As the technological achievements in electronic component packaging continue to develop, circuit design has become more miniaturized, and as a result, has become more difficult to troubleshoot and/or inspect. These achievements allow circuit board manufacturers to take full advantage of the latest circuit design and packaging techniques. With the reliability of electronic production directly related to solder joint quality, the daunting task of troubleshooting printed circuit boards (PCBs) puts a strain on product quality control. The rate of advancements in quality control must be at or above the level of technological changes in order to keep pace. X-ray analysis, initially used as an inspection technique, can provide the capabilities to improve product quality control in an effort to sustain product reliability.

X-ray transmission is an inspection method that utilizes X-rays to view the thin layers of a circuit board as well as its soldered connections. This method exploits the variation in object densities to determine the shapes of different objects. Objects of higher density produce a darker X-ray image. The gray-scaled images that are produced through X-ray analysis can reveal indications of board defects Including:

• Board shorts
• Solder voids
• Solder opens
• Solder bridging
• Missing solder
• Mis-alignment of solder joints
• Minimal electrical clearance violation
• Void percentage with respect to pad area
• Grey value and diameter deviations
• Fuzzy edges (insufficient re-flow)

X-ray analysis is also used to verify that board measurements meet specified manufacturing criteria of acceptance. These measurements include the following:

• Trace width
• Solder ball diameter
• Solder width
• Solder volume
• Oblique view pad wetting analysis

With solder images appearing much darker than other objects, solder voids appear as bright spots within a solder ball or joint (Figure 5-1a). Solder bridging appears as non-uniform dark spots as in Figure 5-1b. Mis-aligned solder joints appear as dark spots either lying beside board traces or board pads (Figure 5-1c). Incomplete board etching, solder bridging, mis-alignment of solder joints, intersecting board traces, or any combination of such, are all probable causes of circuit board shorts.

High level X-ray systems are able to produce two-dimensional (2D) X-ray images of PCBs with simulated three-dimensional (3D) capabilities such as:

• High magnification up to 10,000x
• Oblique views up to 61°
• Voltage levels up to 225kV
• Detail detection down to 200nm (0.2 micron)

The image depth through a PCB sample is controlled by the kilovolt (kV) level and the image contrast is controlled by the micro-amp (µA) level. Systems possessing higher kV levels have an increased board navigation depth. Slanted or oblique views give excellent information in the vertical direction and they enable the user to directly view open BGA solder joints.

High level X-ray systems have adaptable measuring and evaluation software available for quad flat pack (QFP) and micro-lead frame (MLF) type solder joints. Such systems with comprehensive capabilities allow testing for the following:

• Bridges and electrical clearance
• Opens
• Side overhang co-planarity
• Voiding percentage
• Lead footprint
• Joint width and toe overhang
• Quality of heel, side and toe fillet

How does X-ray Inspection Work?
Electrons are emitted from a heated filament in a vacuum tube and are accelerated in the direction of an anode. Upon entering the anode, the electrons pass through a magnetic lens. The magnetic field produced within the magnetic lens redirects the electrons to a single focal point on a specified target. This target consists of a thin tungsten layer deposited on a light metal plate. When the electrons collide with the tungsten layer they abruptly decelerate and an X-ray source is created. The X-rays that pass through a sample board strike the image intensifier where they are translated into gray-scaled images. Since a light source is not present within the sample board and is not used in the imaging process, colored X-ray images are not possible. Figure 5-2 is a basic illustration of the X-ray process.

With the use of automated X-ray inspection systems as opposed to dated manual diagnostic analysis methods, board measurements and possible defects are detected in a minimal amount of time. The benefits of automated inspection are 1) increased consistency in results, 2) near real-time return of results, and 3) the ability to handle a high volume of circuit boards. Assuming proper setup by the user, the results obtained from this method are superior to results obtained from manual inspection but automated equipment can generate considerable setup and processing costs.

Transmissive Radiography Versus Laminographic Radiography
There is much debate as to whether two-dimensional analysis (transmissive radiography), which has a well defined history, is more beneficial than three-dimensional analysis (laminographic radiography). Transmissive viewing of a sample board on an angle relative to the perpendicular plane of the X-ray beam gives a two-dimensional image that can be mapped to a simulated three-dimensional representation (Figure 5-3). Disadvantages of the transmissive method are its difficulty in detecting cracks and its inability to clearly distinguish objects on different layers. The laminographic method allows for the viewing of solder joint volume including the respective component lead and pad. Although laminographic radiography is primarily associated with ball-grid array (BGA) inspection and can provide ball-to-board connection point images, transmissive imaging can determine if there is a defect in the connection itself.

X-ray analysis can be an effective process control tool for the electronics manufacturer. Its ability to reliably detect board defects and to verify critical board measurements during the production process is invaluable in maintaining quality control standards for world class manufacturers. X-ray analysis is a key resource on the EMPF’s demonstration Factory floor due to the advancements in available equipment and the effectiveness of the process. The pace of technological change in electronic assembly design will continue at an aggressive level creating an ongoing need for advanced analytical tools, such as X-ray analysis, on the factory floor.

References
1) Martin,Perry L. Electronic Failure Analysis Handbook. New York: McGraw-Hill publishing, 1999.

2) McClure, Doug (1999), X-ray "How it Works", Nischolet Imaging Systems. http://www.teradyne.com/prods/cbt/products/library/xray/wp_xrayhow.pdf.

3) More than counting black dots (2004), The Technical Knowledge Base For You. http://www.tkb-4u.com/articles/inspection/Xray1/countingdots.php.

4) A guide to solder joint inspection and analysis (2004), Phoenix X-ray.