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Mixed Alloy BGA Assembly: Low-Silver Lead Free BGA Spheres into Tin-Lead & LF Assembly Processes

Presenter: Chrys Shea, Quyen Chu, and Ken Hubbard
Company: Shea Engineering Service, Jabil, and Cisco Systems
Date Published: 6/10/2009

Description: Some ball grid array suppliers are migrating their sphere alloys from SAC305 (3% Ag) or 405 (4% Ag) to alloys with lower silver contents. This change is primarily motivated by the need to improve drop performance, especially for handheld product applications. There are numerous perceived reliability benefits to this move, but process compatibility with SAC305/405 paste (forward compatible) has yet to be fully addressed, or at least not been reported. Further, no data have been published on the compatibility of low Ag solder balls with SnPb paste (backward compatible) for non-handheld applications.

A study has been undertaken to characterize the influence of alloy type and reflow parameters on low-silver SAC spheres assembled with backward and forward compatible pastes and profiles. This study combines low-silver sphere materials with tin-lead and lead-free solders at different combinations of peak temperature and times above liquidus. Solder joint formation and reliability are assessed to provide a basis for developing practical reflow processing guidelines.

In this phase of the experiment, four different low silver alloy sphere materials were assembled with both tin-lead and SAC305 solder pastes. Cross-sectional micrographs of representative BGA solder joint microstructures are analyzed and compared to benchmarked SnPb-SnPb and SAC-SAC baseline microstructures.

Key Words: 

BGA spheres, Low-Silver, Tin-Lead, Lead-free



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