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X-Ray inspection is commonly used at the EMPF for revealing solder joint defects that are invisible to the human eye. The following are defect characteristics most commonly revealed:
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- OPENS are usually caused by insufficient reflow, missing balls, doming, popcorning or contaminated board surface conditions. This defect is found much more easily if the board is tilted on its X and Y axis. This provides a view of the side of the joint, thus rendering a better angle into the suspected area of defect.
(See Figure 1)
- SHORTS are easily found using X-Ray, although on some double sided boards you will see a component from the backside giving the appearance of a solder bridge. This situation can be easily cleared up by rotating the board along the X and Y axis. This will separate your image distinguishing the front side with the backside components. (See Figure 2)
- INSUFFICIENT REFLOW is usually a little more difficult to spot. Some of the characteristics of a solder joint, which has not reflowed properly, include rough grainy appearances on the edges of the solder joint and an irregular shape of the joint.
- DOMING is a defect common when a package has not been stored in nitrogen or other types of non-humidity chambers. The characteristics of the doming effect are the center solder joints of the BGA are slightly smaller than those on the outside edge. When searching for this defect, be certain to bring the entire BGA through the center of the camera. This allows you to eliminate the parallax error caused by the camera lens.
- POTATO CHIPPING occurs when a component's outside edge lifts up from a pad. This will cause the center joints to appear squashed as a result of overheating the component. This defect can be found by rotating the board on its X and Y axis allowing you to see the hourglass appearance of the outside edge and corner solder joints. In addition, a squashed view of the center joints may also be apparent.
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- VOIDS are usually found in boards which have not been above reflow long enough. They are typically created by flux gases that are unable to escape. The best way to view voids is to use a lower voltage and amperage, while the board lays flat along the X and Y axis. An increase in these settings may cause voltage blooming. You can also adjust the X-ray contrast to expose the voids better. As shown in Figure 3, the appearance of a void will be lighter in contrast then that of the solder ball.
- POPCORNING is a violent eruption of moisture from within the package. The eruption may break the die from the substrate, it may pull wire bonds from pads or delaminate the package substrate itself. Plastic packages absorb water from the air. The propensity of the package to absorb water is established through testing. The current industry standard for this testing is J-STD-020B. Packages that have been classified using the standard should be handled in accordance with J-STD-033A. Proper handling requires users to store parts in special vapor barrier bags with desiccant. If packages are exposed to moisture, the factory air, it may be necessary to bake the parts for some time to drive moisture out of the package.
Popcorning of the package may not be visible in X-Ray inspection. X-Rays pass through silicon and epoxy easily. So, if the die is popped off the substrate we may not see that type failure under X-Ray. If the substrate delaminates and blisters, localized swelling may deform some of the solder connections. We have observed this in the form of irregular sized balls around the site of the package failure.
 For more information or details on x-ray inspection at the EMPF, contact the Helpline (610-362-1320 or helpline@empf.org).
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