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Effects of Composition and Isothermal Aging on Microstructure Performance
Effects of Composition and Isothermal Aging on Microstructure Performance
Investigations of alloy composition and isothermal aging on solder microstructure and shear fatigue were performed on Pb-free alloy solder joints.
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Authored By:
Francis M. Mutuku, Ph.D.
Universal Instruments Corporation
Conklin, NY, USA

Babak Arfaei, Ph.D.
Ford Motor Company

Eric Cotts, Ph.D.
Physics and Materials Science Program,
Binghamton University, Binghamton, NY, USA

Summary
Alloy composition, reflow profile and thermal history are all known to affect the microstructure and mechanical properties of Sn-Ag-Cu based solder joints. Secondary precipitates such as Ag3Sn and Cu6 Sn5 interact with dislocations in the Sn grain matrix and thus affect the creep resistance of Sn-Ag-Cu solder alloys. The size, number and arrangement of these strengthening precipitates determine the thermomechanical response of a Sn-Ag-Cu solder joint in service. During typical service, solder joints age with time, temperature and cyclic loading. As the secondary precipitates coarsen, their number density decreases, increasing their spacing and reducing their effectiveness at impeding dislocation motion. This microstructural evolution with aging can degrade the mechanical performance of solder joints and the reliability of the solder interconnects. Sn-Ag-Cu solder strengthening can also be achieved through solid solution alloying elements such as Bi and Sb. Such solid solution strengthening can retain much of its effectiveness after extended aging.

Systematic investigations of the effects of alloy composition and isothermal aging on solder microstructure and shear fatigue were performed on various Pb-free alloy solder joints. The microstructures of these alloys were examined in 20mil diameter solder joints. Isothermal shear fatigue testing was performed on individual solder joints formed with carefully controlled thermal histories. Such testing has been shown to be a relatively quick and reliable method of screening alloys for reliability.

Shear strength, microstructure, and shear fatigue lifetimes of different alloy solder joints were correlated with composition and thermal history. Increasing Ag concentration was observed to increase the area fraction of pseudo-eutectic phase regions and the room temperature shear fatigue life. Shear fatigue life was also observed to decrease with aging time. The rate of degradation with aging increased with increasing Ag content. Alloys containing Bi and other minor elements exhibited less degradation upon aging.

Conclusions
Measurements of the room temperature shear fatigue lifetime of individual solder joints were observed to be sensitive to changes in solder composition and to aging times. While the dependence of the shear fatigue lifetime on solder composition echoed that of the shear strength on composition, the variation of the shear fatigue was much more sensitive. This variation was distinct, unambiguous and large. Shear fatigue measurements also clearly illustrated the variation of the thermomechanical properties of these solder joints with aging. The efficacy of room temperature shear fatigue measurements was clearly illustrated.

The dependence of the thermomechanical properties of near eutectic SnAgCu/Cu solder joints on Ag concentration was clearly illustrated. The volume fraction of the interdentritic phase was found to increase linearly with Ag oncentration for these joints. In a corresponding fashion, the shear fatigue lifetime increased strongly with Ag concentration.

The addition of Bi or Sb to Sn based solder alloys increased their shear strength and shear fatigue lifetimes. Furthermore, these mechanical responses were less sensitive to the effects of long term aging than those for near eutectic SAC alloys. The shear fatigue lifetimes of SnAgCu/Cu solder joints decreased dramatically upon aging at a temperature of 125oC, while solder/Cu joints with Sn based solders containing solid solution dispersoids were relatively stable. In fact, room temperature shear fatigue lifetimes of a SnNiCuBi solder increased upon aging.

Initially Published in the SMTA Proceedings

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