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Characteristic-Life Based Acceleration Transforms for Lead-Free Solder Joint Reliability under Thermal Cycling Conditions
Keywords: Reliability, Acceleration Factors, Pb-Free
The conversion to Pb-free solders has already been implemented in the consumer market. However, the transition to Pb-free solders in the high performance, mission critical networking/server industry has not yet been fully implemented. A significant factor in the slow transition to Pb-free solders is the absence of industry accepted Pb-free acceleration transforms that can translate lab test results into field reliability data. Without such industry accepted acceleration transforms, it is difficult to translate lab test data into reliable predictions of field life performance. In recent publications, some acceleration transforms have been generated, but the data appears contradictory, with no clear fundamental reasons for the differences [1, 2]. In this study, a comprehensive data set has been generated over a wide spectrum of package types, joint size, pitch, die sizes, PCB thicknesses and thermal cycling conditions. The effects of ramp rate and dwell time across different packages is also included. The data has been analyzed to compare with existing Pb-free and SnPb acceleration transform models. The data helps explain contradictions in the existing literature, and offers more comprehensive acceleration transforms for Pb-free solders. Some publications also indicate that the acceleration transforms for Pb-free solders are the same as SnPb acceleration transforms [2]. Comprehensive results of this study also show that in specific conditions, Pb-free acceleration transforms could be even lower than SnPb acceleration transforms. Finally, Finite Element Analysis has also been performed to better understand the effects of the different package design variables on the experimental acceleration transforms. The results have been used to derive recommendations on what package design variables to consider, in ensuring good long term Pb-free reliability. --------------------------------------------------------- References: [1] Pan, N., et. al., “An Acceleration Model For Sn-Ag-Cu Solder Joint Reliability Under Various Thermal Cycle Conditions”, Surface Mount Technology Association International, 2005. [2] Vasudevan, V., Fan, X., “An Acceleration Model for Lead-Free (SAC) Solder Joint Reliability under Thermal Cycling”, Electronics Components and Technology Conference, 2008, pp. 139 - 146.
Mudasir Ahmad, Staff Engineer, Interconnect Design and Reliability
Cisco Systems, Inc.
San Jose, CA

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