Performance Regulation Of Sn-20Bi Lead-Free Solder And Its Solder Joint Characteristics

Sn-Pb solder has been prohibited due to the biological toxicity of lead,while Sn-Ag and Sn-Cu solders have become the primary lead-free solders used in electronic packaging due to their excellent mechanical properties.However,due to their high melting points,they are easy to cause warpage and deformation of the substrate and soldering defects in the high degree of integration and thinness of chip packaging,which cannot meet the needs of the new generation of electronic products.Low-temperature lead-free solder is one of the main measures to solve the above problems.Sn-Bi eutectic alloy is regarded as the most promising low-temperature lead-free solder because of its good wettability and low melting point.However,the high content of brittle Bi phase in Sn-Bi eutectic alloy could affect the reliability of solder joints significantly.There is an urgent need to develop high-performance Sn-Bi low-temperature lead-free solder.In this study,Sn-20Bi alloy with low Bi content has been used as the research object to study the effects on microstructure and properties by adding metal elements Cu,In and non-metallic carbon-based graphene material.The effects of metal and non-metal elements on solder physical properties,the growth and evolution of intermetallic compounds at the interface of solder joints,and the mechanical properties of solder joints were obtained.Based on the first principle,the formation interfacial phases and interfacial bonding mechanism of solder joints were expounded.The service reliability of solder joints was investigated by the isothermal aging experiment,and the influence of composition on the aging resistance of solder joints was explored.The main results are listed as follows:(1 In the single metal element Cu or In addition system,the improvement of properties of Sn-20Bi solder by Cu addition was greater than that by In element.Among them,the overall performance of Sn-20Bi-0.3Cu alloy was optimal,with a wetted spreading rate of 86.6%,a solder maximum tensile strength of 84.6 MPa,an elongation of 17%,and the corrosion rate of0.078 mm/year.Compared with the Sn-20Bi alloy prepared by the same process,its tensile strength was increased by 25.1%,and the corrosion rate was reduced by 61.6%.(2 The grain refinement mechanism and alloy solidification characteristics of Sn-20Bi-x Cu series alloys were studied based on“Edge-to-Edge”model.The results showed that the[（?）（?）23 _Cu6Sn5∥[110 _Sn,(01（?）1 _Cu6Sn5∥(（?）12 _Sn and[（?）223 _Cu6Sn5∥[01（?）_Sn,(01（?）0 _Cu6Sn5∥(1（?）（?）_Sn had a small misfit and were less than 12%in Sn-20Bi-0.7Cu composite solders,indicating that the Cu₆Sn₅ phase has excellent heterogeneous nucleation ability in Sn matrix and can effectively refine the microstructure of the alloy during solidification.(3 Compared with the modification of a single element Cu or In,the synergistic addition of Cu and In has a more pronounced effect on the mechanical properties of Sn-20Bi alloy solder.In Sn-20Bi-Cu-In alloy,the Cu₆Sn₅ phase formed by Cu and Sn plays a heterogeneous nucleation role in the solidification process,which refines the grains and improve the strength and plasticity of the alloy,while In element plays a solid solution strengthening role.The coordinated effects of microstructure refinement,second phase strengthening,and solid solution strengthening together lead to the good overall performance of solder.The wetting spreading rate of Sn-20Bi-0.1Cu-2In alloy is 79.6%,the maximum tensile strength was 88.4MPa and the elongation rate was 17.2%.(4 The structure distribution of the graphene-modified Sn-20Bi alloy prepared by the cladding melting-casting(MC method is very uniform.The Sn-20Bi-0.02GNSs solder prepared by the MC method has the optimal properties:the wetted spreading rate was 81.9%;the maximum tensile strength was 97.3 MPa;and the elongation was 23.3%.Its tensile strength and elongation were close to those of traditional Sn-37Pb solder(the tensile strength is 88 MPa and the elongation rate is 24%,which has excellent application potential.(5 The first-principle study on the interfacial interaction of solder joints describes that the Cu-In covalent bond is formed between the In atoms at the solder joint interface and the Cu atoms on both sides of the interface after In alloying,mainly from the 3d orbit of Cu atoms and the 5p orbit of In atoms.The interfacial energy of Cu(001/Cu₆Sn₅（11（?）0）interface was decreased significantly after In alloying,indicating that the interfacial bonding stability of Cu and Cu₆Sn₅is enhanced,which is beneficial to improve the shear strength of solder joints.The mechanical properties of the solder/copper substrate reflow solder joint showed that the shear strength(50.2 MPa of Sn-20Bi-2In/Cu solder joint is 27.7%higher than that of Sn-20Bi/Cu solder joint.(6 The solder joints with better comprehensive performance in each system have been selected for isothermal aging treatment at 130°C for 360 hours,and it was found that MC Sn-20Bi-0.02GNSs/Cu solder joints had the best aging resistance(the maximum decrease of shear strength is only 14.9%.This phenomenon could attributed to GNSs scattered formation on the surface of Cu₆Sn₅ or between Cu₆Sn₅ grains,hindering the movement of Cu₆Sn₅ grain boundary,which inhibited the growth of Cu₆Sn₅grains at the interface and the formation of Cu₃Sn in the interface microstructure.The shear strength reached 39.4 MPa after aging for 360 h,which was significantly higher than that of the traditional Sn-37Pb solder joint(34 MPa,indicating that the solder joint has good service reliability.In this dissertation,the microstructure,mechanical properties and solder joint service reliability of Sn-20Bi solder are systematically studied,which provides a theoretical basis and data support for the industrial application of Sn-Bi low temperature lead-free solder.