The Effect of Germanium Additions on the Properties of SnAgCu-RE Solder

• Main Authors: Chih-Chien Chi, 紀志堅
• Other Authors: Tung-Han Chuang
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/99286259776542495102

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 96 === Ternary Sn-Ag-Cu alloys have been recommended as potential candidates to replace the eutectic Sn-37Pb alloy as a major Pb-free solder. Due to the higher melting temperature and poorer wettability of the Sn-Ag-Cu solder, many researchers have tried to improve these properties by adding rare earth elements into the Sn-Ag-Cu solder. The studies on adding rare earth elements into Sn-Ag-Cu solder indicate that the mechanical strength and physical properties are, indeed, improved. However, as our laboratory was the first to discover, the phenomenon of amazingly rapid growth of tin whiskers on the surface of solder alloys containing rare earth elements is another problem that needs to be solved. The growth rate at room temperature can reach 8.6 Angstrom/Sec, which is much higher than the values reported in the literature (0.01 Angstrom/Sec~0.1 Angstrom/Sec). It is known that tin whiskers can cause short circuits in solder joints and lead to serious problems with reliability. Our studies of Sn-Ag-Cu-RE solder indicate that “Rare Element Intermetallic Compound” (RE-IMC) oxidation causes that abnormal tin whisker growth, so we added Germanium (Ge), which effectively prevents the surface oxidation of the solder into the Sn-Ag-Cu-RE solder, and observed its effects on the tin whisker growth, physical properties, and mechanical strength. The results of our experiments indicate that a “Ge-Containing Phase” (GCP) exists around the “Rare-Earth Intermetallic Compound” (RE-IMC) and in the matrix of the solder when Germanium addition is greater than 0.5wt.% in Sn-Ag-Cu-RE. The tin whisker growth behaviors resulting from the RE-IMC depend on the amount of Germanium addition in Sn3Ag0.5Cu0.5Ce solder. When the amount of Germanium addition in Sn3Ag0.5Cu0.5Ce solder is greater than 0.5wt.%, the growth of tin whiskers at room temperature, and that of hillocks at high temperature (150℃), can be inhibited. However, the influence of adding Germanium into the Sn3Ag0.5Cu0.5La solder is not significant. The reason why adding Germanium can inhibit tin whisker and hillock growth is that the rate of CeSn3 oxidation in Sn-Ag-Cu-Ce is decreased, but the rate of LaSn3 oxidation in Sn-Ag-Cu-La system is not changed significantly. Furthermore, the addition of a suitable amount of Germanium in Sn3Ag0.5Cu0.5RE solder can improve a lot of mechanical properties of alloy and solder joint (for example: ultimate tensile strength, Vickers microhardness), but excessive addition of Germanium causes a significant degradation of the mechanical strength and wettability.