Research Of Electromigration Of Sn3Ag3Bi3In Line-Type Solder Joints

Under the background of lead-free electronic packaging,Sn-based lead-free solder joints oftern consist of a limited number of Sn grains,showing single crystal or twin crystal structure.Due to the high anisotropy ofβ-Sn,the electromigration（EM）behavior of the joints with limited Sn grains also exhibit the feature of anisotropy.Previous studies showed that the EM reliability of joints can be improved by refining Sn grains.Sn3Ag3Bi3In solder and line-type solder joint were employed in this study,and the joints was reflowed under different solder peak temperature.Microstructure and crystal orientation of joints were characterized by SEM,EBSD and EPMA technology.The mechanical properties of Sn3Ag3Bi3In were compared with Sn3.5Ag and Sn3.0Ag0.5Cu.After that,the EM behavior of the joints with different crystal orientation structure was analyzed.It is found that for Sn3Ag3Bi3In joint,the matrix is whiteβ-Sn with granular Cu₆（Sn,In）₅ and Ag-rich phaseζdispersing in it.IMC at the interface:the thick IMC layer near matrix is Cu₆（Sn,In）₅,while the thin IMC layer near Cu substrate is Cu₃（Sn,In）.In is because In has physical and chemical properties simililar to those of Sn that In atoms take place of Sn atoms in Cu₆Sn₅ and Cu₃Sn lattice positions.After reflow,single crystal,multi-crystalline and polycrystalline joints were found.Twin crystal jonts oftern presents interlaced twin structure.The formation of polycrystalline joints may due to the low degree of undercooling of Sn3Ag3Bi3In.Besides,Sn grains of joints is refined by changing reflow peak temperature.This may due to the higher temperature,the greater the cooling rate which promotes the undercooling of the melt,increasing nucleation rate of Sn.Sn3Ag3Bi3In has higher microhardness and tensile strength than those of Sn3.5Ag and Sn3.0Ag0.5Cu.Besides,EM test of single crystal joints elaborated the dominant effent of high anisotropy inβ-Sn on EM behavior.The EM behavior of twin crystal joints is determined by three orientations of the joint.It is worth that the EM reliability of polycrystalline joint is compariable to that of single crystal joint with c-axis alomost perpendicular to current direction.This may due to the disorder distribution of Sn grain orientation,polycrystalline joint exhibit the feature of‘isotropy’.Therefore,the law of‘atoms diffuse along c axis faster than along a/b axes’does not work here,indicating that the joints with refined Sn grains has good EM reliability.Finally,after EM,the content of In in Cu₆（Sn,In）₅ within joints and at the interface is increased,compared with the initial joints,which may due to the reaction ofζwith Cu and Sn atoms during EM.