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X-ray Inspection of Lead-Free Products
CT imaging shows up otherwise undetectable flaws.
By Devonne McPherson, Customer Support Manager, phoenix|x-ray Systems + Services, Inc., St. Petersburg, FL
X-ray inspection of PC boards is most often performed using two dimensional x-ray systems. 2D x-ray inspection systems consist of an x-ray source, detector, and a multi-axis manipulation system for object maneuverability. Images are taken and seen "as is" with little to no calculation done on the live x-ray image. Many x-ray systems in use today offer automated defect inspection, and this can be performed on either a live or captured image, whichever is most convenient.
X-rays are produced inside an evacuated tube in which a filament is heated until electrons are produced. The electrons are then accelerated towards an anode plate and focused on a very fine spot on a tungsten target using a magnetic lens. At this focal spot, the electron beam is converted into an x-ray beam. The x-ray photons that make up the beam are then received by a detector which converts the photons into a visible live image. If a sample is placed between the focal spot and the detector, part of the radiation will be absorbed by the object and its magnified shadow will be seen on the computer monitor, producing the x-ray image.
X-ray images are defined primarily by their contrast. Different materials within a sample will absorb different amounts of radiation, allowing photons of various power to pass through. This in turn creates bright and dark areas on the detector, the range of which make up the contrast. The amount of radiation absorbed increases with respect to the atomic number of the penetrated material, its density, and thickness. The denser an object the more radiation it absorbs, thus producing a darker image. Because of this, air-filled voids are fairly easy to detect since the voids have a drastic difference in density, material, and thickness compared to the surrounding solder, which causes a significant difference in contrast.
As PC board elements and solder joints become smaller, higher magnification and better image quality is needed. An increase in magnification is achieved when an object is placed closer to the x- ray tube, resulting in a larger shadow of the object at the detector. This same effect can be seen by placing your hand in front of a flashlight at various distances and observing the shadow on the wall. To improve image quality the tube voltage or kV, and the tube power may be adjusted. The kV is the applied voltage between the anode and cathode of the x-ray tube. The higher the tube voltage, the faster, and therefore more energetic are the resultant x-ray photons, which results in a greater ability of the x-ray photons to penetrate matter. The tube power is the product of the tube voltage and filament current. An increase in tube power results in a brighter x-ray source which will result in a brighter image seen on the monitor.
The most commonly used lead-free alternatives are the SAC alloys which are named for their mixture of tin, silver, and copper. There are many different solders containing varying elemental ratios of the SAC alloys, but replacement alloys of any type usually consist of about 97 percent tin. So, the lead in traditional soldering alloys is replaced by tin in the new soldering alloys.
X-ray absorption greatly depends on the atomic number of the material under inspection. The absorption of radiation increases, for the most part, along with an increase in the atomic number of different materials. In general, the higher the atomic number of a given sample, the more radiation it will absorb. Tin has an atomic number of 50, significantly smaller than the atomic number of lead, which is 82. Therefore, we can expect that tin will absorb less radiation than lead, meaning more powerful photons are allowed to pass through the material and strike the detector. What this means for image quality is that the same tube parameters used for inspection of traditional lead solder alloys will result in an overexposed image if used with lead-free alloys. However, this effect can be easily corrected by decreasing the x-ray tube kV and power. The specific values may change according to the x-ray system being used since the properties of x-ray tubes may vary. Although altering the tube parameters may be necessary when inspecting lead-free applications, it can simply be done by the click of a mouse button or turn of a dial, and does not require new x-ray technology.
Advanced Inspection Techniques
There are many other techniques that phoenix|x-ray systems includes to inspect lead, lead-free, or almost any other application. For example, oblique view at highest magnification (OVHM) provides three-dimensional views of an object under inspection. When inspecting the solder joints of a BGA, oblique views can reveal the vertical contours, voids, and the pad wetting distribution of the solder joints. The three-dimensional views reveal object details that would not be visible otherwise, and this is achieved without any loss of magnification.
Powerful software tools are incorporated in phoenix|x-ray systems, and using these tools, it becomes possible to inspect most solder joints automatically. Evaluation algorithms for common types of SMT solder joints include: BGA, CSP, QFP, Gullwing, J-Lead, QFN, MLF, and chip components. With the use of OVHM imaging techniques, even through-hole solder joints, such as PTH and THT, can be automatically evaluated as well. The evaluation criteria can be adjusted to satisfy IPC or company standards. For special applications, users can create their own evaluation routines using the company's X-ray Image Evaluation Environment (XeTU
), which allows the user to set up powerful routines from a comprehensive set of elementary algorithms.
While high resolution x-ray inspection has been a useful tool for non-destructive analysis, some of today's inspection challenges have outgrown conventional x-ray technologies. This is largely because devices are becoming smaller and more complex. phoenix|x-ray's nanoCT technology provides the solution to these problems by allowing the internal visualization of 3D structures with sub- micrometer resolutions down to 200-300 nanometers.
CT, or Computed Tomography, works by taking a large number of two- dimensional x-ray images around a single axis of rotation. The x-ray images are then reformatted as volumetric representations of 3D structures. This volume can be visualized and used for failure analysis, dimensional measurements, and internal visualization of structures. It is also possible to make virtual cuts through objects to visualize their internal structure, replacing traditional destructive slicing at a fraction of the time required. These features make nanoCT a perfect tool for inspecting electric or electronic devices, pore analysis of castings, and even characterization of new materials like ceramics and compound materials. NanoCT offers the capability to view both low-absorbing materials — such as synthetics, composite fibers, or biological matter — as well as the internal microstructures of metal objects, such as the different phases of eutectic solder in lead and lead-free solder joints.
Moving to a lead-free world presents many challenges to the electronics assembly industry. Fortunately, x-ray inspection techniques and technology that have been used successfully for years can continue to ensure manufacturing quality in a non-destructive manner, even in a lead-free environment. The advancement of x-ray technology will provide non-destructive testing solutions for a variety of complex new applications, including materials science, micro mechanics, biology, geology, and micro-electronics. X-ray inspection will remain a primary tool in quality assurance for many years to come, both in present and future technologies.
For more information, contact: phoenix|x-ray Systems + Services Inc., 111 2nd Avenue N.E., Suite 311, St. Petersburg, FL 33701
727-456-1465 fax: 727-456-1466 E-mail: email@example.com Web:
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