Uttara Sahaym

Recrystallization and

Sn-Ag-Cu (SAC) solders are susceptible to appreciable microstructural coarsening during storage or service. This results in evolution of joint properties over time and thereby influences the long-term reliability of microelectronic packages. Accurate reliability prediction of SAC solders requires prediction of microstructural evolution during service. Microstructure evolution in two SAC solder alloys, such as, Sn-3.0Ag-0.5Cu (SAC 305) and Sn-1.0Ag-0.5 Cu (SAC 105), under different thermomechanical excursions, including isothermal aging at 150°C and thermomechanical cycling (TMC) was studied. In general, between 200 and 600 cycles during TMC, recrystallization of the Sn matrix was observed, along with redistribution of Ag3Sn particles because of dissolution and reprecipitation. These latter effects have not been reported before. It was also observed that the Sn grains recrystallized near precipitate clusters in eutectic channels during extended isothermal aging. The relative orientation of Sn grains in proeutectic colonies did not change during isothermal aging.

{\rm Ag}_{3}{\rm Sn}

Sn-Ag-Cu (SAC) solders are susceptible to appreciable microstructural coarsening during storage or service. This results in evolution of joint properties over time, and thereby influences the long-term reliability of microelectronic packages. Accurate prediction of this aging behavior is therefore critical for joint reliability predictions. Here, we study the precipitate coarsening behavior in two Sn-Ag-Cu (SAC) alloys, namely Sn-3.0Ag-0.5Cu and Sn-1.0Cu-0.5Cu, under different thermo-mechanical excursions, including isothermal aging at 150°C for various lengths of time and thermo-mechanical cycling between -25°C and 125°C, with an imposed shear strain of ~19.6% per cycle, for different number of cycles. During isothermal aging and the thermo-mechanical cycling up to 200 cycles, Ag3Sn precipitates undergo rapid, monotonous coarsening. However, high number of thermo-mechanical cycling, usually between 200 and 600 cycles, causes dissolution and re-precipitation of precipitates, resulting in a fine and even distribution. Also, recrystallization of Sn-grains near precipitate clusters was observed during severe isothermal aging. Such responses are quite unusual for SAC solder alloys. In the regime of usual precipitate coarsening in these SAC alloys, an explicit parameter, which captures the thermo-mechanical history dependence of Ag3Sn particle size, was defined. Brief mechanistic description for the recrystallization of Sn grains during isothermal aging and reprecipitation of the Ag3Sn due to high number of thermo-mechanical cycles are also presented.

Particle Redistribution During Thermomechanical Treatment of Bulk Sn–Ag–Cu Solder Alloys

The images presented in Fig. 1a and b of Mackay et al. [1] were not obtained from samples produced using the specific conditions described in the Experimental section on page 1477. Replacement figures from samples prepared under the conditions described in the text [1] using a deposition current of 35 mA/cm are provided in this Erratum. Owen and Norton have recently shown [2] that nanoneedle structures can be produced using a range of deposition conditions; the material shown in the corrected figure was obtained with the best known conditions at the time of submission. The analysis of the structures and the way that these needles interact with, e.g., Li [3] are not affected by the error since the analysis in this paper was carried out on material prepared under the conditions described in [1].

Microstructural evolution and some unusual effects during thermo-mechanical Cycling of Sn-Ag-Cu alloys

During service and/or storage, Sn-Ag-Cu (SAC) solder alloys are subjected to temperatures ranging from 0.4 to 0.8 Tm (where Tm is the melting temperature of SAC alloys), making them highly prone to significant microstructural coarsening. The microstructures of these low melting point alloys continuously evolve during service. This results in evolution of creep properties of the joint over time, thereby influencing the long-term reliability of microelectronic packages. Here, we study microstructure evolution and creep behavior of two Sn-Ag-Cu (SAC) alloys, namely Sn-3.0Ag-0.5Cu and Sn-1.0Cu-0.5Cu, isothermally aged at 150°C for various lengths of time. Creep behavior of the two SAC solders after different aging durations was systematically studied using impression creep technique. The key microstructural features that evolve during aging are Ag3Sn particle size and inter-particle spacing. Creep results indicate that the creep rate increases considerably with increasing inter-particle spacing although the creep stress exponent and creep activation energy are independent of the aging history.Copyright

Erratum to: Template-free electrochemical synthesis of tin nanostructures

The microstructure of Sn-Ag-based solders undergoes continuous evolution during service due to isothermal and strain-enhanced aging. The precipitate particle size and interparticle spacing increase during service, resulting in the evolution of mechanical properties. In the accompanying paper (Part I), the effects of thermal-mechanical history on the microstructure and creep behavior of Sn-3Ag-0.5Cu and Sn-1Ag-0.5Cu were studied experimentally. The creep response of solders under all microstructural conditions follows a power law, with the stress exponent and activation energy being independent of composition and microstructural condition. However, the preexponent depends on the alloy composition and thermal/mechanical history, and is related to the precipitate size (or spacing) within the eutectic. The solder creep rate increased linearly with increasing particle size for the solder with higher alloy content (e.g., Sn-3Ag-0.5Cu) but nonlinearly for the solder with lower alloy content (Sn-1Ag-0.5Cu). In this paper, a composite model for creep of solders, explicitly accounting for both the eutectic and proeutectic constituents of the microstructure, is developed to rationalize this experimental observation. The creep rate of the eutectic microconstituent is proportional to the precipitate size. Hence in Sn-3Ag-0.5Cu, where creep is controlled by the continuous network of eutectic in the microstructure, the creep rate is nominally proportional to the precipitate size. In Sn-1Ag-0.5Cu, where the eutectic is discontinuous and a large proportion of proeutectic β is present, the creep rate increases at a less-than-linear rate with increasing precipitate size. The analytical composite model is shown to predict the creep behaviors of both solders well.