Study On Electromigration Behavior And Mechanism Of Cu₆Sn₅ Reinforced Sn-based Single-crystal Solder Joints

With the rapid development of global information industry,the number of components in integrated circuits has begun to grow rapidly beyond Moore's law to meet the characteristics of miniaturization and multifunction of electronic products.With the increasing packaging density of electronic products and the decreasing size of components,the interconnection solder joints,which are important components of the packaging system,bear high current density and Joule heat during service,which accelerates the electromigration of solder joints.As a result,electromigration has become an important reliability issue.The approach of solder composite is one of the effective ways to improve the comprehensive reliability of solder joints.The mechanical properties of solder joints can be effectively improved by adding Cu₆Sn₅reinforcement into the solder matrix.Thus,Cu₆Sn₅ is chosen as reinforcement to improve the electromigration reliability of solder joints.To explore the mechanism of Cu₆Sn₅ reinforcement inhibiting the electromigration behavior of solder joints is one of the research focuses of this topic,which could propose an effective method for improving the electromigration reliability of solder joints.In addition,Sn is the main component in lead-free interconnection solder joints,and the anisotropy of Sn has an important influence on the electromigration behavior of solder joints.However,the orientation of Sn crystals is often neglected.Thus,a systematic study of the effect of Sn crystal orientation on the electromigration behavior of composite solder joint and its mechanism are the other research focus of this topic,which sets a theoretical foundation for a better understanding of the effect of Sn crystal orientation on the electromigration behavior.The main research contents and results are as follows:1.According to the crystal orientation analysis of Sn-based solder joints,the influence mechanism of c-axis in solder joints on the electromigration behavior of solder joints was studied.The determination method of Sn crystal orientation is established for Sn-based lead-free solder joints.The orientation of Sn crystal in solder joints can be determined by determining the angle between the c-axis of Sn crystal and the direction of electron flow?angle??and the c-axis direction.In addition,the effect of c-axis direction on the electromigration behavior was studied for Cu₆Sn₅-reinforced Sn3.5Ag composite solder joints.The surface microstructure and internal microstructure of the solder joints during electromigration were investigated.It was shown that Cu atoms were accumulated on the surface which c-axis pointed and were formed to IMC,while Cu atoms were consumed on the surface which the opposite of c-axis pointed,accompanied by the dissolution phenomenon.What's more,the influence mechanisms of the c-axis direction of Sn crystals on the diffusion direction of Cu atoms and IMC distribution during electromigration were expounded.2.The effects of different angles?on the electromigration behavior of Cu₆Sn₅-reinforced Sn3.5Ag composite solder joints were studied.For the solder joints with angles?of 30^o,45^o,60^o,75^o and 90^o,the relationship between angles?and the thicknesses of IMC layer at the anode interface,the growth of IMC in the solder matrix and the dissolution phenomenon at cathode interface during electromigration were analyzed from the observed surface of c-axis and its opposite direction pointed.The results showed that with the decrease of angle?,the thickness of anode IMC layer increased,the growth rate of IMC in the solder matrix increased and the cathode dissolution phenomenon became more serious.As a result,the phenomenon of electromigration became more serious.In addition,the relationship between diffusion coefficient of Cu atom and angle?was established.Moreover,the influence mechanisms of angle?on the thickness of anode IMC layer,the growth of IMC in the solder matrix and the cathode dissolution phenomenon during electromigration were described.3.By comparing the electromigration phenomena of Sn3.5Ag solder joints without Cu₆Sn₅ reinforcement,the effects of Cu₆Sn₅ on the electromigration reliability of solder joints were investigated.The electromigration behavior of Sn3.5Ag solder joints with angles?of 30^o,60^o and 90^o was studied.In addition,based on the influence mechanisms of the c-axis direction and angle?on the electromigration of solder joints,by comparing and analyzing the anode IMC layer,the growth of IMC in the solder matrix,the cathode dissolution phenomenon and the electromigration lifetime in Sn3.5Ag solder joints and Cu₆Sn₅-reinforced Sn3.5Ag composite solder joints under the same angle?.The results showed that the addition of Cu₆Sn₅ reinforcement could effectively inhibit the growth of IMC at the anode interface and in the solder matrix,delay the cathode dissolution phenomenon and the formation of voids,and prolong the electromigration life of solder joints.4.Based on the microscopic analysis of the solder joints,the strengthening mechanism of improving the electromigration reliability of solder joints by adding Cu₆Sn₅ reinforcement under high current density load was studied.For Cu₆Sn₅-reinforced Sn3.5Ag composite solder joints,by observing the phase interface of IMC/Sn at different electromigration stages,the evolution process of IMC phase interface in the solder matrix and its hindering effect on electromigration process were expounded.Blocked by IMCs,Cu atoms accumulated at phase interface and formed IMCs,which reduced the amount of Cu atoms in diffusion in the solder matrix.In addition,the IMC diffraction patterns in the solder matrix were obtained,and the crystal structure of IMC was Cu₆Sn₅ superstructure in hexagonal system.There were two states in the process of electromigration:complete ordered superstructure and superstructure of"transition state".When the“transition state”transformed to complete ordered superstructure,Cu₆Sn₅ absorbed the Cu atoms outside the phase boundary,and indirectly consumed the diffusion Cu atoms in the solder matrix.The Inhibition mechanism of Cu₆Sn₅ reinforcement on the electromigration of the solder joints was expounded.Cu atoms in diffusion were fixed on IMCs in the solder matrix by means of hindrance and absorption,thus reducing the number of Cu atoms reaching the anode interface of solder joints during electromigration,which increased the concentration of Cu atoms in the solder matrix,increased the concentration ladder of Cu atoms in solder matrix/cathode interface,and slowed down the diffusion speed of Cu atoms from the cathode interface to the solder matrix,as a result,the reliability of electromigration of solder joints was improved.