Study On New Tin-based Lead-free Electronic Solder Sn-Ag-Cu-RE-P

With the European"WEEE"and"RoHS"directives, the provision and the regulation made by American, Japan, China and other countries promulgated and implemented, the trend of being lead-free for electronic products has reached an agreement all over the world. Several lead-free solder with different compositions were studied by researchers, among these compositions Sn-Ag-Cu family has been commonly regarded as the most possible candidate for replacing the tradition lead-containing solder in terms of their good wettability and remarkable mechanical property. In this paper, to obtain an electronic lead-free solder with low price and good property, the preparation method and process of Sn-Ag-Cu-RE-P solder which designed by adding trace RE and P elements into Sn-3.0Ag-0.5Cu solder were studied. The physical property, mechanical property and wettability of Sn-Ag-Cu-RE-P solder along with the property of the soldered joint were mainly investigated in this paper, especially the effect of adding small amount of RE and fast cooling process on the solder.The results indicated that the microstructure of Sn-Ag-Cu-RE-P solder under common cooling process were made up ofβ-Sn dendrites and eutectic structures between them and intermetallic compounds Ag3Sn and Cu6Sn5 distributed on them. While under fast cooling process, the microstructure of Sn-Ag-Cu-RE-P solder were changed greatly that it became finer: the dendrites replaced by equiaxial structure, the sizes of Ag3Sn and Cu6Sn5 decreased notably. RE could refine the microstructure of Sn-Ag-Cu-RE-P solder and the refinement effect was more obvious under common cooling process compared with that under fast cooling process. The microstructure of Sn-Ag-Cu-RE-P solder was very fine and even by both the effect of adding RE and fast cooling process.In all the Sn-Ag-Cu-RE-P solders obtained under different cooling processes, there were no low melting point phases which do harm to reliability of the joints formed with the addition of trace RE and P elements. Adding RE and fast cooling process had little to do with the temperature of solidus and liquidus, but it can decrease melting range properly and thus improve the liquidity. Meanwhile, the density decreased which means certain low-density advantage of the solders. Though the conductivity of the solders went down slightly, the Sn-Ag-Cu-RE-P solder still had good conductivity.The addition of RE can greatly increase the shear property of the solder joint of Sn-Ag-Cu-RE-P electronic lead-free solder with its shear strength increased 35.84%～51.26%, the highest shear strength was 44.9MPa. Aging at 150℃, the growth of the IMC layer at the solder joint of Sn-Ag-Cu-RE-P solder was mainly decided by diffusion mechanisms. The existence of RE element can defer the growth of IMC layer according to the aging test results.