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X-Ray Inspection: A User's Point of View
December 31, 1969 |Estimated reading time: 10 minutes
Inspection and test have been the standard tools of product-quality verification. However, traditional inspection relies on "seeing" solder interconnections via the human eye or camera, a task more difficult when they are under the component.
By W. James Hall, Phil Zarrow and Chester Lowe, Ph.D.
With area-array packages, primarily ball grid arrays (BGA, Figure 1), assuming a larger role as product components, it is vital to validate their interconnection integrity and reliability. The ultimate goal is to achieve high yields in the manufacturing process and low failure rates in the field under operating conditions. This requires proper soldering of all interconnections including good wetting of the printed circuit board (PCB) pads and proper fillet formation and solder bump collapse. Shorts, opens, inadequate solder, excessive voiding and solder cracks are all possible conditions that can impede functionality or degrade reliability. Area-array package rework is difficult, time consuming and should be avoided whenever possible. When it is required, however, imaging is critical in guiding the process and confirming the integrity of the repair
Figure courtesy of FeinFocus.
Figure 1. Comparison of X-ray tube-intensifier configurations. Electromagnetic beam focusing can achieve smaller focal-spot sizes.
To ensure consistent quality, validation through solder interconnection inspection is vital. X-ray images can provide these data during setup and process optimization and through in-process inspection to confirm the stability of assembly and ultimately product quality. These images must provide accurate details of solder joint shape and consistency for all components on both sides of double-sided assemblies, regardless of construction complexity.
The primary objectives of X-ray imaging are to provide views or images of both the outer shapes of the solder joints and the continuity of the core materials. Proper solder volume, fillet formation and wetting are indicated by shapes and shading of outer surfaces. Voids and cracks are potential defects within the interior structure of the solder materials.
The basic process consists of "shooting" X-rays through the area of interest (i.e., the solder joint) and recording the patterns of diffusion. The chief problem with the process is the effects of other materials within the assembly, which lie between the area of interest (solder joint) and the X-ray source and/or the detector or image-capture device.
X-ray Capabilities
An X-ray inspection system must incorporate an appropriate combination of power, resolution and oblique-angle viewing to image as many types of defects as possible. X-ray power must be sufficient to "project" the X-rays through all of the materials on both sides of the area of interest between the source and the detector.
Resolution is the capability to identify small features within the image. Required resolution levels depend on the size of the object of interest as well as on the analysis being performed. Resolution is based on the focal spot size of the system; a smaller spot size can generate higher resolution images. Open tube designs using electromagnetic beam focusing typically can achieve smaller focal-pot sizes (Figure 1).
Photo courtesy of FeinFocus.
Figure 2. X-ray of BGA balls and wire bonds. Interference from other materials often can be moved out of line with the main image.
Angled viewing with X-ray inspec-tions provides two distinct yet interrelated capabilities:
- It permits viewing of different surfaces of the solder joint of interest, such as the fillet shape at the PCB interface.
- Interference from other materials often can be moved out of direct line with the primary image (Figure 2).
The combination and interaction of these features is essential to effective imaging. In particular, resolution must be maintained throughout the full range of angled viewing and the entire area of the PCB. If resolution is lost due to mechanical interference between the product and the X-ray source or detector, then overall system effectiveness can be compromised significantly. The total combination of power, resolution and flexible angled viewing must be capable of imaging to permit identification for prevention of undetected defects or "escapes." Finally, resolution must be sufficient to permit accurate fault identification without generating "false calls."
X-ray Speed
X-ray inspection can be time-consuming, particularly inspecting a specific assembly for the first time. Two-dimensional (2-D) systems, however, provide the inherent advantage of producing actual images in a short time. As actual images are viewed in real time, no programming is required to generate them. But the operator must perform some or all of the following steps: assembly loading and mounting; moving the X-ray image to the area of interest; focusing; adjusting magnification; adjusting voltage and current levels; adjusting contrast and brightness; activating and adjusting image-processing functions; and finding acceptable angles of viewing to display the required features.
These steps can affect the productivity of an X-ray inspection system significantly and, hence, not all systems are equal in this arena. Simplifying or automating them is important to cost-effective utilization. Improved speed can permit faster setup of new products or it can provide the time for more extensive inspection coverage when used in-line.
About Automation
Automation features are valuable for systems used in production to inspect multiple units of the same product. In such cases, it is less than ideal that the operator must repeat each setup step for each image regardless of the speed. The most productive feature is the ability to record the initial manual steps into a program that then can be repeated automatically for each successive assembly. The basis for this is complete computer control over the full range of functions starting with basic horizontal X-Y locations and extending to full angular motion control and image-processing functions.
To be most effective, programs should incorporate all the adjustments and configurations, permitting the operator to view sequential images and perform inspection decisions. Advanced (and more desirable) systems provide "programming" capabilities such that once the operator has identified the best angles and image-processing parameters for viewing a specific solder joint or series of joints, that information can be saved and applied automatically to the inspection of future units.
Modes of operation will permit control of the progression from one image to the next, or automatically timed sequences of images requiring operator interaction only when a defect is observed in a specific image. If a system is designated for use in production, then speed of setup (of the "learn" mode) is less important so long as the final automated program can produce sequential images with delays of no more than a few seconds.
Photo courtesy of FeinFocus.
Figure 3. Display screen showing board-layout map. A top-down image of the assembly provides a reference without requiring the operator to divert his attention by looking into the X-ray chamber.
The final step in simplifying operator tasks is the capability to automate the location and alignment tasks for repetitive inspections. With the addition of advanced, numerical image-evaluation algorithms, again linked to pre-established and programmed locations and viewing angles, many actual quality decisions can be automated to reduce the requirement to view each location and to make the "good" or "bad" decision (Figure 3).
Ease of Use
One area where ease of use is particularly important is in the setup of the system for a new or different product. Process verification is vital for attaining effective assembly manufacture using BGA components. Because 100 percent inspection often is cost-prohibitive, many production lines rely on on-line process control of validated processes. X-ray is a validation technique usually requiring inspection of unique test boards as well as initial production samples. Frequent setup also is required in a "high-mix" environment, such as in typical contract manufacturing environments where many different products may be assembled on one line serviced by one inspection system.
Photo courtesy of FeinFocus.
Figure 4. Display screen showing board-layout map. A top-down image of the assembly provides a reference without requiring the operator to divert his attention by looking into the X-ray chamber.
Ease of use starts with basic product handling. Fixtures should be simple to operate. Product location can be effected mechanically or, ideally, through vision-alignment of fiducials or other board features. The next step is to constantly be aware of what location (horizontal X and Y) is being viewed. An on-screen, top-down image of the overall assembly with numerically defined crosshairs provides an easy-to-use reference without requiring the operator's eyes to leave the screen to look through a window into the X-ray chamber (Figure 4).
A Clear Image
From an operator's viewpoint, the value of the X-ray system depends on the quality of the images it can produce and the speed and simplicity with which those images can be achieved. The ease of producing a clear image is greatly facilitated by the auto-focus algorithms available on most systems. Simplification or full automation of voltage and power levels also reduces the skill and experience levels required to achieve usable images, and thus increases productivity.
Perhaps the most significant enhancement to 2-D X-ray systems is user-friendly "off "or "oblique" angle-viewing. When properly optimized, angled viewing can display virtually all the features and possible defects in bump or column-type solder joints. From the standpoint of ease of use, the issue is the time and effort required to find the right angle and to focus for specific solder joints. For complex assemblies or because of the interference of other metals within the assembly, a wide range of angles may need to be investigated above or below the inspected joint. This has been difficult and tedious, if not impossible, with traditional X-ray systems, which often require realignment, recentering and refocusing after each angular change.
New systems have virtually solved these problems. Using a combination of hardware design and software compensations, these units permit continuous scanning through a nearly full hemisphere of viewing angles while keeping the areas of interest in the center of the screen, in focus and at a relatively constant resolution (i.e., magnification). These systems permit focusing for quickly identifying defects in a relatively "stable" image as it is rotated through all possible viewing angles.
The ultimate goal of any inspection system is to find defects (or to ensure their absence). Shorts and missing solder bumps typically are easy to identify even with only vertical viewing. The identification of subtle defects such as cracks and fillet shapes can be facilitated by a comprehensive set of image processing or enhancement tools such as filters, edge enhancers and numerical evaluation tools. User friendliness is the key — one must be able to easily turn image-processing features on and off, combine them, and control their critical parameters.
Reliability
The ultimate value of an X-ray inspection system is founded on the reliability of the results. This translates into a requirement for repeatable images so that fault-threshold values (upper and lower specification limits) can be defined accurately. This is the most important factor in minimizing "false calls" and "escapes." Naturally, since the operator makes most final decisions, there is always the influence of human perception. It has been well documented that visual evaluations are not 100 percent repeatable even by highly trained and experienced inspectors. To minimize this, the inspection machine must introduce as little variation as possible, which starts with the stability of the X-ray beam itself.
The "targets" that actually produce the focal spot within the X-ray tube are all subject to degradation. Optimized tube design can minimize this. Some designs provide for easy replacement of the target area while others require complete tube replacement. Recalibration can minimize variations due to "wear" on the target, but these can be time consuming and affect productivity if they are required at frequent intervals. Viewing parameter repeatability also can contribute to the overall reliability of inspection results, especially in in-line situations where the same products are being inspected on a continuous basis.
The value of an X-ray inspection system depends on its ability to identify defects. This can translate to reduced testing, repair costs and field failures. This minimizes or prevents defects throughout the manufacturing processes, increasing yields and reducing rework.
Conclusion
X-ray inspection technology for electronics assembly has come a long way. However, as with traditional visual inspection, X-ray inspection is a somewhat subjective activity and, in many cases, it may never be possible to confirm the quality of all solder joints in all locations with 100 percent certainty. Double-sided assemblies and other complex geometries have continued to increase the "degree of difficulty" in clearly viewing all interconnections. The challenge today for the user is to achieve the highest level of defect identification in the shortest time at a reasonable cost. Improved capabilities and functionality incorporated into current, advanced X-ray systems facilitate significant steps toward this goal.
Acknowledgement
The authors acknowledge NETCO, Circuit Technology Center and FeinFocus for the use of their X-ray systems in the preparation of this article.
W. James Hall and Phil Zarrow may be contacted at ITM Consulting, Durham, N.H. 03824; (603) 868-1754; E-mail: ITM@ITM-SMT.com; Chester Lowe, Ph.D., may be contacted at FeinFocus USA Inc., 76 Progress Dr., Stamford, CT 06902; (203) 969-2161.