| With the development of electronic packaging technology,chip integration has been continuously improved,and the feature size of interconnected solder joints in the chip has been further reduced,resulting in increasing temperature gradients at both ends of the solder joints,which is more likely to induce directional migration of atoms in low-temperature packaging solders(Thermomigration effect,TM).TM causes changes in the microstructure of the solder joint.In the service process of solder joints,changes in the microstructure of the solder joints due to TM effects will inevitably lead to the deterioration of the mechanical properties of the solder joint,which has a negative impact on the service reliability of the solder joint.Therefore,research on the evolution of the microstructure and mechanical properties of micro solder joints in low temperature packages under TM process is of great significance for evaluating the reliability of solder joints.In this paper,Sn-58 Bi low-temperature packaging solder and Cu and Ni substrates were used to prepare three different solder joints with a height of 10?m,which are Cu/Sn-58Bi/Cu,Ni/Sn-58Bi/Ni,Cu/Sn-58Bi/Ni.The microstructure evolution of the Bi phase and intermetallic compound(IMC)was investigated under 125? which temperature gradient of 1600?/cm for 200,400,600,and 800h;The relationship between the shear behavior of the solder joints and the microstructure evolution under unequal time is analyzed.As a comparison,this paper also studies the microstructure evolution and shear behavior of solder joints under isothermal aging conditions at 125?.The important conclusions are followed:With the extension of the TM time,the Bi atoms in the Cu/Sn-58Bi/Cu solder joint continuously migrated to the cold end,and an almost continuous Bi band was formed at the cold end after 800 h.Both the small Bi phase and the continuous Bi band can effectively inhibit the migration of Cu atoms dissolved at the hot end of the solder joint,so that the interface IMC in the cold and hot ends can grow symmetrically.The thickness of the interface IMC reached a relatively optimal value after TM 200 hours,at which time the shear strength of the solder joint was the best.Ni/Sn-58Bi/Ni solder joints have no obvious TM behavior during the entire TM process.The IMC thickness of the cold and hot ends grows slowly and symmetrically with time.Ni as the base substrate can effectively inhibit the TM behavior of Bi phase.After TM,the shear strength of the solder joint gradually increases with time,but the shear strength of the solder joint after isothermal aging gradually decreases with time.By comparing the shear strength of TM and isothermal aging,it can be seen that there is a critical thickness between 0.63 and 2.62 ?m for the thickness of IMC to maximize the shear strength of the solder joint.Under the TM effect,the evolution of the Bi phase in Cu(Hot)/Sn-58Bi/Ni and Ni(Hot)/Sn-58Bi/Cu solder joints is faster than that of Ni / Sn-58 Bi / Ni solder joints,but slower than Cu/Sn-58Bi/Cu solder joints.The shear strength of the two solder joints under TM conditions is similar,and both increase gradually with time.Regardless of whether Cu is used as a hot-end substrate or a cold-end substrate,the thickness of the IMC on the Cu side is larger than that on the Ni side.Under the TM effect at the same time,comparing the three solder joints,it can be seen that Ni/Sn-58Bi/Ni solder joints have the best effect of suppressing Bi phase roughening and migration behavior,and the highest shear strength.Under isothermal aging conditions,the microstructure morphology evolution speed of the three solder joints is relatively fast,and the shear strength decreases rapidly with time.Among them,Cu/Sn-58Bi/Cu solder joints evolved the fastest,and the shear strength decreased the most. |
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