The Effects of Aging on the Reliability of Lead Free Fine-Pitch Electronics Packaging

Abstract

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 BGAs with three different board finishes (ImSn, ImAg and SnPb) were evaluated. The aging temperatures were 25oC, 55oC, 85oC, and 125oC, applied for a period of 6 months. Subsequently, the specimens were thermally cycled from -40oC to 125oC with 15 min dwell times at the high temperature. Weibull analysis of failures vs. cycle number show a ~ 50% reduction in package lifetimes when aged at 125oC compared to room temperature, with less dramatic but measurable reductions in lifetime at 85oC and even 55oC. 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 125oC (85oC) 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 (IMC) during the aging, particularly on the pad side. In a second portion of this work, longer aging with more different type packages have been investigated. The degradation rate becomes slower compared with 6 months aging, but continuous reduced lifetime cycles are observed. The microstructures of these packages have been examined and correlated to their mechanical properties and reliability performance.