| Due to the harmful effects of lead on humans and the environment,traditional Sn Pb soldering materials are no longer widely used.Sn-58Bi lead-free soldering material has become one of the best alternatives to Sn Pb soldering materials because of its low melting point and low cost,which is similar to Sn Pb soldering materials.However,its micro-solder joints can be abnormally coarse in the Bi phase during service and lead to component failure,so research on improving the reliability of micro-solder joints is of great significance.Multi-walled carbon nanotubes(MWCNT)are a recognized effective reinforcing phase,and it was also found that metallic Ni has a certain strengthening effect on Sn-based soldering materials.Therefore,in this paper,MWCNT and nickel-plated carbon nanotubes(Ni-CNTs)were selected as reinforcing phases to improve the microstructure of soldering materials and the mechanical properties of micro-solder joints using Sn-58Bi lead-free soldering material as the substrate.In order to better illustrate the failure mode of micro-solder joints under actual service conditions,this paper also clarifies the creep deformation mechanism of micro-solder joints by performing creep accelerated damage experiments on the composition-optimized micro-solder joints and systematically studying the growth evolution of interfacial intermetallic compound(IMC)and tensile creep failure behavior under the action of single or two fields of electricity,heat and force.When MWCNT was used as the strengthening phase,the results showed that:with the increase of MWCNT,the melting point of the composite soldering did not change significantly,and the spreading area tended to increase first and then decrease,and reached the maximum at the addition of 0.01%,which was 42.23 mm~2.MWCNT not only refined the microstructure of the soldering material but also inhibited the growth of IMC at the interface through diffusion strengthening.Due to the load transfer effect of MWCNT,the tensile strength of the micro-solder joint was improved,and the average tensile strength was highest at 0.01%MWCNT,which was 50.07 MPa.Due to the load transfer effect of MWCNT,the tensile strength of micro-solder joints was improved,and the highest average tensile strength of 50.07 MPa was achieved at 0.01%MWCNT,and the fracture location was inside the soldering,and the fracture mode was mixed fracture.When Ni-CNTs were used as the strengthening phase,the results showed that:with the increase of Ni-CNTs,the melting point of the composite soldering had a small increase,and the spreading area tended to increase and then decrease,and reached the maximum at the addition amount of 0.05%,which was 48.45 mm~2.Compared with MWCNT,Ni and Sn would react to form Ni-Sn compounds,the appropriate amount of Ni-CNTs could refine the solder matrix structure.CNTs can refine the solder matrix structure,and the interfacial IMC after the addition of Ni-CNTs is slightly thicker than that of the Sn-58Bi-0.01MWCNT micro-solder joints.Meanwhile,the tensile strength of the micro-solder joints can be improved with the addition of Ni-CNTs at 0.05%,reaching a maximum of 54.39 MPa,which is higher than that of the Sn-58Bi-0.01MWCNT micro-solder joints.The average tensile strength of the micro-solder joints was increased by 8.6%,and the fracture location was inside the soldering material,and the fracture mode was ductile fracture.This indicates that the bonding between the carbon nanotubes and the solder matrix was enhanced after nickel plating,and the Ni-CNTs had a better enhancement effect than MWCNT.The results of the creep properties of micro-solder joints show that:thermal aging and electromigration lead to a decrease in the steady-state creep lifetime of micro-solder joints because they promote the growth of interfacial IMC.For Sn-58Bi-0.05(Ni-CNTs)micro-solder joints,the steady-state creep activation energy decreases and the lifetime decreases as the temperature increases,the tensile stress increases,and the current density increases.Under the experimental conditions,the creep parameters are as follows:the stress index n is 4.23,the activation energy Q is 44.76 KJ/mol,and the creep constitutive equation is established.It can be judged that the creep deformation is mainly controlled by the dislocation climbing,and the creep fracture is located inside the soldering material,and the fracture mode is mixed fracture. |
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