Effects of Mn nanoparticle addition on wettability, microstructure and microhardness of low-Ag Sn-0.3Ag-0.7Cu-xMn(np) composite solders

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Abstract
Purpose: The purpose of this paper is to investigate of the effects of Mn nanoparticle addition on the wettability, microstructure and microhardness of SAC0307-xMn(np) (SAC: Sn–Ag–Cu; x = 0, 0.02, 0.05, 0.1 and 0.3 Wt.%) composite solders. Design/methodology/approach: The SAC0307-xMn(np) composite solders were prepared by mechanically mixing different weight percentages of Mn nanopowders into the SAC0307 solder paste with rosin flux. In this study, the wettability of the solders was studied using contact angle and spread ratio methods. Afterward, the microstructure of the solders was investigated using scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffractometry. Moreover, the microhardness of the solders was studied. Findings: The wetting process of SAC0307-xMn(np) composite solders was found to experience four stages. Adding a small amount of Mn nanoparticles (x = 0.05 Wt.%) could improve the wettability compared to Mn-free solder. Beyond this level, the wettability deteriorated. The addition of Mn nanoparticles significantly refined the size and spacing of Ag3Sn grains in the solder matrix. When 0.1 Wt.% Mn nanoparticles was added, both the average size of the Ag3Sn grains and the spacing between the Ag3Sn grains decreased significantly and approached minimum values. Beyond this amount, the size and spacing between Ag3Sn grains increased slightly but remained smaller than those in the Mn-free solder matrix. The refined Ag3Sn grains increased the microhardness of the Mn-containing composite solders by 6-25 per cent, in good agreement with the prediction of the classic theory of dispersion strengthening. Originality/value: The paper demonstrates that Mn nanoparticle addition could improve the SAC0307-xMn(np) solder wettability and reduce the grain size and spacing between Ag3Sn grains. The enhancement of the solder microhardness shows good correlation with the microstructure.

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