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Changing BGA Solder Ball Metallurgy
December 31, 1969 |Estimated reading time: 3 minutes
Portable electronic devices often are subjected to drop or shock conditions. Brittle lead-free solders, such as commonly used ball alloys Sn/Ag/Cu (SAC 305/405) with 3-4% silver are especially susceptible to intermetallic failure. To improve drop-and-shock resistance, some manufacturers provide BGAs with SAC balls containing lower amounts of silver. This article discusses the benefits and challenges resulting from a change in BGA solder ball metallurgy.
By Karen Reid and Girish Wable, Jabil
In portable devices, such as cell phones, PDAs, DVDs, and MP3 players, stress can cause intermetallic failures, as can impacts common to shipping-and-handling. According to one component manufacturer, customers have received BGA packages with missing solder balls because they had fallen off the BGAs during shipping.1 Several part-change notices (PCNs), as well as technical publications, have been released by major semiconductor manufacturers.2-7 Currently, these PCNs have a limited scope; however, the changes are expected to become effective across many different package types.
Lowering the silver content may increase compliancy of the bulk solder joint by reducing its elastic modulus and intermetallic thickness. Migrating to low-silver solders for BGA spheres is claimed to also provide other benefits, including:1, 3-6
- Alter failure mode and location;
- Slower intermetallic growth under temperature aging (at 150°C);
- Lower copper dissolution in SMT joints due to addition of dopants;
- Improved wetting due to suppression of Sn oxidation;
- Reduced silver/tin platelets;
- Reduced surface roughness (improves ease of inspection);
- Eliminates underfilling, in some cases;
- Lower prices due to reduced silver.
Some challenges result from this change in metallurgy. First, the SAC 305/405 melting range is 217-221°C, while lower-silver alloys melt at 217-228°C. Existing lead-free reflow profiles may need to be re-optimized, in some cases. Peak temperatures for all packages and solder joints on the board must be re-evaluated to determine the impact of this change. Some possible, but unverified, changes to the lead-free reflow process are an altered peak temperature and time above liquidus (TAL) for reflow/rework; change in SAC 305/405 paste volume; and a re-evaluation of specific paste chemistries for higher process temperature effects, i.e. flux volatization.
If a lead-free BGA is used in a tin/lead process, significant effort is needed to determine the optimum reflow solution. Changing from SAC 305/405 to a low-silver solder alloy may require a thorough re-evaluation of mixed-soldering process solutions. Introducing a low-silver BGA without changing a profile may reverse the performance benefit seen in drop testing. Several vendors are releasing PCNs that indicate that they do not expect to change part numbers, as they do not see a change in form, fit, and function. Not all OEMs and EMS providers agree, and will request that suppliers assign new part numbers to devices that undergo any change in metallurgy.8-9 The potential effect on the JSTD-020 qualification temperature also may need to be reviewed. Table 1 shows examples of alloy selection for BGAs.
Conclusion
Lower-silver solders may provide improved performance against drop and shock stress. However, a change in metallurgy will necessitate the reevaluation of many manufacturing and business processes. BGA manufacturers and OEM/EMS providers must continue studying the effects of this change, especially in cases where traceability is not controlled. This change, in conjunction with the limited availability of lead-free soldering data for high-reliability applications may further delay full lead-free implementation.9
REFERENCES
- J. Caers, Z. Xiujuan, J. Kloosterman, “Philips Lead-free Solder Ball Adhesion Improvement in BGA-packages,” Philips Applied Technologies, March 2006.
- STMicro Product/process Change Notification, Sept. 2006.
- L. Garner, S. Sane, D. Suh, T. Byrne, A. Dani, T. Martin, M. Mello, M. Patel, R. Williams, “Finding Solutions to the Challenges in Package Interconnect Reliability,” Intel Tech. Journal, V. 9, Iss. 4, Nov. 9, 2006.
- R. Lee, Wha Soo Sim, Jong Keun Jeon, Heui Seog Kim, “Sn2.5Ag0.5Cu Lead-free Solder Balls with ‘GE and Ni’,” IEEE, 2005.
- R. Pandher and S. Athavale, “Reflow Profile Optimization for Lead-free (SAC) Alloys in BGA Applications,” Electronic Tech. Conf., IEEE, 2006.
- Weiping Liu, Ph.D and Ning-Cheng Lee, Ph.D; “Novel SACX Solders with Superior Drop Test Performance,” SMTAI, Indium, Sept. 2006.
- National PCN 200474101-B, April 2007.
- iNEMI, “iNEMI Members Call for Unique Part Numbers to Differentiate Ball Metallurgies on Pb-free BGA Components,” May 2007.
- Henshall, et. al., “Manufacturability and Reliability Impact of Alternate Pb-free BGA Ball Alloys,” June 2007.
Karen Reid is technical coordinator, Advanced Manufacturing Technology (AMT) Group, Jabil. Girish Wable is senior engineer, AMT, Jabil. For more information, contact Tom Cipielewski, director of AMT at tom_cipielewski@jabil.com.