level of detail and resolution needed for the inspection of low-density structures and ultra-small features common in today's electronics. The technology is an integration of tube and sophisticated software for controlling performance aspects such as short- and long-term stability, image contrast, brightness and amount of radiation.
Nanofocus X-ray inspection systems are particularly suited for applications consisting of submicron components, circuitry and assemblies such as with MEMS and MOEMS, but also with wafer-level packaging. In such instances, the resolution and sharpness required to detect defects in solder bumps and interconnects can be met only with nanofocus tube design and system technology.
 |
null
 Figure 3. Determination of focal spot with interaction volume dimensions. |
null
A notable application for nanofocus technology is inspecting packages with non-filled die attach, such as thermal adhesive to hold a microchip in place. Detecting the slight difference in contrast attributable to the adhesive mandates the resolution of a nanofocus tube. Such tubes can also be used to check the silver particle loading in electrically conductive adhesives. This capability ensures that the paste is homogeneous and loaded with sufficient particles to achieve the desired conductivity.
 Figure 4. Distances from focal spot to and from object to image plane play a key role in determining geometric magnification. |
null
Delaminations in packages can be an "unseen" problem with microfocus X-ray systems, depending on the degree of delamination. Such a defect, if unnoticed during X-ray inspection, can result in a failure during functional testing without the cause being determined. With a focal spot under 1 µm, nanofocus systems can detect submicron cracks and flaws in silicon packaging and fine bonding wires (below 25 µm), as well as solder whiskers that might otherwise be invisible to the X-ray inspection system.
 Figure 5. Relationship of focal spot dimension and geometric sharpness. |
null
Finally, the development of nanofocus technology has led to improvements in the design of microfocus tubes and systems. As a result, electron beam emissions are more controlled, with fewer aberrations, and imaging is "cleaner" and sharper.
Future of X-ray Inspection
A need exists for both microfocus and nanofocus tubes for real-time radiographic imaging. For contract manufacturers, where inspection requirements can vary from microfocus applications that demand high X-ray output to nanofocus applications that demand high resolution, multifocus X-ray tubes are suitable. The tubes incorporate a high-power mode for dense structures that require high intensities for inspection, such as castings, weldings and machined parts. Switching modes is a matter of a keystroke or a mouse click at any time during the inspection process.
For many applications, microfocus tubes provide sufficient resolution, contrast and magnification for the respective inspection task. For smaller components and denser circuitry where feature recognition requires a focal spot smaller than that of a microfocus tube, then nanofocus tubes are a preferred solution. The growing applications of MEMS and MOEMS indicate a continuing trend toward miniaturization, which can only mean a bright future for nanofocus radiography.
References
For a complete list of references, please contact the authors.
NORBERT DANEKE, Ph.D., X-Ray physical engineer, and BIANCA SCHANKLIES, marketing, may be contacted at feinfocus, Roentgen-Systeme GmbH, Im Bahlbrink 11-13, 30827 Garbsen, Germany; +49(0) 5131/7098-59-0; e-mail: [email protected].