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Alloy Composition and Aging on the Survivability of Lead-Free Solders



Alloy Composition and Aging on the Survivability of Lead-Free Solders
A comparison of failure modes for different packaging architectures at elevated test temperatures and vibration has been presented in this study.
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Authored By:


Pradeep Lall, Ph.D., MBA, Vikas Yadav, Di Zhang
Auburn University
NSF-CAVE3 Electronics Research Center and Department of Mechanical Engineering
Auburn, AL, USA

Robert Kinyanjui, Ph.D., Brad Palmer, Jesse Jangula
John Deere Electronic Solutions, Inc.
Fargo, ND, USA

Summary


Electronics in modern automotive platforms are being used to facilitate a number of operational and safety functions including energy generation, transmission, collision avoidance systems, lane departure warning systems, antilock braking systems, vehicle stability assist, global positioning, navigation and drive assist systems. During operation, automotive electronics in underhood environments may be simultaneously exposed to high temperatures in the neighborhood of 150-200 Celsius in addition to vibration over a sustained period of time. Typical failure modes may include solder fatigue, copper trace or lead fracture. In prior studies, SnAgCu solders which are increasingly used in automotive electronics have shown susceptibility to evolution of the material properties under exposure to high temperature. However, the reliability of the solder alloys under combined exposure to vibration at high automotive temperatures is not well understood to a level which will allow the development of models for lifeprediction.

In this study a variety of packaging architectures fabricated with SAC105 and SAC305 alloys have been studied under exposure to random vibration and range of temperatures in the neighborhood of 25-125 Celsius. Spectral content of the printed circuit board assemblies has been studied using accelerometer output. All the parts on the board assemblies have been monitored for resistive opens using a high-speed data-acquisition system. FE simulation using global-local finite element models is thus correlated with the system characteristics such as modal shapes, natural frequencies and displacement amplitudes for every temperature. The solder level stresses have been extracted from the sub-models. Stress amplitude versus cycles to failure curves are obtained at all the three test temperatures.

A temperature and vibration-amplitude dependent-version of the Basquin power law damage relationship has been developed using the test data. Material properties of the PCB at test temperatures have been measured using tensile tests and dynamic mechanical analysis. A comparison of failure modes for different packaging architectures at elevated test temperatures and vibration has been presented in this study.

Conclusions


been studied in two test vehicles under simultaneous temperature and vibration. Board assemblies have been subjected to two acceleration levels of 10g and 14g over temperature range of 25 Celsius to 125 Celsius. Resistance of the components has been measured during the test till failure. The board assemblies have been modeled using finite elements to extract the stress amplitude in the solder joints during exposure to simultaneous temperature-and-vibration. Volume averaged solder stresses at high stress location of every component is plotted against its experimentally obtained cycles to failure. S-N curves for SAC305 are obtained based on Basquin power law relation between stress amplitude and stress reversal cycles. S-N curves at 25C, 75C and 125C are thus obtained for SAC305.

A temperature dependent form of the Basquin power law has been presented for the SAC305 solder under simultaneous temperature-vibration. The temperature dependence of the fatigue strength coefficient and fatigue exponent has been computed from the SAC305 S-N curve relation using Arrhenius relationship. The temperature dependent forms of the fatigue coefficient and fatigue exponent have been implemented in Basquin power relation. The temperature dependent form of the Basquin Power Law can be used for the life prediction of the SAC305 solder joint reliability under exposure to temperature and vibration.

Initially Published in the SMTA Proceedings

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