Research On Effects Of Minute Amount Of Rare-earth Element Ce On Properties Of SnAgCu Alloy And Reliability Of Soldered Joints

According to the WEEE & RoHS mandates issued by EU, lead and other hazardous materials will be prohibited to use in household electrical appliances since July 1, 2006, so researches and developments on lead-free solders which are substitutes for Sn-Pb solders are coming into a practical stage. Taking the most promising substitute of Sn-3.5Ag-0.5Cu solder that added rare earth element Ce as the research object, investigations were made to explore the rules of effect of different rare earth contents on the physical properties, mechanical properties and reliability of Sn-3.5Ag-0.5Cu soldered joints. Effects of the addition of Ce on microstructures of Sn-3.5Ag-0.5Cu alloy and that of solder/Cu substrate were also analyzed in this paper.Results show that in all the examined Sn-Ag-Cu-Ce solders, no low-temperature eutectic peak was observed by DSC curves, which implicated that no low-temperature eutectic phase was produced while added small amount of rare earth Ce to the solder and benefited to form reliable soldered joints. The liquid temperatures of Sn-Ag-Cu-Ce alloy lie in the scope of 218℃～221℃. Results also show that the density of the solder will decrease and its electrical conductivity will be improved if Ce is added into Sn-Ag-Cu lead-free solder, which indicates that the alloy has an excellent electrical transmit ability. According to the orthogonal tests,the optimum wettability can be achieved under conditions of 260℃soldering temperature, 0.05wt.% Ce content in Sn-Ag-Cu alloy, substrate with Sn-Bi coating and N2 protection.Tensile strength and shear strength of soldered joints were tested by means of STR-1000 micro-joint tester, the optimum strengths (including tensile strength and shear strength) can be achieved when the content of Ce in the solder is about 0.03 wt.% which it is because the crystal grains of the solder become finer when added Ce, as a result, the tensile strength of Sn-Ag-Cu-Ce alloy is larger than that of Sn-Ag-Cu alloy and elongation percentage of Sn-Ag-Cu-Ce alloy is much bigger. When the content of Ce is exceeded the optimum value and upto 0.1wt.% (in this paper), the brittle hard rare earth compound phase was emerged, which is adverse to the properties of Sn-Ag-Cu-Ce alloy.Based on the interactions of elements in the alloy and the relevant binary phase...