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Step 9: Test & Inspection
December 31, 1969 |Estimated reading time: 5 minutes
As BGA usage increases on printed circuit board (PCB) assemblies, more contract manufacturers are implementing X-ray inspection into their test methodology. This inspection method can non-destructively check not only the quality of the optically hidden BGA solder joints, but also can be used to confirm overall production process quality, such as the level of voiding within solder joints.
By David Bernard
New users of X-ray inspection often will use IPC-7095, Design and Assembly Process Implementation for BGAs as the initial basis for BGA inspection criteria. However, the X-ray images contained within this publication are several years out of date in comparison to what can be provided by today's X-ray inspection systems, affordably and by less skilled operators. In particular, 2-D X-ray inspection provides enhanced resolution, higher magnification, oblique-angle views, automated fault analysis and higher penetrating powers to improve the analytical quality of X-ray in BGA inspection.
Within X-ray system development, for example, the more prevalent use of open-style X-ray tubes compared to closed tubes has immediately provided substantially greater magnification to the X-ray user, together with a minimum of 5X better resolution. Add to this the ability to observe all around the BGA at oblique angle views by tilting the detector instead of the sample without compromising the magnification, and the user can inspect the interfaces between the BGA solder ball and the board. If oblique views are unavailable from an X-ray system, the shape and bulk of the solder ball itself will obscure the joint interface, making detection of open joints (opens) virtually impossible. As devices continue to shrink in size, the need for oblique views and high magnification becomes critical.
Figures 1a and b Digital 16-bit X-ray image of a single BGA solder ball at an oblique angle. The location and size of the interfaces of the ball to the board, and the ball to the device, can be seen as ellipses within the ball. The ellipses have been hand colored in Figure 1a for better clarity. A large void also can be seen with its position clearly adjacent to the lower interface.
The most dramatic and recent development in X-ray system im-provement is use of digital detectors to produce the X-ray image. Such detectors permit X-ray systems to have up to 16-bit grayscale sensitivity, allowing the slightest density variations to be observed. In particular, this capability can make it possible to see the shape of the actual joint interfaces within collapsible BGA solder balls. Figures 1a and b show an example where the joint interfaces, together with a large void, can clearly be seen within the solder ball. This level of inspection would not be possible at lower grayscale sensitivity and/or lower magnification. The presence, or absence, of the ellipses (visible in the photos) can help to indicate joint quality — an absent "ellipse" compared to neighboring solder balls strongly suggests an open joint.
Using these new X-ray system capabilities, a suggested regime for manual inspection of BGAs is:
- Adjust the X-ray tube kV and power to achieve a suitably contrasted image for the operator. This will be sample dependent, with higher tube-accelerating voltages (kVs) necessary for thicker boards/devices, as well as devices with or within metallic covers or heat sinks.
- Inspect the BGA at low magnification from above (i.e., not obliquely). This will clearly highlight gross defects:
- Solder bridges that may have occurred between BGA balls. Bridging can be caused by excessive paste volume being deposited, or so-called "popcorning" of the device because of incorrect MSD control or thermal handling of the item prior to placement as described in IPC-7095. (Figure 2)
- Missing solder balls within the BGA matrix. These are indicated by a ball image that is less dense. (Figure 2)
- Misalignments between the BGA balls and the pads on the PCB. (Figure 3)
- Presence and frequency of voiding within the BGA joints.
- Presence of foreign objects within the BGA matrix, or that of solder "splashes" or contamination.
- Inspect the whole BGA at various oblique angle views at low magnification. This should indicate:
- Any variation in the ball shape within the BGA, indicating opens or other faults (see IPC-7095). A range of different oblique views is suggested, as the suitable viewing angle may differ in components.
- Misalignment between ball and pad.
- Increase magnification and repeat straight down, and oblique angle, views of any suspect areas determined in Steps 1 through 3. If required, make measurements of the BGA. For example, X-ray systems may allow manual and/or automated calculation of the BGA voiding percentage and ball diameters to help identify suspect locations.
- Criteria for acceptable voiding percentages within BGA balls, and its meaning for joint reliability still is a hot topic of debate within the industry. However, a general suggestion is that a level of voiding that exceeds 20 to 25 percent within the BGA balls indicates a process problem. IPC-7095 includes suggested values for acceptance criteria used to set limits on the percentage of voids allowed.
- At high magnifications, differences in ball shape and size in the BGA can be identified and used to confirm earlier diagnosis.
- With experience, the characteristic shapes of open joints become more obvious. Figure 4 shows that the shape of a solder ball that has not been reflowed is different from that of the reflowed ball. Such shape variations can be quickly taught using "good" and "bad" examples to allow deskilling of the inspection process.
- For a comparison, look at the interfaces of single BGA balls shown in Figure 1.
Figure 2 X-ray image of BGA at low magnification. Red arrow indicates location of a bridge, and the blue arrow indicates location of missing solder balls. Figure 3 X-ray image of BGA at low magnification. Red arrow indicates area of misalignment between BGA and pads.
Characteristic ball shapes (Figure 1) does not guarantee the presence of faults. For example, the circuit configuration within the PCB may result in a ball shape that is of concern, but by looking at the ball at different oblique views, the joint can appear to be as good as the rest of the BGA.
Figure 4 Oblique angle view X-ray image of BGA where the solder ball, indicated by the red arrow, has not reflowed, remaining circular in comparison to the reflowed ball (blue arrow). The red arrow shows the separation between the solder ball and the paste that has not been reflowed.
To speed BGA inspection, consideration also might be given in the design phase to include wetting indicators within BGAs. Solder can be encouraged to partially flow down the track for a short distance, creating a characteristic ball shape after reflow that can be seen in the X-ray image. Any joints that do not follow this shape can then be identified.
Conclusion
X-ray inspection of BGAs gives better-resolution analytical images at higher magnifications and oblique angles, allowing a far more sensitive and rigorous interrogation of quality of the BGA joints. Once BGAs have been inspected, X-ray inspection can be used to look at other components on the board.
References
For a complete list of references, contact the author.
David Bernard, X-ray systems product manager may be contacted at Dage Precision Industries Ltd., Rabans Lane, Aylesbury, Buckinghamshire, HP19 8RG, England, +44 1296 317800; Fax: +44 1296 435408; E-mail d.bernard@dage-group.com.