Study On Microstructure And Orientation Of Micro-solder Joint Under Thermomigration

The development of microelectronic packaging technology from two-dimensional?2D?packaging technology to three-dimensional?3D?packaging technology promotes the continuous reduction of the characteristic size of micro-solder joints.At the same time,the characteristics of high power chip will cause serious Joule heat problem,which will cause large temperature gradient in solder joints and induce directional migration of metal atoms,namely thermomigration effect.Thermomigration can significantly affect the interfacial reaction of micro-solder joints and change the morphology and crystal orientation of solder joints.The change of crystal morphology and orientation will affect the reliability of solder joints,and then affect their service performance.At the same time,the reduction of the characteristic size of solder joints makes the number of grains in solder joints decrease sharply.On the one hand,the distribution of beta-Sn in solder joints is single or several grains;on the other hand,the proportion of intermetallic compounds?IMC?at the interface is greatly increased,but the number of IMC grains is greatly reduced.This leads to obvious anisotropy in structure and properties of solder joints.Therefore,it is of great significance to study the morphology evolution and orientation characteristics of micro-solder joints under thermal migration.In this paper,the microstructural evolution and crystal orientation characteristics of Cu/Sn/Cu and Cu/Sn/Ni micro-solder joints with solder joint height of 7?m were studied under the conditions of median temperature 110?and temperature gradient 1100?/cm after 200 h,400 h,600 h and 800 h respectively.At the same time,the samples were aged at110?for 200 h,400 h,600 h and 800 h,respectively,to compare the effect of thermomigration on the structure of solder joints.The main conclusions are as follows.Microstructure and morphology of Cu/Sn/Cu solder joints show that thermomigration can significantly change the interface IMC morphology and make the interface IMC grow asymmetrically at both ends of the solder joints.The interfacial layer of cold end IMC is increasing,and the grain morphology changes from scallop to polygon.The thickness of IMC interface layer at hot end decreases at first and then tends to be stable,and the grain of Cu₆Sn₅ is scallop-like.In addition,the Cu₃Sn grains in the solder joints remain elongated strips perpendicular to the copper substrate.Microstructure and morphology of Cu/Sn/Ni solder joints with Cu as hot end and Ni as cold end show that thermomigration can significantly change the IMC morphology of the solder joint interface.The interfacial IMC at cold and hot ends of solder joints grows asymmetrically,the interfacial layer of cold end?Cu,Ni?₆Sn₅ IMC increases continuously,and the grain size of bulk?Cu,Ni?₆Sn₅ increases.The thickness of IMC interfacial layer at hot end changed little,and the grain size of?Cu,Ni?₆Sn₅ was almost unchanged.Microstructure and morphology of Cu/Sn/Ni solder joints with Ni as hot end and Cu as cold end show that the evolution rate of interface?Cu,Ni?₆Sn₅ IMC structure is small.The thickness of IMC layer at cold end interface increases slowly,and the grain size of bulk?Cu,Ni?₆Sn₅ increases.The thickness of IMC at the hot end interface has little change,and the grain size of?Cu,Ni?₆Sn₅ with small or granular morphology is relatively stable.The results show that the{0001}crystal face of Cu₆Sn₅ grain has preferred orientation distribution characteristics.The<0001>orientation distribution of Cu₆Sn₅ grains was preferred during thermal migration from 0h to 600 h.The orientation of preferred orientation of Cu₆Sn₅ grain changes during 800 h thermomigration.The results show that the{0001}crystal faces of?Cu,Ni?₆Sn₅ grains have preferred orientation distribution characteristics.Under the condition of thermomigration,the crystal orientation of Cu/Sn/Ni solder joints with Ni as hot end and Cu as cold end shows that the grain orientation of?Cu,Ni?₆Sn₅ at both cold and hot ends of solder joints is different.The grain orientation of cold end?Cu,Ni?₆Sn₅ is similar,while that of hot end?Cu,Ni?₆Sn₅ is quite different.After aging,the grain orientation of Cu/Sn/Ni solder joints with Ni as hot end and Cu as cold end?Cu,Ni?₆Sn₅ at both cold and hot ends did not show significant difference.The results show that the c-axis orientation of beta-Sn grains is different under the conditions of thermomigration and isothermal aging.The angle between the c-axis orientation of the beta-Sn grains and the height direction of the solder joints is larger under isothermal aging condition,while the angle between the c-axis of the beta-Sn grains and the height direction of the solder joints is smaller under thermomigration condition.The results of study on orientation difference angle of each crystal structure in solder joint show that the main grain boundaries of Cu₆Sn₅,?Cu,Ni?₆Sn₅ and Cu₃Sn are large angle grain boundaries,and there are a lot of twins in Cu₃Sn grain.The main grain boundaries of beta-Sn are small angle grain boundaries,and the orientations of the intergranular crystals are similar.