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Using Underfill to Enhance Lead-free Drop Test Reliability
December 31, 1969 |Estimated reading time: 8 minutes
With two months until the RoHS deadline, one would assume that most issues pertaining to lead-free manufacturing would have been resolved. Manufacturers have tested and retested materials, evaluated process parameters, and implemented full lead-free manufacturing. While the use of underfills has been known to increase reliability of lead-free solder joints when compensating for CTE mismatches, they also show promise for mechanical drop test and vibration reliability of handheld devices.
By Dan Maslyk, Mark Privett, and Brian Tolen, Ph.D.
With the RoHS deadline just two months away, there are still questions remaining regarding the reliability of Sn/Ag/Cu (SAC) alloys, as compared to tin/lead alloys. Some issues related to unexpected lead-free solder joint failures have been studied and addressed. Most notably, the reliability of lead-free solder joints in relation to failures arising from coefficient of thermal expansion (CTE) mismatches has been resolved through the use of certain types of underfill materials that provide increased stability and reduced device stress.
Other challenges with lead-free solder joint reliability exist beyond thermal-cycling stresses, particularly with chip-scale package (CSP) and wafer-level chip-scale package (WLCSP) devices within handheld products. As the industry learns more about devices in the field that have been manufactured using lead-free processes, additional data suggests that using robust underfill materials with increased flexibility, modulus, fracture toughness, and adhesion deliver extremely high levels of drop-test protection for advanced CSP devices. These components are being pushed to the limits with increasing demands on functionality, resulting in finer pitches and a higher likelihood of failures. Regardless of these issues, end users expect these products to be dependable, even when manufactured in a lead-free process. These demands are forcing electronics assemblers to find new ways to ensure device reliability; the use of robust underfill materials is proving to be one of the most capable systems for achieving these goals.
Figure 1. Stress strain curves for tin/lead and SAC alloys.
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Counterintuitive Nature of Lead-free
When the electronics industry began researching lead-free manufacturing, it was believed that the move would enable more-reliable solder joints. Bulk testing of SAC systems showed increased strength (Figure 1) over tin/lead alloys. A tin/silver alloy displays more than two times the creep strength of tin/lead alloys (Table 1). Therefore, if bulk properties of a solder alloy were used as the primary indicator of solder joint reliability, one may assume that lead-free assemblies would have higher reliability. But that is not the case. Bulk-alloy tests have shown poor correlation to lead-free assembly reliability. To get a complete picture of lead-free solder joint robustness, other factors such as design, materials selection, and process dynamics must be evaluated.
When these factors are taken into account, data reveals that lead-free solder joints exhibit more vulnerability to failures due to CTE mismatches, particularly in area-array components. Forces arising from CTE mismatches make solder joints more prone to cracking, so some joint creep is an advantage. The increased likelihood of board warpage due to higher lead-free peak temperatures, combined with the lower ductility of lead-free alloys, make failures and joint cracking more probable. Some package types, such as SOICs and QFPs, are more resistant to this because component leads are able to flex, but chip-type components have limited flexibility. When the board warps, fractures in the solder joints of CSP devices are more likely, unless countermeasures are taken to minimize stress on the solder bumps. Using underfill materials spreads the stresses across the entire surface area of the CSP, reduces concentrated stresses at the joints, and provides additional environmental protection to the component and individual solder bumps.
Although a significant body of work has been completed to confirm that thermal cycle reliability may be improved with underfill usage, drop test, or physical shock reliability, is just as important to users of handheld devices as thermal-cycle reliability. New data prove that certain types of next-generation underfills provide more protection than the reduction of joint stress arising from thermal-processing issues.
Lead-free CSP Drop Test Reliability
By nature, lead-free solder joints are more brittle than tin/lead predecessors.1 Because of this, lead-free solder joints have an increased tendency to crack and break, especially when subjected to external forces such as dropping. In mobile phones, PDAs, MP3 players, and other mobile communications devices, this is precisely what happens. Couple this with the fact that these devices are populated with very fine-pitch components and it would appear that lead-free solder joint failures in these products is inevitable.
Figure 2. Drop test results of tin/lead vs. lead-free with and without underfill.
Just as underfill materials have provided stability to CSP devices for failures arising from higher-temperature lead-free processing requirements, new testing has revealed that certain types of underfills also provide protection against failures due to physical shock during drop.
Drop Test Reliability of Tin/lead vs. Lead-free
Earlier drop testing of traditional tin/lead solder joints to lead-free solder joints on a 0.5-mm pitch, 8-mm body size CSP device with 132 I/0 and a 3-row perimeter were conducted using a 6-meter drop onto stainless steel. The tin/lead paste used was Sn63, and the lead-free solder paste was SAC387. To evaluate the protection offered from underfill materials, testing was performed on both tin/lead and lead-free non-underfilled CSPs, CSPs underfilled with older-generation materials, and CSPs underfilled with a new-generation, flexible underfill system. Test results confirm that tin/lead solder joints have better reliability than lead-free joints, and that underfilling increases the reliability of lead-free solder joints significantly. As can be seen from test data, while older-generation underfills helped improve lead-free solder joint reliability, newer-generation materials that can absorb more physical shock are more robust and protective for certain applications (Figure 2). Solder joint characteristic life parameters (characteristic life is defined as the point at which 63.2% of the devices have failed) were assessed based on drop test data, again revealing that the newer-generation, more flexible, fluxing underfill demonstrates better performance than the older-generation, more rigid underfill (Table 2).
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A Universal Platform for Drop Test Standards
The test method previously described is used by some labs to test reliability to drop testing. Every electronics manufacturer has developed its own, custom-designed drop test requirements; an approach that has made similar materials and product comparisons virtually impossible. Because of this confusion, and the inability to make valid, equal materials and product evaluations, the JEDEC JESD22-B111 drop test standard was developed. Using this standard, OEMs, contract electronics manufacturers (CEMs), and materials suppliers can make meaningful comparisons.
Figure 3. JEDEC JESD22-B111 test board.
By definition, JEDEC JESD22-B111 provides a common test platform for handheld electronics products that fall under consumer and portable market segments. Because these products are more prone to being dropped, electrical failures can occur, and may result from various failure modes, including cracking of solder interconnections between the component and the board. The test method provides a standard test vehicle design (Figure 3), and drop-test parameters in terms of pulse width and G forces (Figure 4). This combination enables users to know the force being applied to any single site when the test is performed according to the specification.2
Figure 4. Diagram of a drop tester (l) and pulse energy (r) for JEDEC JESD22-B111.
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Drop Test Reliability of Lead-free WLCSPs
Additional analysis was performed to for drop test reliability of a lead-free WLCSP device. Testing to the JEDEC 22-B111 standard, a 7- × 7-mm WLCSP device with 0.4-mm pitch and 192 I/Os processed using SAC305 was evaluated. As with the earlier CSP devices, underfilling dramatically improved lead-free solder joint reliability in the WLCSP. The newer-generation, flexible underfill material demonstrated advantages over the older-generation, rigid underfill material (Figure 5).
Figure 5. Drop test results, per JEDEC JESD22-B111 on lead-free WLCSPs.
Traditional underfills provide some level of drop test protection for lead-free solder joints and, because they are filled to reduce material CTE, they tend to deliver improved thermal-cycling reliability. However, these systems are more rigid and crack easier, necessitating the development of newer materials that can absorb more physical shock and deliver higher levels of reliability. Test data presented illustrates that the improved flexibility, modulus, and fracture toughness of next-generation underfill materials increases protection dramatically.
Conclusion
Although lead-free manufacturing will be the industry norm, many challenges remain with lead-free solder joint reliability and long-term stability. While certain devices such as CSPs were originally designed to be underfill-free products, recent discoveries regarding their behavior in relation to lead-free processing has dictated a new look at reliability improvements. Underfills have proven their usefulness in protecting lead-free solder joints from failures arising from thermal cycling challenges. A new generation of underfill materials is demonstrating improvements in drop test reliability for CSP and WLSCP devices used in handheld, mobile communication products.
Consumers must continue to push the envelope - demanding more functionality in smaller products. Though this would be challenging with traditional tin/lead processes, decreasing device sizes and subsequently smaller pitches, combined with lead-free manufacturing, make these requirements more difficult. New materials technology in the form of next-generation, flexible underfill systems are responding to the call, and overcoming what was once believed to be an insurmountable obstacle. Today’s material developers must continue to anticipate future manufacturing requirements and provide innovative solutions for the unique challenges posed by advancing technologies. As long as that is the case, consumers will reap the benefits of reliability, advanced functionality, and convenience afforded by today’s sophisticated electronics products.
REFERENCES
1 Chiu, C.; Zeng, K.; Stierman, R.; Edwards, D.; and Ano, K.; “Effect of Thermal Aging on Board Level Drop Reliability for Pb-free BGA Packages,” 54th ECTC Conference, 2004.
2 JEDEC Standard No. 22-B111, JEDEC Solid State Technology Association, July 2003.
For a complete list of references, please contact the authors.
Acknowledgments
The authors thank Renzhe Zhou of the electronics group of Henkel for drop test data of tin/lead devices.
Dan Maslyk, applications engineer, the electronics group of Henkel, may be contacted at (949) 789-2500; e-mail: dan.maslyk@us.henkel.com. Mark Privett, senior applications engineer, the electronics group of Henkel, may be contacted at (949) 789-2500; e-mail: mark.privett@us.henkel.com. Brian J. Toleno, Ph.D., application engineering team leader, the electronics group of Henkel, may be contacted at (949) 789-2500; e-mail: brian.toleno@us.henkel.com; Website: www.henkel.com.
Following is a list of additional resources within the electronics assembly industry targeting underfill. Please contact each company directly for more information.
Zymet East Hanover, N.J. (973) 428-5245 www.zyment.com
the electronics group of Henkel Irvine, Calif. (949) 789-2500 www.henkel.com
Indium Corporation of America Clinton, N.Y. (315) 853-4900 www.indium.com
Emerson & Cuming Billerica, Mass. (978) 436-9700 www.emersoncuming.com
Cookson Electronics Jersey City, N.J. (201) 434-6778 www.alphametals.com
Ablestik Laboratories Rancho Dominguez, Calif. (310) 674-4600 www.ablestik.com
3M Electronics Markets Materials Division St. Paul, Minn. (651) 733-1110 www.mmm.com