Enhance your BGA x-ray inspection process: understanding x-ray signatures allows operators to rapidly diagnose the BGA assembly process and improve quality control.

The use of x-ray inspection for quality control of ball grid array (BGA) bonding has become widespread in surface-mount assembly. The numbers of equipment suppliers and features have proliferated, but the understanding of what a BGA x-ray image means has not advanced very far beyond the simple defects of shorts and missing balls and the misplaced concerns over small solder voids (Figure 1).

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Signature identification of x-ray images is a new means of extending the usefulness of x-ray inspection beyond the obvious defect library mentioned. In addition, signature identification can provide rapid, effective x-ray inspection of BGAs with just a modest investment in x-ray inspection equipment.

Anatomy of the BGA Bond

Before describing signature identification, you first must understand the BGA's anatomy and how the x-ray image relates to the assembly and reflow processes. The anatomical features that must be considered before, looking at the x-ray image are:

* Type of package. The type of package, such as plastic BGA (PBGA) and ceramic BGA (CBGA), will provide information on the thermal effects anticipated, such as thermal expansion, heat capacity and tendency to deform (Figure 2). PBGAs in particular will physically deform during reflow if they are not properly baked out. Deformation effects can be detected in an x-ray image.

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* Solder paste. If solder paste is used, such problems as screening too much or too little paste can occur. These problems can be observed in an x-ray image.

* Printed circuit board (PCB) construction. High heat capacity boards can affect reflow temperatures and thermal expansions. These conditions can be observed in an x-ray image.

Typical Reflow Stages

By combining real-time x-ray inspection with a rework station, we have actually observed and recorded using a patented process the following reflow stages as they were occurring. The three stages of reflow (Figure 3) are as follows:

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* Stage A-Preheat. Component placement before reflow can result in BGAs being aligned or partially aligned with their corresponding pads. As shown in Figure 3, the standoff height is 1 mm. The solder balls under the component lid are all perfectly spherical in shape, and their surfaces are smooth, yet dull in reflectivity.

* Stage B-Initial Collapse. When the reflow process begins and the solder's melting point (183[degrees]C or higher) is reached, gravity causes the initial drop of the BGAs. The standoff height is now 0.8 mm, and the solder ball shape has changed to become vertically elongated. The solder ball surface becomes rough with a matte finish. During initial melting, solder balls will start wicking to their pads, and, when all the balls have melted, the package will be pulled into accurate alignment with the pads.

* Stage C-Final Collapse. When the peak reflow process temperature is reached, complete and proper wetting of the entire pad is accomplished. At this point, a second drop of the BGAs occurs. The standoff height decreases to 0.5 mm. The solder ball changes once again to a round shape, and the surface becomes smooth and shiny.

During preheat, the standoff height is equal to the ball height, and the diameter of the bali's x-ray image is equal to the nominal ball diameter. During initial collapse, the standoffheight drops to approximately 80 percent of the initial ball height. During final collapse, the standoff height drops to approximately 50 percent of the initial ball height. The diameter of the ball's x-ray image increases approximately 17 percent, resulting in an increased projected area of 37 percent.

The changes in size and uniformity of the ball's x-ray image after reflow are key to an intelligent and informed understanding of the final x-ray image. At final collapse, if all ball x-ray images are uniformly circular and equal in area to within 10 percent to 15 percent, the probability is very good that no defects are present in the ball bonds. This condition is called uniformity (Figure 4). Uniformity provides the first and most important characteristic in the use of x-ray inspection to rapidly determine the quality of a BGA bond.

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However, when non-uniformity exists in the BGA x-ray image, variations can be observed that are either random or that have some type of pattern. Both random variation and patterned variation provide clues as to the possible defect or process problem and what may be causing it.

Signatures and Their Meanings

With this understanding, we can now look at some x-ray image signatures and speculate on what went wrong in the reflow process. Some possible problems include: insufficient reflow; cold solder joints; doming or dishing; and random differences in bond areas.

* Insufficient reflow. When reflow is not sufficient, several signatures can be observed (Figure 5). Elliptical bonds can result when the BGA, upon placement, is not exactly registered with the pads. When the initial melting stage occurs, the solder will wick to the pads, creating an elliptical image until full alignment occurs.

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However, if full reflow is not achieved, full alignment will not occur, and some or all of the x-ray images will appear elliptical. Elliptical images will also be observed if the pad pitch is not matched to the BGA pitch.

* Cold solder joints. The cold solder joint signature can appear as one or more bond images with irregular edges around their perimeters (Figure 6).

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* Doming or dishing. Doming or pop-corning and the opposite effect, dishing or potato chipping, are both caused by physical deformation of the BGA (Figure 7). Plastic BGAs are particularly susceptible to these conditions if the moisture is not baked out before placement.

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In doming, &lamination occurs under the die area, causing the plastic package to dome in the center. As a result, the ball bonds under the die are compressed and enlarged. In dishing, the BGA edges curl up, pulling and thinning the bonds on the perimeter.

* Random differences in bond areas.

When a signature of random differences in ball bond areas is observed, closer observation usually reveals that a corresponding random distribution of solder voids is also present.

When examining x-ray images for solder voids, be aware of possible voltage blooming (Figure 8). Voltage blooming is a distortion of the size of a void caused by the x-ray inspection machine itself. It is not an actual solder joint problem; a void exists, but it is not as large as it appears in the x-ray image.

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Because solder voids are easily detected with x-ray inspection, they are usually classified, without a clear understanding, as defects. However, some studies have shown that some solder voids actually improved the reliability of the solder bond. The question then remains as to what level of solder voiding is acceptable. This issue is addressed in detail in IPC Standard 7095, Design and Assembly Process Implementation for BGAs.

Effectively Using BGA Signatures

The signature identification concept can provide rapid, effective x-ray inspection of BGAs with just a modest investment in x-ray inspection equipment.

X-ray systems can be classified as being appropriate for either production environments or laboratories. In production environments, when signatures are employed, image magnifications up to 40 times are usually adequate. In laboratory environments, the concern is failure analysis; rapid inspection is not required, so magnifications of 1,000 times or more may be used.

Employing signature identification entails detecting x-ray image variations of more than 15 percent deviation from the nominal bond diameter. This difference can be detected by an operator, but computer image processors can more quickly and repeatedly flag deviation signatures. Figure 9 shows some examples of computer-processed flagging.

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Endoscopic Inspection

Even if a signature is identified, a defect is not necessarily present, nor does a signature precisely determine the cause of a defect. However, a signature does indicate that a process problem exists that can eventually lead to defects. So, if an x-ray image looks suspicious, get a second opinion.

An effective technique for a second opinion is the use of a side-viewing optical microscopic probe (endoscope). Several different endoscopic designs are commercially available that produce a magnified optical, video image of the ball itself (Figure 10).

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Endoscopic inspection of a BGA bond has proven to be an invaluable adjunct to x-ray inspection. Endoscopes can positively identify cold solder joints, poor contact, cracks and flux residue. However, endoscopic inspection is often cumbersome and time-consuming.

When used together, endoscopes, realtime x-ray systems and signature identification are the most effective means of inspecting BGAs, flip chips and chip-scale packages. X-ray inspection and signature identification very quickly qualify the BGA bonds as being either acceptable or suspicious. Suspicious bonds can then be examined off-line using an endoscope to determine the exact cause of the solder irregularity. Knowing the cause can point to the appropriate process correction.

Conclusion

Although x-ray inspection is widely used for the quality control of assembled BGAs, the full benefit cannot be realized until signature identification is also used. By understanding x-ray signatures, operators can more rapidly diagnose the process and make any corrections.

Gil Zweig is president of Glenbrook Technologies Inc., Randolph, NJ; e-mail: [e-mail].