|Zero degrees tilt at 60kV showing no voids.
Years of BGA imaging experience have resulted in a complete understanding of the actual BGA placement process. With a solid understanding of the placement process, software engineers have been able to develop image analysis software that is easy to use, while providing the relevant information needed to monitor the entire placement process.
X-ray inspection of electronic components and BGAs requires a delicate balance of x-ray tube power, x-ray tube focal spot size and x-ray image detector resolution.
A number of small details have been discovered over the years revealing how the data should be analyzed. One of these small details is the use of the expanded grayscale that is provided by the Flat Panel Detector. The typical void inspection routine is to do pattern matching; the software looks for a BGA ball expecting a size that was predefined in the software. If the ball size didn't meet the predefined size and had voiding of a set circular size, the ball was rejected — being under-sized or in some cases, too large. The software had very limited capability to adapt the inspection routine to voiding that was small or not circular.
The use of the expanded grayscale Flat Panel Detector allows the software to measure actual grayscale at a pixel level. This type of measurement provides a more detailed and accurate analysis of the solder balls and a more complete picture of the placement process.
Flat Panel Detectors
Huge advances have been made in the ability to convert x-ray photons efficiently into visible light that can be captured and displayed on a computer monitor. Over the past few years, the use of Digital Flat Panel x-ray image detectors has become the standard practice in all high-end x-ray inspection systems. With the continuing increase in computing power at a reasonable cost, the use of Digital Flat Panel x-ray image detectors has allowed innovative x-ray equipment manufactures to include the advantages of Flat Panel Detectors in affordable x-ray systems.
The advantages of Flat Panel x-ray image detectors:
- Increased overall resolution by eliminating the 256 grayscale image intensifier, which also eliminates the light loss associated with the CCD camera coupling and signal conversion from analog to digital. The conversion was made necessary so the image could be used and stored in the image processing computer.
- Digital Flat Panel (DFP) image detectors provide a minimum of 4096 shades of gray along with megapixel counts.
- DFP image detectors provide the optimum grayscale and pixel resolution to allow the operator to capture an image and send it to the image processing computer for analysis.
X-Ray tubes are designed based on overall resolution (focal spot size). This measurement will determine the basic resolution of the x-ray system. The downside to smaller focal spot sizes is the lack of power available in overall wattage. Years of actual use have determined that the optimal focal spot size and power rating for an x-ray system inspecting BGA components is 90kV with a 5µ focal spot size with 9 watts of total power.
The main advantages of the small focal spot x-ray tubes are their ability to use oblique angle viewing (tilted views) at high magnifications. This type of viewing will easily display solder reflow, ball collapse (ball size) and locations of voiding. The use of oblique angle X-ray views at high magnifications will often help the inspection of the joint interfaces by allowing a better observation of the interface; the interface might otherwise be obscured if the view is only top-down.
When to Tilt
Zero degree tilt provides a quick overview to check for voids, bridges and missing balls over the entire BGA component at lower magnifications. The use of at least 80kV will allow for adequate penetration of the component and substrate, while producing the best contrast-grayscale for accurately analyzing a BGA component. Evidence of complete reflow of solder paste can be measured at the zero degree viewing angles when high magnification is used. Solder joints on any one device should have uniform shape and size and surface appearance which depends on the process, materials and the soldering atmosphere.
Oblique angle viewing can be accomplished in two ways; the first is to rotate the sample on a tilting table, but unfortunately, this technique has a few disadvantages. The first disadvantage is that the sample has to be placed in a fixture to keep the sample in a stable position as the table is tilted. Another major disadvantage: the sample is moved away from the x-ray tube, resulting in a lower overall magnification with a loss of detail.
The loss of magnification is further exacerbated by the area of interest being moved out of the viewed image. This requires that the operator find the area of interest, and this whole procedure makes the proper inspection of BGAs difficult.
The ideal method to obtain the 45° oblique angle is to tilt the x-ray image detector while keeping the sample on a fixed plane; this technique allows the x-ray tube to be in close proximity to the sample table, without losing any magnification. Another advantage is that area of interest location is automatically compensated for by the computer-controlled stage, allowing the operator to concentrate on the solder joint analysis. This results in more consistent inspection results.
|45° tilt at 90kV high magnification showing voids.
The 45° tilt with high magnification provides the ability to see ball shape and pinpoints where the voiding is located in solder paste connection. This view requires the x-ray system to have the capability to penetrate the component and substrate (capability of at least 90kV) and have sufficient x-ray tube resolution to get mid-to-high magnification ranges needed to see small voids and solder joint discontinuities.
A 70° tilt provides the ability to see if the voids are breaking out of the solder joint, which will cause solder joint failure. This oblique angle also allows the operator to see wetting of the solder flux. Oblique angle viewing also helps to isolate components when BGAs are mounted on both sides of the circuit board.
Advancements and the relatively low cost of computing power have allowed manufacturers to provide an incredible amount of automation — not only for the image capture and analysis, but also to the actual x-ray system. It's not uncommon to see entry-level x-ray systems with full programmable (point-and-click) CNC control and stage functions. These systems also can have the ability to see fully automated BGA inspection routines for basic ball size, circularity and percentage of voiding.
The Auto BGA feature allows the process engineer to define the inspection tolerances. These include all system settings such as magnification, oblique angle viewing and the optimal kV settings for multiple circuit boards and components. The inspection routine and tolerances are saved in the software database for future use by an operator; this allows an operator to click on a predetermined inspection routine for ease of use while providing more consistent inspection results.
Using transmission x-ray, the x-rays pass completely through the sample to produce an image. This function is called "penetration" and is the key to discovering any voiding in a BGA solder ball. Typical kV settings in the 75-to-80kV range are required to produce BGA ball images that contain enough useful information. These data provide the x-ray software grayscale algorithms with useful statics to accurately measure the percentage of voiding.
|Zero degrees tilt at 90kV with voids.
The percentage-of-void function in the image processing software is used calculate the percentage of void from a zero tilt position. This position also allows for measurements of ball size and circularity. Advanced image processing software allows the operator to automatically or manually test a single BGA ball as well as a full BGA component for percentage of voiding, ball size and circularity, using with drag-and-drop boxes. The software also has an Auto BGA function that will inspect the entire BGA and provide pass/fail reporting that can be saved as an XML or Excel file. Advanced software algorithms determine voiding based upon grayscale and not the shape of the ball or the shape of the individual voids.
Opens and Solder Ball Size
All BGA solder balls should be broadly circular in appearance, consistent in size and shape. Advanced auto inspection routines will measure and compare the ball size and provide a pass/fail result based on the predetermined tolerances. The software will also easily detect bridging and suspected opens based on the overall circularity of the solder balls. Solder balls that are not circular tend to be an indicator of an incomplete or cold solder joint.
The advantages of the small focal spot x-ray tubes, Digital Flat Panel x-ray detectors and smart adaptive image processing computers have always been used in higher cost x-ray inspection systems; now manufacturers are incorporating these high-end features in affordable smaller form factor x-ray systems.
Contact: Scienscope, 5751 Schaefer Ave., Chino, CA 91710 800-216-1800 or 909-590-7273 fax: 909-494-5513 Web: http://www.scienscope.com