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(April 11, 2007) SAN JOSE, Calif. — The EMS Forum released "Guidelines for Suppliers Transitioning to RoHS-compliant BGA Packages with Low-silver SAC Alloys," with notes on necessary reflow profile adjustments and proper notification of low silver (Ag) content. According to the EMSF, some component suppliers are providing BGAs with SAC solder balls containing lower Ag content that SAC 305/405. This change affects assembly and reliability, but may improve mechanical shock resistance and reduce failure rates.
The melting range of low-Ag SAC alloys jumps 7°C higher than SAC 305/405, encompassing 217–228°C. The exact metallurgy of BGA solder balls with lower Ag content varies from one component manufacturer to the next, according to the EMSF guidelines, so users may need to review JSTD-020 qualification temperature, peak reflow temperature, and other process specifications.
EMS providers assembling mixed-alloy — lead-free components with tin/lead solder, or the reverse — also must re-evaluate reflow profiles when assembling a lower-Ag BGA on a board. BGAs with SAC alloy variations are not considered backward-compatible, notes the guidelines. Limited long-term reliability data is available for partially or fully mixed SAC 305 and low-Ag SAC alloys for BGAs.
The forum outlines several logistical steps that ensure assemblers using these SAC-alloy variations document and disclose related RoHS and process data properly. All changes from leaded to lead-free, and within a lead-free process, should be documented using a product change notice (PCN). EMSF considers the change in solder metallurgy to lower-Ag content to be major, and documentation necessary. The forum requires a part-number change when BGAs switch from one SAC alloy to another.
Manufacturers producing lead-free or RoHS-compliant products should provide a product roadmap for customers, with planned changes and a timetable, in addition to availability and life-cycle data for lead-free parts. This applies when solder alloys are changed as well. Prior to a new product introduction (NPI), manufacturers should make sample devices and qualification data available to the customer, according to the guidelines.
To identify incoming lower-Ag BGAs for EMS providers, device datasheets should clearly indicate solder composition, maximum component temperature rating, recommended and absolute reflow limits, and moisture sensitivity. The EMSF encourages associations and consortia to devise universally recognized labels to identify lead-free products, and endorses JEDEC JESD97 as a standard.
Coordinated by Dongkai Shangguan, Ph.D., the EMS Forum comprises representatives from Celestica, Flextronics, Jabil, Plexus, and Sanmina-SCI.
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