A new approach to X-ray
06/01/2002
BY ADRIAN WILSON
Contract manufacturers and printed circuit board (PCB) assemblers are realizing that an X-ray system is a required tool rather than a "nice to have" system in their arsenal of inspection tools. The main impetus for this is the necessity to inspect bonds hidden under devices such as area array packages and J-leaded components. The nature of the interconnections on these products makes it difficult to verify bond quality with other technologies.
While the need for an X-ray system is easy to understand, it can be more difficult to decide what type of system is most appropriate for a given application. There is a trade-off between fast, automated in-line systems and slower, but more thorough, off-line process control and failure analysis tools. Another factor is that off-line systems can be used as both a process control and as a failure analysis tool, while in-line systems are only for process control.
The Ideal X-ray System
In a perfect world, X-ray systems would be both fast and thorough by providing:
- High resolution and magnification
- A wide spectrum of fault detection
- Fully automated fault detection
- High throughput
- Reasonable cost.
Such a system would use an image acquisition/inspection technology that would facilitate the automated inspection of all potential process errors, including difficult process faults, such as open ball grid array (BGA) bonds or bad wetting, that are now only identifiable by high-end failure analysis systems. These systems would be equipped with a fully automated board handling system that would allow it to be interfaced to the production line.
Current X-ray System Applications
For most high-volume production lines, an in-line system is preferred because it can maintain line volumes. However, because of the throughput required and the link into the conveyor systems, only the basic measurements of alignment, shorts, missing balls, pitch errors, ball formation errors and spare or displaced balls can be tested. Defects that require tilting or oblique views to detect, such as voiding, improper wetting and open or lifted balls, are time consuming and must be done offline using a separate failure analysis (FA) system. Typically these FA machines are located away from the production floor, resulting in delays in the process feedback. These delays can cause potentially hundreds of bad boards to be produced, requiring rework and creating a concern for throughput and cost.
 Table 1. Analysis speed of different fault types. |
Most low- and medium-volume production floors tend to use the best system they can afford, from low-priced off-line manual inspection systems to semi-automated failure analysis systems. These systems are either manual, which requires the operator to move the board assembly under joystick control and make subjective failure determinations, or semi-automatic, where the board assembly is moved automatically from position to position. Semi-automatic systems also have some degree of automated analysis, which is often tailored to the inspection of BGAs and other area array packages. These systems are generally on or near the production floor and can provide immediate process control, although test times tend to be long.
Figure 1 summarizes the criteria that users employ to select current X-ray technology for use on the production floors and failure analysis labs.
Another Solution
A new X-ray system has merged a failure analysis system with an acquisition/inspection technology and a board handling system to create an automated failure analysis machine. Part of a new generation of
X-ray inspection systems, systems like this have a higher throughput than manual or semi-automated failure analysis systems. They also offer more analysis, so variances can be caught before failures begin.
 Figure 1. A summary of the criteria used in selecting an X-ray inspection system. |
One development that allows for fast analysis is an ultra- high-resolution open X-ray tube. Tubes like this have a number of features, including a wide angle of X-ray emission that allows a system to quickly capture oblique view at highest magnification (OVHM) images.
OVHM imaging differs from normal 2-D inspection by providing an oblique view of bonds without tilting the board. As shown in Figure 2, moving the image intensifier through an arc rather than tilting the board away from the tube achieves an OVHM image. Because the distance from the source to the board remains constant, a system using OVHM technology achieves an oblique view without losing the high magnification needed for wetting analysis. This technique can provide insight into area array package faults, such as opens, by providing 2-D image quality with depth (Figure 3).
Compared to an automated 3-D imaging system, a 2-D X-ray image with OVHM typically has a higher resolution. A 3-D image is basically created by rotating the sample, and the image chain that results in essentially a series of 2-D images with low magnification. While this allows slice-by-slice analysis of the component, joint or board, it takes time to accumulate the image and magnification is typically limited.
Throughput and Automation
Table 1 shows the approximate analysis speed for various fault types using the new X-ray system. All faults can be analyzed concurrently, with the exception of open balls because of the need to view the component in an oblique view. This means that the actual analysis time is the longest of the speeds listed in Table 1. For example, the identification of the first six faults would take 250 ms per ball. Thus, a typical 256-pin BGA could be fully tested in two and half minutes, not including the time to load and manipulate a board. Such factors are board-dependent.
 Figure 3. (a) High-magnification 2-D image of a CSP, vs. (b) a 2-D with OVHM image of the same bond showing an open. |
Although not fast enough to be a true in-line system, such systems are well-suited for the high throughput of medium- and high-value board production lines. Systems are equipped with a fully automated board handling system with a SMEMA-compatible in/out port. This allows for simple wheel-up and wheel-away testing applications.
Additionally, the system software inspects the area array board assembly faults fully automatically: shorts, missing balls, spare balls, array pitch error, ball voiding, ball form error and open ball bond/bad wetting. A new image analysis software package is the key to these measurements. A new algorithm was developed to determine whether a ball is open or connected by how much the solder has flowed during the reflow process. In-line systems cannot do this measurement, and failure analysis systems rely on operator interpretation. The algorithm allows the user to select and adjust pass/fail levels and then identifies the integrity of each ball with a certainty of 0 to 100 percent, improving the analysis and speeding up the inspection. AP
Adrian S. Wilson, president, can be contacted at Phoenix X-ray Systems + Services Inc, 3883 Via Pescador, Unit A, Camarillo, CA 93012; 805-389-0911; Fax: 805-445-9833; E-mail: [email protected].
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