An A-to-Z Guide to X-Ray Inspection, Part II Print E-mail
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Written by Dr. David Bernard   
Monday, 31 December 2007 19:00

Practical recommendations for production sampling and viewing various part types.

Ed.: This is the second of a two-part article; part one was published in December.

Various procedures are suggested as a standardized approach for x-ray inspection of specific component types on assembled PCBs in a production environment. It is recommended that a sample of two boards be examined from each batch for normal in-process inspection. Inspection should also occur when changing temperature profiles or when setting new product introduction profiles. This information can be valuable for future reference, particularly when using new component types.

Voiding is a common fault detected by x-ray inspection. Voids are the presence of air bubbles or other nonmetallic material trapped within the solder joint. Voiding in production is usually caused by either a fault in achieving the peak reflow temperature or the board spending insufficient time above the liquidus temperature of the solder paste. The question of what level of voiding is acceptable is hotly debated. SnPb and Pb-free solder joints will almost always have some level of voiding. However, the size and quantity of voiding seen will depend on the sample and the x-ray system’s detection sensitivity.

X-ray inspection should be conducted after rework of any area array devices, land grid array (LGA) or quad flatpack no-lead (QFN) components. This will quickly confirm rework quality nondestructively and help establish correct rework process profiles.

BGA voiding. The presence of voids and their quantity are process quality indicators. In particular, if the voiding level over time tends to increase or decrease, this is often seen as a sensitive indicator of changes in the production process. BGA void percentage is calculated on an x-ray inspection system by totaling the lighter pixels (void pixels) within each solder ball and presenting them as a percentage of the total number of pixels within the entire solder ball area. This calculation is achieved by looking at the ball from the top-down, but recognizes that it gives values based on 2-D data for 3-D spheres. The calculation requires setting suitable greyscale thresholds to define the solder ball outline and void pixels.

Two primary types of voiding can be present: process or “bulk” voids and interfacial voids. Process voids are often relatively large and positioned in the middle of the ball or associated with one of the solder ball interfaces, pad side or device side. Process voids often can be reduced in size by slightly increasing the time above liquidous for a few seconds, thereby permitting volatiles to fully escape during reflow. Interfacial voids are smaller than process voids and are typically associated with the solder ball interfaces, most often to the pad or device. The position of these voids can be confirmed by observing the solder ball at different oblique angle views within the x-ray and observing how the voids move relative to the surrounding materials.

Limited greyscale capability of earlier x-ray systems and detectors was insufficient to see the interfacial voids; only the larger process voids could be seen. It is suggested there always has been some level of interfacial voiding in BGA solder joints, but this could not be detected with older systems. Newer digital x-ray systems can detect more than their older counterparts because of advances in x-ray tube technology (Figure 1).

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It is suggested within the industry that any voiding at a BGA device interface may be more detrimental to overall joint quality than if the voiding is in the bulk of the solder ball or at the pad interface. The known situations where interfacial voiding may cause production issues are within the failure mode described as “champagne” voiding. It is suggested substantial interfacial voiding at the pad interface from the effects of the board surface finish can cause champagne voiding. Digital x-ray inspection, with its greater greyscale sensitivity compared to its analog counterpart, can distinguish differences between interfacial voiding and bulk voiding and emphasize the presence of non-reflowed or open joints (Figure 2).

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‘Wetting indicators.’ To assist BGA inspection in confirming that good reflow has taken place or highlight the presence of open joints, it is possible to modify the BGA termination pads during design so they include “wetting indicators.” Wetting indicators make BGA solder joint inspection much easier because a wetting indicator is a minor, deliberate change to the BGA joint shape that can be easily seen in the x-ray image. For example, this could be achieved by designing pads in elliptical forms instead of circular shapes. Alternatively, an area of the trace from the mounting pad could be left exposed from the design of the solder mask. The result of either modification is to permit the solder to deliberately wet away from the main pad in a controlled manner during reflow, causing a characteristic solder ball shape compared to “standard” reflow. This characteristic wetting shape will be obvious within the x-ray image and make any non-reflowed joints much easier to identify.

Not all pads need be defined this way. It is sufficient to have a single example of this type of reflow indicator near each corner of the BGA together with a few in the center of the device. Failures are most likely to occur in these two areas. This approach will give confidence the reflow process is efficient.

X-ray inspection systems should make void percentage measurements on BGAs automatically so that any trend in the voiding level over time can be identified. Measuring the void percentage level within BGAs not only ensures current production quality meets necessary standards, but also is able to monitor any subtle changes that may occur in the process. With LGAs and QFNs, care should be taken to inspect the center area of these devices because they are being used to dissipate heat. Excessive voiding in this area will mean a reduction in the die attach area, which results in a reduction of the heat dissipation capacity of the device.

QFP inspection should begin from one corner of a device and scan around all four sides (Figure 3). Attention must be paid to the presence of heel fillets, side fillets and, if possible, toe fillets on gullwing leads. Toe fillets will not always be visible during inspection due to the lack of wettable area on the lead tip. Heel fillets should be consistent in size and are the area that will be subject to stress during mechanical or thermal cycling. Voiding may also be present under the lead. Heel fillets will be visible on all gullwing leads, ideally with toe fillets present, as well as J-lead terminations having fillets on the back and front of the lead.

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Inspecting discretes. X-ray inspection of passives should be left until last, as they normally will be satisfactory if all other parts are confirmed as completely reflowed. As a result of their small mass, they are likely to reflow before any other component and therefore less likely to exhibit voids. However, they may exhibit voiding following double-sided reflow.

When a chip component has successfully been soldered, there will be evidence of a fillet on the end terminations and possibly on the side terminations. The solder joint area under the chip termination should also be assessed.

Small actives such as SOT23, SOT89 and SOIC devices are also less likely to exhibit poor reflow. These devices’ low mass makes their complete reflow relatively straightforward. It is possible to see voiding on SOT89 components on the center termination. As the size of transistor packages increases, their power handling tends to increase; therefore, any substantial voiding in the termination may affect heat dissipation from the device.

Advanced packages. Inspecting semiconductor packages places a greater demand on x-ray inspection and provides a unique set of challenges. Traditional analysis using 2-D x-ray imaging is often limited with advanced package types because the multiple layers within the device are seen at the same time. This can be confusing since multiple dies and multiple layers of wire bonds appear to overlap each other in a 2-D image.

Another challenge of packaging inspection is interfacial and plating void detection within microvias. While difficult to detect with normal 2-D x-ray imaging or cross-sectioning, these defects can be determined with computerized tomography (CT) inspection (Figure 4).

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Recent advancements in CT technology have greatly improved imaging speed and resolution of the most detailed features. Because of these improvements, CT has become an ideal inspection methodology for complex 3-D packages since it generates a 3-D model of the entire electronic package. The resulting 3-D model can be viewed in real-time so that interconnections normally obscured by other joints or components within the package can be diagnosed, ensuring complete package inspection.

As component technology continues to drive the miniaturization of devices, the need for x-ray inspection becomes more apparent. Traditional means of inspection are still necessary, yet are less effective than x-ray for detecting and resolving manufacturing defects with today’s advanced packages and area array devices.

Dr. David Bernard is product manager x-ray systems at Dage Precision Industries (dage-group.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Wednesday, 02 January 2008 11:47
 

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