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A direct and deleterious effect on packaging reliability has been observed during elevated temperature isothermal aging for fine-pitch ball grid array (BGA) packages with Sn-1.0Ag-0.5Cu (SAC105), Sn-3.0Ag-0.5Cu (SAC305), and Sn-37Pb solder ball interconnects. Package sizes ranging from 19 mm with 0.8-mm-pitch BGAs to 5 mm with 0.4-mm-pitch μ BGAs with three different board finishes (ImSn, ImAg, and SnPb) were evaluated. The aging temperatures were 25°C, 55°C, 85°C, and 125°C, applied for a period of 6 mo. Subsequently, the specimens were thermally cycled from -40°C to 125°C with 15-min dwell times at the high temperature. Weibull analysis of failures versus cycle number show a ~ 50% reduction in package lifetimes when aged at 125°C compared to the same at room temperature, with less dramatic but measurable reductions in lifetime at 85°C and even 55°C. In contrast, the reliability performance of Sn-37Pb is much more stable over time and temperature. The degradation was observed for both SAC alloys on all tested package sizes and board finishes. For the 19-mm SAC105 case, for example, there was a 53% (32%) reduction of characteristic lifetime at 125°C (85°C) compared to room temperature aging. The trends were in the expected directions; namely, the reliability was reduced when using higher aging temperatures, smaller solder balls, and SAC105. The dominant failure mode can be associated with the growth of Cu6Sn5 intermetallic compounds during the aging, particularly on the pad side.
Components, Packaging and Manufacturing Technology, IEEE Transactions on (Volume:2 , Issue: 8 )
Date of Publication: Aug. 2012