Study On Low Ag Sn-Ag-Zn Lead-free Solder And Its Interfacial Reaction

Because of the legislative concerns of environmental protection and the requirement of modern high-integrated microelectronics development, the research and development of high-performance lead-free solder has become one of the critical issues in electronic and materials area. Among various lead-free solder systems, Sn-Ag solders have been recognized as the most promising substitute of Sn-Pb solders, but they have some weakness such as high melting point, high cost. Currently, the researchers around world have been improving the performance of Sn-Ag solders through the addition of the third component, Sn-Ag-Cu and Sn-Ag-Zn solders attract the most attention. Sn-Ag-Cu solder owns good wettability, but poor soldering interfacial reliability and high cost, on the other hand the system has been patented by foreign country mostly. Sn-Ag-Zn solder, especially the low Ag Sn-Ag-Zn solder is a merging research hotpot, which exhibit low melting point, good mechanical performance and ideal interfacial reliability. However the research system of low Ag Sn-Ag-Zn solders has not been completed, especially the interfacial reliabliltiy.The paper has explored the research of the performance belonging to various low-Ag Sn-Ag-Zn solders and the interfacial microstructure of selected Sn-2Ag-2.5Zn soldered on different substrates(Cu, Ni, Ni-W(3%W and 10%W).The study shows that: Sn-2Ag-2.5Zn solder is the optimized component, which owns relatively better mechanical performance, wettabiliy and microstructure. The microstructure of Sn-2Ag-2.5Zn is equiaxed crystal with fine ?-Sn+AgZn eutectic.Double layer IMC formed when Sn-2Ag-2.5Zn soldered on Cu substrates and aging at 150℃, which are Cu5Zn8 and Ag3 Sn in sequence next to the Cu substrate. The IMC growth rate of Cu5Zn8 is only 1.1-18 m2/s under the condition of 150℃ aging, and after aging for 200 h, the thickness of Ag3 Sn is only 2.5μm. The double layer IMC suppress the growth of each other.When Sn-2Ag-2.5Zn soldered on Ni substrates, a thin and smooth Ni3Sn4 layer formed at the interface. After aging for 600 h, the thickness of Ni3Sn4 is only 1μm and the smoothness is ideal, with no gathering appearance of other alloy elements, and it demonstrates the good barrier performance of Ni layer.After soldering on the Ni-W barrier layer, it formed Ni3Sn4 layer which thickness is 0.5-0.8μm. Through the aging process, Sn diffused into the barrier layer and Ni out of the barrier layer, which incurred the transformation of the Ni-W layer into bright layer. With the consumption of the Ni-W layer, the thickness of Ni3Sn4 increased and the smoothness of Ni3Sn4 decreased. After the whole Ni-W layer transforming into bright layer, Sn atoms diffused continuously to reacted with the Cu substrates underneath, forming Ni3Sn4 and Cu6Sn5 in sequence.