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Factors Impacting the Reliability of Ultralow Silver Lead Free Alloys
Factors Impacting the Reliability of Ultralow Silver Lead Free Alloys
The delta of the thermal cycle and dwell at each end of the thermal cycle are determined to be significant contributors.
Analysis Lab

Authored By:
Neil Poole, Ph.D.
Henkel Electronic Materials, LLC
Irvine, CA, USA

Brian J. Toleno, Ph.D.
Henkel Electronic Materials, LLC
Irvine, CA, USA

Mark Curie, Ph.D.
Henkel Electronic Materials, LLC
Irvine, CA, USA
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Summary
Although the price of silver has stabilised, it remains the most expensive element of the lead free Tin silver Copper (SAC) alloys. Lowering the level of silver has a number of effects beyond simply the cost of the alloy. It has been shown that decreasing the silver can improve the drop/mechanical shock resistance, a desirable parameter for many hand held applications, but at the cost of an increase in liquidus and a drop in thermal fatigue resistance. Although this impact has been studied for low silver alloys 1-3% little work has been don on ultralow silver alloys (less than 1%).

In this paper the key parameters impacting the thermal fatigue of SAC0307 are studied, as well as the delta of the thermal cycle being important the peak temperature and dwell at each end of the thermal cycle are determined to be significant contributors to the ultimate performance of the solder joint. The contribution was determined to vary with the component tested. Thus care should be taken in extrapolating the thermal cycle data from one test vehicle to an actual circuit assembly.
Conclusions
All of the changes in reliability observed above can be explained by changes in the level of plastic deformation generated in each thermal cycle as per the Coffin-Manson equation. These changes in plastic deformation are most likely due a combination of changes in the creep resistance at different temperature, variable levels of induced strain in the joint and the rate of strain relief.

The data yielded viable linear models for both components, suggesting that it may be possible to generate simple models, for components, to predict the characteristic life, under given conditions. It would be desirable to extend the work to include the impact of component size and to have additional data to better understand the impact of cross terms.
Initially Published in the SMTA Proceedings
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