Study On Reliability Of Sn-Bi Micro Solder Joints Under Thermal Migration

In the post-Internet era,the development of artificial intelligence is becoming more and more mature,and the society will enter the era of intelligence,which will make the communication between people and equipment,equipment and equipment become more and more frequent.All of this has promoted the miniaturization of integrated circuits and electronic terminals,leading to the continuous demand for higher input/output density,smaller feature sizes and better performance requirements for electronic packaging.The increase in packaging density and the reduction in the size of solder joints make the Joule heating phenomenon of micro solder joints more serious,and the heating problem causes the temperature of the micro solder joints as heat dissipation channels to rise and the temperature gradient at both ends to become larger.Therefore,it is necessary to study the reliability of micro solder joints with temperature gradients.Sn-Bi solder has received extensive attention and research due to its low melting point,better wettability and higher tensile strength.In this thesis,three solders of Sn-15Bi,Sn-35Bi,and Sn-58Bi are selected,and three different hot end materials of Cu,Ni,and Co are used respectively.The cold end materials are all Cu to make linear micro solder joints.Under the three conditions of solid-state thermal migration,isothermal aging,and liquid thermal migration,the evolution of the morphology and composition of the micro-solder interface compound,the migration of hot-end atoms,the evolution of the morphology of the solder,and the migration of Bi atoms are studied.The heat migration behavior of Sn-Bi series solders and excellent hot-end barrier materials are obtained.According to the simulation calculation results and actual measurement results of Ansys Workbench,we found that Ni and Co as hot-end materials can effectively reduce the temperature of the micro solder joints during solid-state thermal migration,and the temperature range is between 10-20?.At the same time,the total heat flux of the solder joints can be reduced.When Cu is used as a hot-end material for solid-state thermal migration,as the content of Bi in the solder increases,the dissolution and diffusion of the hot-end Cu atoms are inhibited.When the solder is Sn-15Bi,the dissolution and diffusion of Cu atoms at the hot end are very obvious.The compound at the hot and cold ends grows asymmetrically.The diffusion flux of Cu atoms after 30 days reaches 5.81×10¹⁴ atoms/cm²s.But no obvious segregation of Bi phase occurred.When the solder is Sn-35Bi,after 10 days,the Bi phase is obviously segregated near the interface compound at the cold end,the Bi phase near the hot end is reduced,and the dissolution and diffusion of Cu atoms are obvious.As the migration time increases,the Bi phase at the cold end becomes coarser,and the dissolution and diffusion of Cu atoms at the hot end increase.When the solder is Sn-58Bi,significant Bi phase segregation occurs after 20 days,and the Cu atoms at the hot end did not dissolve until 30days.When Ni and Co are used as hot-end materials for solid-state thermal migration,none of the three solders has the phenomenon of Ni atoms and Co atoms migrating to the cold end.In addition,no obvious asymmetric growth has occurred in the interface compounds at the hot and cold ends,and the solder joints are stable and reliable.When the solder is Sn-35Bi and Sn-58Bi,the Bi phase segregation at the cold end can be observed.However,it is much slower than when Cu is used as a hot end material under the same conditions,which is manifested by a longer time for Bi segregation and a smaller size of the Bi phase near the cold end interface.And the phenomenon is more obvious when Co is used as a hot end material.In summary,Co as a hot-end material exhibits the best thermal migration resistance.When Cu is used as the hot end material for liquid thermal migration,no Bi phase segregation occurs,and the interface compound at the cold and hot ends grows asymmetrically.And with the increase of Bi content,the diffusion flux of Cu atoms decreases.When the solder is Sn-15Bi,the diffusion flux of Cu atoms is 1.30×10¹⁶ atoms/cm²s.When the solder is Sn-35Bi,the diffusion flux of Cu atoms is 8.15×10¹⁵ atoms/cm²s.When the solder is Sn-58Bi,there is no obvious dissolution phenomenon in the hot end Cu substrate,and only a small amount of Cu atoms migrate from the hot end to the cold end.The conclusion is consistent with the solid-state thermal migration that the increase of Bi content in the solder can effectively slow down the dissolution and diffusion of Cu atoms at the hot end.When Ni and Co are used as hot-end materials for liquid thermal migration,except for the Co/Sn-58Bi/Cu structure,the Ni atoms and Co atoms of the other structures have migrated to the cold end.In summary,it is concluded that it is consistent with the solid-state thermal migration that Co as a hot-end material exhibits the best thermal migration resistance.