Elementary, my dear rework technician: uncovering the mysteries of 2-D x-ray inspection.

Reworking high mix/low-volume ball grid array (BGA) packages requires the discipline and attention to detail of a hard-nosed detective. One essential tool for the successful sleuth is x-ray inspection. Rework departments examine BGAs after rework with visual and x-ray inspection, but some expertise is needed to use x-ray technology effectively. Since most systems use two-dimensional (2-D) x-rays, instead of the higher-end 3-D x-rays, the canny sleuth must learn how to use this tool to repeatably perform BGA rework.

The 2-D x-ray systems provide valuable, non-destructive information that is not directly visible. In certain conditions, x-ray image quality and detail may be limited, so a degree of operator skill is necessary to identify defects. A typical 2-D x-ray image shows features on the top and bottom sides of the board and possibly some metal features in the internal structure. On double-sided, multilayered assemblies, "shadowing" between features on the top and bottom sides can cause confusion. Some materials and defects are difficult to see using x-ray, but can be detected through telltale indicators.

The rework technician might consider the evaluation process as a reflow profile case to he solved. Inspection is an information gathering exercise, as we look for clues to indicate whether reflow at the site was successful. Using visual feedback, x-ray data, equipment heat and airflow indicators and thermocouple information, we determine if we have succeeded in properly reflowing the BGA. We then declare our reflow profile problem solved or make the necessary adjustments. If the operator is fortunate enough to have access to in-circuit testing, his/her reflow question can be easily answered. But, for the average gumshoe, something is usually missing from the information puzzle; x-ray inspection helps fill that void.

Here are some hints on how to proceed with x-ray inspection. Directly after reflow, visually examine the outer rows of balls. Look for visual defects and carefully observe the location and form of good balls and connections. Next, note how the good connections look in the top-down x ray image and compare them to ball/joint images that are in places we cannot manually see. By comparing the known shapes with the unknowns, we can further build our case for the success of our reflow process. Irregularities in ball shape may indicate defects that should be explored in more depth.

Next, turn up the intensity of the x-ray image and look for voids. The higher intensity gives a great view of the inside of ball; the lighter white areas inside the balls are voids. If you identify voids, determine if they fall within the limitations stated in IPC A 610 or IPC 7095.

Two-dimensional x-ray systems will not indicate opens; however, inconsistencies in the image patterning can provide clues that a problem exists. Does an image of greater circumference indicate a component ball flattening out over unflowed paste? Does the narrowing of a ball or trailing off of the solder dog bone connection indicate mask breakdown at the site? Do your observations agree with your visual inspection of the site prior to placement? Does your thermocouple information lead you to think you might have under- or overflowed the location?

Even systems capable of looking at individual layers do not always show the finer problems, such as fissures in the board or the ball/pad junction. Taking into account reflectivity issues and the fuzziness of x-ray imaging, inspectors must have a sharp detective's eye and a data collector's mind to know what to look for.

Some problems are easy to assess with x-ray technology. Shorts are usually easy to detect as one ball graphically bleeds into another. Another easily detected problem is the dramatic breakdown of solder mask at the pad location.

X-ray inspection provides invaluable help to the detective in need. The technology can detect problems in visually inaccessible areas, including shorts, wicking of solder down its attached via or circuit, unwanted foreign materials under the component, extraneous solder balls, bleeding of the vias through the board, paste-clogged via holes and voids. X-ray inspection is not perfect but is essential for reliable BGA rework. Just having the tool is not enough; the training, experience and attitude of a regular Sherlock Holmes makes the most of this invaluable technology.

Jeff Ferry is president

of Circuit Technology

Center, Haverhill,

MA; [phone];

www.circuittechctr.com