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|Isothermal Aging Effects on the Thermal Reliability Performance of Lead-Free Solder Joints|
|Keywords: Isothermal aging, Pb-free solder, Reliability|
|The microstructure, mechanical response and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging. Electronics assemblies built with Sn-Ag-Cu (SAC) solder joints are often exposed to elevated ambient temperatures for prolonged periods of time. Thus it is important to understand the impact of isothermal aging effect on the long term reliability. Such effects for lead free solder joints are especially important for applications in harsh environment electronics such as high performance computing, automotive, aerospace and defense applications. A direct effect on packaging reliability has been observed during elevated temperature isothermal aging for fine-pitch ball grid array (BGA) , Chip Scale packages (CSPs) and Quad Flat No-Lead package (QFN) 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 and 12 months. Subsequently, the specimens were thermally cycled from -40oC to 125oC with 15 min dwell times at the high temperature. It was found that the thermal performance of lead-free fine-pitch packages significantly degrades up to 65-70% after aging at elevated temperature. The dominant failure mode can be associated with the growth of Cu6Sn5 intermetallic compounds (IMC) during the aging, particularly on the pad side.|
|Jiawei Zhang, Research Assistant