The increased use of Ball Grid Array (BGA) and Flip Chip component packages provides many unique advantages over conventional leaded components. Their relatively small outline, coupled with their increased performance characteristics, make these components an ideal package to incorporate into many electronic designs. In spite of their advantages, the use of these advanced component packages presents a unique set of challenges to the electronics manufacturer, not only in process development but also in inspection techniques used to verify acceptability of these components. Visual inspection techniques are no longer applicable when dealing with array packages and verification of solder connections between the component and substrate is critical.

This article examines the use of fluoroscopic or "continuous real time" X-ray inspection techniques and provides graphic examples of the more common defect conditions found when processing area array component packages. Although some of the examples shown are easily discernable asdefect conditions, others are more subtle and require closer inspection of the X-ray image to detect any possible defect conditions.

Before examining the defect conditions and their corresponding images, it is important to understand the physics of the wetting forces involved in the creation of acceptable, reliable solder connections under these area array packages. During the initial preheating stage, the component balls and solder paste remain in tact, while the flux activators begin to chemically scrub oxidation from the solderable surfaces. As the temperature increases above the melting point of the solder, the component balls and the solder paste begin to reflow together. The weight of the component begins to collapse the melting solder connections.

Finally, during the reflow portion of the thermal profile, ample time above the liquidus point of the solder is provided allowing the solder connections to fully melt, causing a drop in component standoff height. During this stage, the surface tension of the molten solder may physically pull the component into alignment and may self center the component to the land pattern of the substrate.

By understanding the physics of the molten solder connection, we can draw some conclusions regarding the appearance of properly wetted solder connections under X-ray analysis. The main characteristic to look for is uniformity of the connections (Figure 1). If the X-ray image shows all connections uniformly circular and relatively equal in area, this is a good indication of complete and proper reflow of all the solder connections associated with the array component.

Knowing that uniformity is indicative of proper wetting and reflows, let's now examine some of the more common soldering defects associated with BGA and Flip Chip component packages.

Figure 2 shows a typical X-ray image of solder bridging between adjacent BGA connections. Some of the probable causes for this condition are smearing of the solder paste during the stenciling operation, misregistration of the component, excessive flux paste and warping of the component during reflow.

With careful handling of the BGA components, the component solder balls are rather robust. It is possible however to damage a component interconnect to the point of catastrophic failure. As shown in Figure 3, the damaged BGA component was not inspected prior to placement and the missing sphere is visible under X-ray analysis.

X-ray images of component interconnects exhibiting solder voiding are not as readily discernable as those showing bridging and missing balls. Figure 4 shows an example of solder voiding. Because the X-ray beam is transmitted through the solder mass, less dense areas of the interconnects appear as faint or white areas. These less dense areas are voids in the solder connection and can present future reliability issues with the component interconnections. It is important to note that solder voiding will only be visible under higher X-ray powers of approximately 60kv or greater and lower power X-ray equipment may not reliably detect the presence of solder voids. Solder voids are usually an indication of an improperly adjusted thermal profile and are actually pockets of entrapped flux gases that were not able to escape from the molten solder connection during reflow. Poor quality solder paste, as well as component and land contamination, can also cause this defect.

We can understand how important the thermal profile is to successfully soldering area array packages. An improperly adjusted thermal profile can lead to the next series of defects we will examine. Figure 5 shows indications of insufficient reflow. The solder connections were either not brought to a high enough temperature or above the melting point for a sufficient amount of time to completely melt the entire solder connection. The oblong shape of the solder connection is attributed to the wetting force of the molten solder. As the connection becomes molten, the surface tension of the molten solder tends to pull towards the land area. If all of the component interconnections are not completely molten for a sufficient length of time, some joints will attempt to self-center while others do not.

Figure 6 shows a cold solder connection under the BGA component. The reasons for this defect are either insufficient reflow, insufficient solder paste, or contamination. In either case, the component ball failed to wet sufficiently to the land to form a reliable solder connection. It should be noted that the best chances of detecting this type of solder defect under X-ray analysis is to present the assembly at an angle to the

X-ray beam. An angle of 45 degrees is usually sufficient for this purpose. Irregular edges around the perimeter of the joint are indicative of this defect condition.

All defects dealt with in this article until now have focused on the failure of the interconnect between the component and substrate. We will now examine a failure mode of the actual component package known as "popcorning," which manifests itself in warpage of the component package. All area array packages have limitations on the amount of allowable exposure to moisture in an ambient environment. Any absorbed moisture in the component package must be eliminated prior to exposure to the high temperature reflow process. If this entrapped moisture is not eliminated, it will rapidly boil during the reflow process and the resulting steam pressure may cause internal component damage. As the component package warps, the coplanarity of the component to the substrate is compromised, resulting in less than acceptable solder connections. Evidence of this condition can be seen in Figure 7.

The component package warped during reflow causing the outer edges of the component to lift. As a result, the center of the array is forced down into the solder connection forming irregular shaped balls while the outer connections physically separate from the component causing fractured, intermittent and unreliable solder connections as shown in Figure 8.

It is apparent that one of the most reliable and cost effective methods of inspection of BGA and Flip Chip package solder interconnections is through the use of fluoroscopic X-ray imaging. As we have seen, some defect conditions are easily detectable while others require greater attention to detail and an understanding of the image presented. Through a partnership between the EMPF and Glenbrook Technologies, one of the world's leading manufacturers of X-ray inspection equipment, we are able to diagnose defect conditions as presented above and also provide inspection training to the personnel responsible for making the decisions regarding acceptability. If your process requires the use of X-ray inspection or training of inspection personnel using fluoroscopic techniques, please do not hesitate to contact the EMPF Helpline at 610-362-1320 for additional information on the services provided at our facility.