X-ray inspection systems for printed circuit boards (PCB) are getting more and more industry attention as manufacturers seek new methods to increase process efficiency and improve product quality. The heightened interest is a result of today’s increasingly smaller component packages with finer-pitch, higher-density boards, limited visual and electrical access, and higher customer expectations for product quality. Some recent advancements in X-ray inspection systems also make them beneficial in just about any high-tech manufacturing situation.
When investigating X-ray inspection systems, a manufacturer will be faced with a variety of terms and nomenclature that may not be immediately understandable. It’s useful to know some basics of the X-ray language to more efficiently determine which type of system is most appropriate for a particular manufacturing environment.
2-D and 3-D Inspection
Two-dimensional (2-D) systems, also referred to as transmission systems, produce a single image much like a basic medical X-ray. It creates a top-down image that sees through components and substrate to reveal solder-joint defects. Because they take only one image from one viewpoint, transmission systems usually are fast. They also can have relatively high-resolution so it is possible to use them for fine-pitch components. They are effective in finding defects for all joint types on single-sided boards, although through-hole joints and ball grid arrays (BGAs) can cause some difficulties.
Since 2-D technology offers just a top-down image of the board, a transmissional image cannot effectively reveal voids in the barrel of a through-hole joint. In the case of BGAs, the ball blocks the view of the solder connect (called an occlusion).
For the same reason, transmission systems cannot distinguish between the top and bottom sides of a doubled-sided board. The image they produce would be of the two sides overlapping, making analysis difficult at best. Boards often are tilted in 2-D machines to attempt to reduce these limitations.
Three-dimensional (3-D) X-ray systems view a solder joint from multiple angles to construct a cross-sectional image. The image from these views shows the solder joint at a particular height, called a slice.
Collectively, these multiple slices look at various depths of a single joint and detect faults or weaknesses at any point in the solder. With this capability, cross-sectional X-ray systems detect defects in the barrel of a through-hole solder joint and the solder connects of BGAs. They do not rely on the view from a single angle. These systems can inspect high-density, double-sided boards with any component and solder-joint type.
Different Slicing Techniques
There are three approaches to 3-D X-ray inspection: laminography, tomography, and tomosynthesis. These technologies produce cross-sectional images but implement the slicing concept differently.
Laminography uses a rotating X-ray beam to view every possible angle of a solder joint. A high-resolution camera combines the continuous angle images into one 3-D image of the solder joint at a specific depth. Because these systems do a hardware integration of the angle images to produce a single image, they are relatively fast, and the images are clear and easy to analyze. Multiple points of a solder joint can be sliced and analyzed.
Tomography, also known as digital tomography, creates a mathematical representation of the solder joint. Because of the extreme amount of computations performed, this approach is quite slow and not often used in manufacturing environments.
Tomosynthesis uses detectors and an X-ray source to take at least eight separate discrete angle images at points on a circle around a solder joint to approximate laminography. Then filters are used to mathematically combine the separate images into a single image for analysis. Increasing the number of discrete images, although time-consuming, produces a clearer final image that is easier to analyze.
Manual and Automated Systems
Both transmission and cross-sectional X-ray process test systems can be used in manual or automated modes. In manual systems, the operator must decide whether a defect is present. Some defects will be obvious, but some will require a judgment call.
A manufacturer’s requirements for repeatability—the measurement of how reliable a good-joint/bad-joint call is—must be considered. An AT&T study showed a typical decision repeatability of only 44% for even the same inspector.
Automated X-ray inspection (AXI) systems use Gerber or CAD data to develop inspection sites for each PCB. The manufacturer sets defect thresholds that the system uses to automatically determine whether a solder joint is out of specification. A repeatability rate of more than 95% is achievable from AXI systems.
The price of manual X-ray systems may be tempting, but the trade-off in terms of repeatability and speed must be considered. Inspection rates of 60 to 90 joints per second make 100% inspection possible with AXI systems compared to the sampling method typically used with the slower manual systems.
Copyright 1998 Nelson Publishing Inc.
February 1998
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