SMTA International Conference Proceedings


LOW-SILVER BGA ASSEMBLY PHASE II – REFLOW CONSIDERATIONS AND JOINT HOMOGENEITY FOURTH REPORT: SENSITIVITY TO PROCESS VARIATIONS

Authors: Chrys Shea, et al.
Company: Shea Engineering Services, Jabil, Cisco Systems, Cookson Electronics, Amkor Technology, and Hewlett-Packard
Date Published: 10/4/2009   Conference: SMTA International


Abstract: 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.

Process compatibility concerns stem from the fact that the low-silver SAC replacement alloys have higher melting temperatures than SAC305, approximately 227°C as compared to 221°C. Certain families of electronic assemblies, such as consumer portables, are often heat-sensitive and are reflowed in the low end of the established lead-free peak temperature range, typically 230-235°C. The temperature difference between the spheres’ melting temperature and the peak reflow temperature raises questions about the reliability of the solder joints that are formed under this tight thermal margin. These are similar to the concerns raised with the backward compatibility of SAC305/405 spheres with tin-lead solder processes. Some of the solutions identified in the lead-free ball/tin-lead paste scenario may apply to the low-silver SAC ball/SAC305 paste combination, but they require review for their applicability with this new set of mixed metals.

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.

This will be the fourth report in a series of results that are published as they become available. This report probes the effect of assembly process noise on the formation of partially mixed solder joints that were formed with peak temperatures lower than the melting point of the ball alloy. Particular focus is placed on the 0.5mm and 0.8mm pitch devices.

Key words:Lead-free, low silver, BGA, reflow, mixed metals



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