Microstructural Aspects of Fatigue Parameters of Lead-Free Sn-Zn Solders with Various Zn Content

Abstract

The study includes the results of research conducted on selected lead-free binary solder alloys designed for operation at high temperatures. The results of qualitative and quantitative metallographic examinations of SnZn alloys with various Zn content are presented. The quantitative microstructure analysis was carried out using a combinatorial method based on phase quanta theory, per which any microstructure can be treated as an array of elements disposed in the matrix material. Fatigue tests were also performed using the capabilities of a modified version of the LCF method hereinafter referred to in short as MLCF, which is particularly useful in the estimation of mechanical parameters when there are difficulties in obtaining many samples normally required for the LCF test. The fatigue life of alloys was analyzed in the context of their microstructure. It has been shown that the mechanical properties are improved with the Zn content increasing in the alloy. However, the best properties were obtained in the alloy with a chemical composition close to the eutectic system, when the Zn-rich precipitates showed the most preferred morphological characteristics. At higher content of Zn, a strong structural notch was formed in the alloy because of the formation in the microstructure of a large amount of the needle-like Zn-rich precipitates deteriorating the mechanical characteristics. Thus, the results obtained during previous own studies, which in the field of mechanical testing were based on static tensile test only, have been confirmed. It is interesting to note that during fatigue testing, both significant strengthening and weakening of the examined material can be expected. The results of fatigue tests performed on SnZn alloys have proved that in this case the material was softened.

Comparative Studies of Microstructure and Fatigue Life of Selected Lead-free Alloys

“…It should also be noted that during fatigue tests the examined material can undergo the effect of strengthening or weakening [12]. The results of fatigue testing indicate that, similar to the general tendency observed in binary Sn-Zn alloys, also the examined SnZnCu1 alloys undergo the weakening effect.…”

“…The method has been successfully verified for a variety of materials including heterogenous microstructures and has already been described in detail as regards both its operating principle and measurement procedure [11,12].…”

“…The limit of accommodation (Ra), defined as a limit stress above which the stabilization of permanent deformation occurs no longer [11,12], was also assessed. The key parameter in MLCF method is the fatigue strength under rotary-bending conditions Zgo, determined from an experimental graph (Fig.…”

“…10. Experimental curve for fatigue strength evaluation [11,12] The parameters b, c, n', K and max were determined based on the same methodological assumption which were adopted in [11,12].…”

“…In the authors' opinion, particular attention deserves the fact that from a methodological viewpoint the structural examinations were carried out using phase quanta theory [10], while the assessment of fatigue life was based on a modified low cycle fatigue test (MLCF) [11,12].…”