| Electronic industry faces a major problem of reliability. With the deeper studies on lead-free solders, there are two ways can improve the reliability: eliminating the disadvantages for the mechanical properties of the solder joints caused by intermetallics and overcoming the phenomena of the thermal fatigue that emerged during the thermal cycling due to the difference of coefficient of thermal expansion (CTE) between the packaged components and the printed circuit board. In this thesis it can be seen that developing self-adaptive lead-free solders will make the two ways available and fixed on the Sn-Ag-Zn solder (Zn as the third element).The structures of the eutectic Sn-3.7%Ag-0.9%Zn (weight percent) solder at different cooling rates (slowly cooled, water cooled and rapidly cooled) were studied in this thesis. Theβ'-AgZn intermetallic compounds (IMC) with simple-cubic structure was observed in the water cooled eutectic Sn-Ag-Zn solder by X-ray diffraction (XRD) and electron diffraction (ED) analyses. The Ag3Sn IMC particles nucleated and grew up by the first precipitated AgZn IMC rods. Heated at 200℃with time extending, the arborescent structure vanished gradually and Ag3Sn IMC as well asζ-AgZn IMC precipitated in the rapidly cooled eutectic Sn-Ag-Zn solder. At room temperature the metastableβ'-AgZn firstly changed toβ-AgSnZn whose thermodynamic stability is rather close toβ'-AgZn's, and then changed to stableβ-AgZn. As a result, the phase transformation fromβ'-AgZn toβ-AgZn didn't detected in the water cooled eutectic Sn-Ag-Zn solder during the DSC test.In addition, the micro-structure of the Sn-3.4%Ag-0.5%Zn (weight percent) solder was compared with the eutectic Sn-Ag-Zn solder's and the eutectic Sn-Ag solder's. With the increase of Zn content, the formation ofβ-Sn dendrites were greatly suppressed and fine precipitates uniformly dispersed in the surrounding matrix.
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