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Comparing Digital and Analog X-ray Inspection for Component Analysis
December 31, 1969 |Estimated reading time: 8 minutes
Increasing use of BGAs and chip scale packages (CSP) within products, which provide increased functionality at the same or smaller size, has reinvigorated the need for X-ray inspection.
By David Bernard and Steve Ainsworth
X-ray helps determine joint quality beneath these packages without sacrificing the device. Furthermore, as legislative and commercial requirements for lead-free manufacture get stronger, the need to determine quality dramatically increases because the historical benchmarks for solder joints created with tin-lead solder no longer apply. Lead-free and shrinking package size have demanded that X-ray system manufacturers improve the quality and function of their offerings.
Initially, system improvement concentrated on higher X-ray tube magnification and resolution. This was followed by the ability to examine the solder joints at oblique angles, critical for inspecting BGA/CSP components, without compromising magnification. The most recent development in X-ray systems has been improvement in the X-ray detector to improve the quality and resolution capabilities of the analytical image.
Analog and Digital X-ray Detectors
Originally, all X-ray inspection images were produced on film, requiring wet chemistry. This also meant analysis could not be provided in real time. Medical industry requirements resulted in the development of image intensifiers (II) as X-ray detectors. IIs produce X-ray images in real time and have been adopted by X-ray system manufacturers for printed circuit boards (PCB) and semiconductor applications.
In an II, an X-ray-sensitive phosphor faces the X-ray tube. X-rays pass through the sample, hit the phosphor and are converted into visible photons. The photons are captured and displayed onscreen via a CCD camera. The image's dark area shows the location of more dense material, such as solder, and the lighter areas are shown as less dense materials in the sample, such as the board material (Figure 1).
Figure 1. 0.3-Mpixel analog intensifier X-ray image of a PCB.
Older CCD cameras acquired data at 25 frames per second (fps) at 8-bit (256) grayscale levels; hence the "real time" terminology. They are capable of around 640 × 480 pixels, or 0.3 Mpixels, per frame image and capture the visible photons from the entire II phosphor surface. In addition, the larger the diameter of the II phosphor, the lower the resolution of the captured image. As an example, a 4" diameter image intensifier can have a resolution of up to 50 line pairs per centimeter (50 lp/cm). This is the standard available from most X-ray manufacturers and often is termed an "analog" detector as the image information from the CCD camera is in analog format.
Improved image resolution, image size and grayscale sensitivity are required for current and future inspection applications. As a result, a variety of other detectors are available and used as alternatives to the analog II. In addition, improved image clarity at all magnifications allows a better analysis (Figure 2). The improvements these alternative detectors bring to X-ray inspection are particularly important for samples with little inherent X-ray contrast difference. Here, the much greater grayscale sensitivity of these detectors allows better visual separation of features within applications such as non-conductive die-attach, voids in packages, molding compounds and glob-tops, microvia inspection, etc.
Figure 2. 1.3-Mpixel digital intensifier X-ray image of the PCB shown in Figure 1.
These alternative detector types can be classified under the same generic term of "digital detectors." However, a number of different digital technologies are used. Their similarity is that the image data output is digital, not analog. Traditionally, digital detectors were available as expensive options to X-ray systems, but recently, certain digital detectors have been provided as standard.
Digital vs. Analog
Both analog and digital detectors have the same basic parts. An X-ray sensor converts incoming X-rays into another medium that can be measured or imaged, usually including some type of amplification. This is followed by an analog to digital (a-d) converter for computerized image processing. The major difference between analog and digital is where the a-d conversion takes place (Figures 3 and 4).
Figure 3. Analog detector-based X-ray system schematic.
In the analog system, an analog signal exits the detector to be converted within the image-processing computer. With digital detectors, the a-d conversion takes place within the detector, and only a digital signal comes out of the unit. At first glance, this digital and analog difference seems to be trivial. However, the real difference comes from the nature of the digital X-ray sensor, together with its resolution, speed of operation and grayscale sensitivity.
Figure 4. Digital detector-based X-ray system schematic.
Digital detectors typically used for PCB X-ray inspection include an II using a digital CCD camera — a digital image intensifier, or a CMOS flat panel detector.
Digital II vs. CMOS Flat Panel
The digital II uses the same X-ray-sensitive phosphor in the analog intensifier, but a digital CCD camera of higher pixel count, grayscale sensitivity and improved resolution replaces the analog CCD camera. The CMOS flat-panel detector uses a similar phosphor to convert incoming X-rays into visible photons. However, instead of a CCD camera to capture the resulting picture, a 2-D silicon photodiode array is used. The optical photons from the phosphor create storage of electric charge at each pixel in the photodiode array according to the light intensity received. The charge is periodically transferred to corresponding data lines and a digital image can be recovered. A comparison of the features and performance of the two detectors can be seen in the table, with the equivalent analog intensifier information for reference. The most commonly available detectors within commercially available X-ray inspection systems also are shown.
Performance of both digital detectors appears substantially better than the analog intensifier. Looking at the comparison be-tween the two digital detectors further:
- The digital intensifier has a 30 percent greater image size compared to the CMOS flat panel (433 percent more than the analog)
- The CMOS flat panel resolution is 10 percent greater than the digital intensifier (266 percent more than the analog)
- The digital intensifier has two or four times the grayscale resolution of the CMOS flat panel (128X more than the analog)
- The digital intensifier has >600 percent faster frame rate acquisition than the CMOS flat panel (analog has the same as the digital intensifier).
Much larger CMOS flat panels are available but add substantially to the X-ray system's total price, and exhibit a relatively high level of manufacturing defects. These defective areas must be disabled and removed from data capture to prevent failed detector lines within the X-ray image. The removal of these defective sensors is not necessarily perfect, and the computer makes a "best guess" for values that should be contained in the sensor. At high contrast levels, these defects become visible within the final, analytical images and can retard analysis.
The greater the number of pixels within the X-ray image, the more detail available for analysis, allowing inspection of BGAs and other components at lower magnification. As a result, a larger area is covered with a single image, resulting in faster inspection without compromising inspection integrity.
The small difference in resolution between the two digital detectors is difficult to see because the final image is computer processed and output on either an analog or digital monitor, enhancing the final image from what emerged from the detector.
Binning the CMOS Flat Panel
The biggest difference between the digital II and CMOS flat panel is frame rate, the speed at which the detector acquires images. Analog and digital intensifiers can acquire image data in real time at 25 frames per second. The CMOS flat panel only is able to acquire images at around four frames per second. This is caused by the delay in response time of the photodiode array to the generally low intensity X-ray signals acquired during inspection. The photodiodes must build a sufficient charge before sending an acceptable image.
Using the CMOS flat panel within an X-ray inspection system, therefore, means that several seconds of data acquisition must elapse before sufficient detail is available for the operator to view. The CMOS flat panel takes six times longer to acquire the same quantity of frames compared to a digital II. During X-ray inspection this delay translates into delays in moving the device under inspection.
CMOS flat panels can use a "binning" technique to raise the frame rate by as much as four times. Although the frame rate increases closer to real time, it only is achieved by updating the signal captured to a subset (e.g., 0.25) of the pixels in the array in every cycle — i.e., sending less information at each cycle. Even though data acquisition speed has increased, it still takes the same time as to acquire an adequate image for analysis from all pixels. If binning is used, the resolution of the images produced is reduced to half of its standard value (i.e., 80 lp/cm falls to 40 lp/cm). As a result, binning does not offer additional benefits. Using a digital image intensifier does not require binning as data are being captured and presented in real time.
Conclusion
As devices and features requiring inspection continue to shrink in size, the next step in improving the analytical and X-ray quality of BGA, CSP and other devices is to use X-ray inspection systems with digital detectors. The increase in image size, resolution and grayscale levels these detectors provide enable higher quality inspection, often at lower magnification, allowing for better overall test quality at an increased throughput.
The two digital detectors available for today's X-ray inspection systems are the digital II and the CMOS flat panel. As technology improves, the CMOS flat panel may offer better resolution and faster detection in the future. To achieve this, pitch patterns on silicon must reduce, yield must improve and, most important, the price must fall.
References
For a complete list of references, please contact the authors.
David Bernard, X-ray systems product manager, and Steve Ainsworth, engineering manager may be contacted at Dage Precision Industries Ltd., Rabans Lane Aylesbury, Buckinghamshire HP I9 8RG, England; +44 (0) 1296 317800; Fax: +44 (0) 1296 435408; E-mail: d.bernard@dage-group.com or s.ainsworth@dage-group.com.