This month issue An Independent Journal Dedicated to the Advancement of Chip - Scale Electronics September - October 2000 Email the editor

By John Goings, Sonix Inc., Springfield, Va.

Figure 1. Unlike SAMs in the past that were oriented toward failure analysis labs, this Sonix UHR-2000 scanning acoustic microscope can be geared to production environments.

To date, scanning acoustic micro-scopy has not been widely used for inspecting CSPs. Currently, X-ray inspection is the most popular method for CSP applications, including component-to-board joint inspection. For many types of CSP inspections, however, the SAM (Figure 1) offers a superior imaging method.

Although there are notable exceptions, the SAM's ability to identify very slight density variations and air gaps makes it the preferred inspection method when the suspected defect results in a thin, planar delamination.

On the other hand, X-ray inspection is often the preferred method for solder ball and die attach voiding and CSP-to-board attachment quality.

Acoustic microscopes have traditionally been found in failure analysis laboratories where throughput is not a major issue; recently, however, interest is growing in the use of acoustic microscopes in a production or lot qualification role.

In the past, the widespread use of acoustic inspection has been limited, due to its relatively slow throughput rate, slow scanning speeds and available analysis software.

Recent technological and material handling advances allow SAMs to provide the throughput necessary to be appealing in production environments. It is important for even those familiar with SAMs to give the technology another look.

This article discusses the use of the scanning acoustic microscope to inspect the most common CSP failure modes, including die cracking, underfilled CSPs and inter-laminate failure.

Figure 2. SAM image of die cracking generated with a 260 MHz, 5.9 mm transducer

Die Cracking

Die cracking is a critical failure mode for all types of packages, including CSPs. Conventional failure analysis tools, like destructive cross-sectioning, are laborious and destroy all possibility of conducting further studies on the device.

In addition, hairline fractures can be very difficult to confirm by cross-sectioning. X-ray inspection for die cracking, though feasible, is often difficult and falls prey to subjective image interpretation. SAMs, however are ideally suited for finding die cracking in CSPs.

Figure 2 shows an acoustically generated image created using a 260 MHz 5.9 mm focal-length transducer. Note that the red arrow points to a crack within the silicon die. This shattered die was likely caused by die stress induced by substrate warpage.

Underfilled CSPs

While underfilling of CSPs is not too common, the use of underfill is growing. One area of great interest, in terms of CSP reliability, is the CTE mismatch between the component substrate and the PWB.2

Underfilling, because it reduces or equalizes the CTE mismatch and aids in the structural support of the solder joints by reducing stress, appeals to many users. Furthermore, the process is particularly valuable in high-reliability environments.

The SAM image in Figure 3 shows underfill voiding/delamination in a CSP. (For demonstration purposes, a dummy CSP without encapsulant was used.) Other nondestructive failure analysis methods, X-ray among them, are unable to distinguish the subtle density variations between underfill and air.

SAMs, on the other hand, are very sensitive to even the thinnest of air gaps and can be imaged quite easily. Underfill voids located away from solder joints are not as problematic as defects adjacent to joints.

Figure 3. SAM image of an underfilled CSP employing a 260 MHz, 5.9 mm focal length transducer

When located near the joint, voids have an increased impact on joint integrity. (In Figure 3, the blue arrow points to a void in a noncritical area, while the red arrow points to a "halo"-type underfill void touching a joint.3)

SAMs also possess the ability to calculate the percentage of disbond automatically and output an accept/reject result. The result can also be based on the level of defect location criticality.

Figure 4 shows a magnified area of the lower-left corner of the image in Figure 3. The red arrow points to a bright white area signifying an underfill void commonly referred to as a "halo" defect.

These defects are frequently caused by "creep stress" or a lack of flow around a particular solder joint.4 The structural integrity and electrical functionality of this joint is most likely compromised. Upon stressing, electrical shorting is more likely to occur in areas with a lack of underfill.

Figure 4. SAM image of underfilled CSP. This is a close-up of the lower-left corner of the image in Figure 2.

Inter-Laminate Failure

One of the more common CSP failure mechanisms is the delamination of the inter-laminates of thin-film substrates.

When CSPs are designed for portable electronics, a small degree of substrate flexibility is desirable, because it enables the substrate to tolerate harsh drop testing and mechanical fatigue environments.5

Polyimide tape is among the more commonly accepted CSP substrate materials. As a consequence of polyimide's flexibility, the likelihood of inter-laminate separation is increased, especially around intermetallics where bonding is most critical.6

The air gap created by delamination within the layers can be easily observed with a SAM. Again, conventional inspection methods like X-ray have difficulty distinguishing the density variation between the laminate and an air gap.

Similarly, electrical test cannot provide a clear indication of long-term component reliability, making SAMs the preferred inspection tool for this application.

Figure 5 is an acoustic image showing a separation of the inter-laminate within a thin film. The red arrow points to an area of delamination within the inter-metallics of the package. The weak layers within the polyimide tape apparently caused this delamination.

Figure 5. Scanning acoustic micrograph of a CSP imaged through the substrate using a 110 MHz, 8-mm focal length transducer

Conclusion

Acoustic microscopes can be used to inspect a variety of structural and internal reliability issues in CSPs. This technology has proven to be especially valuable for the inspection of die cracking, underfill voiding and inter-laminate delamination failure modes.

SAM technology has carved out a niche in CSP inspection through its ability to image nondestructively while displaying even the thinnest air gaps. Many researchers have previously documented the usefullness of SAMs for finding defects such as popcorning, and poor attach bond quality.7-8

References

1. J. Sigmund and M. Kearny. "Acoustic Microscopy of Flip-Chip Packages," Proc. Pan Pacific Conference 1999, p. 52.

2. J. Clech, "An Expert Looks at the Issues," Chip Scale Review, November-December 1999, p. 41.

3. A. Babiarz and J. Semmens, "Inside Underfill," Circuits Assembly, August 1997.

4. M. El-Ghor, M. Peterson, et. al., "Evaluation of a 64-Pin MicroStar BGA Package," Proc. Pan Pacific Conference 1999, pp.98-105.

5. Y. Chiou, T. Chen, et. al., "A Simple Flex-CSP and its Reliability," Proc. Pan Pacific Conference 2000, p. 326.

6. W. Lawton and J. Barrett, "Characterization of Chip-On-Board and Flip-Chip Packaging Tech-nologies by Acoustic Microscopy," Microelectronics Reliability 1996, Vol. 36, No. 11, 12, p. 1803-1806.

7. Sigmund and Kearney

8. Lawton and Barrett

Acknowledgment

The author acknowledges contributions by Cheryl Hartfield, Texas Instruments, for insights into the use of acoustic microscopes for CSPs.