Thermomigration In Cu/In-48Sn/Cu Micro Solder Joints And Its Effect On Soldering Interfacial Reactions

The progressively smaller size of micro solder joints in electronic packaging technology has created a severe challenge for the reliability of micro solder joints.The formation of Intermetallic Compound(IMC)at the interface of micro solder joints during the soldering reaction is the key to realize a reliable metallurgical interconnection between the solder and the Under Bump Metallization(UBM)layer.However,due to the brittle nature of IMC at the interface,which will reduce the mechanical properties and reliability of the micro-solder joint,therefore,its thickness and morphology must be effectively controlled.In the manufacturing and service process of micro solder joints,the temperature difference between the two ends of a joint tends to induce a temperature gradient(TG)in the micro solder joint,which then drive the directional diffusion of metal atoms and can cause the redistribution of elements,i.e.Thermomigration(TM),which will significantly affect the growth behavior of interfacial IMCs and the reliability of micro interconnects.In this paper,Cu/In-48Sn/Cu micro-solder joints were investigated,focusing on the thermomigration behavior and solder interface reaction of micro-solder joints at an average temperature of 140? and a temperature gradient of 1200?/cm and at an average temperature of 160? and a temperature gradient of 1200?/cm,and compared with the solder interface response at the corresponding temperature to reveal the interfacial IMC growth evolution,Cu and In atom thermomigration direction as well as the kinetics of IMC and the mechanism of Kirkendall void formation,during isothermal reflow and reflow under temperature gradient.The results of the study are as follows.(1)As-prepared Cu/In-48Sn/Cu micro solder joints form two kinds of IMC at both sides of the interface:Cu(In,Sn)₂ and Cu₂(In,Sn).The as-prepared microstructure of the solder consists of two phases:Sn-rich?-phase InSn₄ and In-rich?-phase In₃Sn.Both phases were uniformly distributed within the solder,and no elemental segregation occurred at the solder/IMC interface.(2)For isothermal reflow at 140oC,both Cu(In,Sn)₂and Cu₂(In,Sn)IMCs were present at the interface of the micro solder joints,and the phenomenon of Cu(In,Sn)₂ IMC spalling into the solder was observed,and the growth kinetics analysis concluded that the growth of Cu(In,Sn)₂ and Cu₂(In,Sn)phases was mainly controlled by grain boundary diffusion.For isothermal reflow at 160oC,the interface IMC gradually changes from the as-prepared[Cu(In,Sn)₂+Cu₂(In,Sn)]to Cu₂(In,Sn)due to the stronger diffusion ability of Cu atoms than In and Sn atoms,which enhances the reaction between Cu atoms and Cu(In,Sn)₂ to form Cu₂(In,Sn),and its growth was mainly controlled by bulk diffusion;meanwhile,the growth of Sn,In and Cu atoms have different diffusion fluxes in both fine-grained and coarse-grained Cu₂(In,Sn)sublayers,leading to the formation of Kirkendall voids within the Cu₂(In,Sn)IMC layer.(3)Two kinds of IMCs,Cu(In,Sn)₂ and Cu₂(In,Sn),are present at the interface of both ends of the micro-solder joints after reflow under two temperature gradients.Under the effect of temperature gradient,In and Cu atoms TM from the hot end of micro-solder joints to the cold end,resulting in an asymmetric growth phenomenon of IMC at both sides of the interface,i.e.,IMC at the cold end has a larger thickness.After reflow,there was no segregation of Sn-rich or In-rich phases at the interface of micro-solder joints,and both were still uniformly mixed and distributed within the solder.Under the effect of TM,the growth of IMC at the cold and hot ends was mainly controlled by grain boundary diffusion at an average temperature of 140?,while the growth of IMCs at the cold end was mainly controlled by bulk diffusion and the growth of IMCs at the hot end was mainly controlled by grain boundary diffusion at an average temperature of 160?.