Thermomigration In Single Crystal Cu/Sn/Cu Micro Solder Joints And Its Effect On Interfacial Reactions

With the development of electronic products and packaging devices towardsminiaturization,3D packaging technology with multi-chip vertically stacked is developing rapidly.Herein the diameter of solder bumps is downsizing continuously,the current density is increasing rapidly,and the Joule heating problem is prominent.As a result,temperature gradient is easily generated across micro solder joints,which triggers the directional movement of metal atoms and thus has a great influence on the interfacial reactions of the micro solder joints.At the same time,the number of interfacial intermetallic compand(IMC)grains in micro solder joints is also reducing,whereas the proportion of IMC to the whole micro solder joint is increasing.A full IMC solder joint may even be obtained.In this case,the micro solder joints definitely show a strong anisotropy in both physical and mechanical properties,which has an increasingly prominent impact on the performance and reliability of micro interconnections in 3D packaging.The present work focuses on the thermomigration behavior and soldering interfacial reactions in polycrystalline Cu/Sn/polycrystalline Cu(Cu/Sn/Cu),single crystal(001)Cu/Sn/polycrystalline Cu((001)Cu/Sn/Cu)and single crystal(011)Cu/Sn/polycrystalline Cu((011)Cu/Sn/Cu)micro solder joints at 267 oC and under a temperature gradient of 150 oC/cm.The purpose of this study is to reveal the effects of thermomigration(temperature gradient)on the morphology evolution and crystal orientation of the interfacial IMC in the above micro solder joints.The following conclusions are drawn in the present work:1.Study on interfacial reactions in Cu/Sn/Cu micro solder joints.After thermomigration,the size of the Cu6Sn5 grains at the cold end interface increases obviously,and the morphology has changed from the initial scallop-like to the short rod-like or prism-like.However,these grains array randomly.Moreover,the cold end Cu6Sn5 grains have certain preferred orientation in the temperature gradient direction,and this orientation relationship is more obvious with the increase of reflow time.The Cu6Sn5 grains at the hot end interface remain scallop-like and their size changes little during the reflow process.2.Study on interfacial reactions in(001)Cu/Sn/Cu micro solder joints.After imersion soldering,the Cu6Sn5 grains display regular prism-like along two perpendicular directions,and distribute homogenously on the whole(001)Cu surface.After isothermal reflow,theprism-like Cu6Sn5 grains at the Sn/(001)Cu interface gradually transform to scallop-like,and the resulting scallop-like IMC grains are of different orientations.That is,no preferred orientation is detected.After thermomigration(with(001)Cu substrate as the cold end),the Cu6Sn5 grains on(001)Cu substrate has changed into large prism-like grains parallel to each other,and the prismatic morphology is maintained.Further more,the Cu6Sn5 grains at the Sn/(001)Cu interface keeps preferred orientations along RD(consistent with the temperature gradient direction),TD and ND directions,with their < 1120 > pole being parallel to the direction of the temperature gradient.3.Study on interfacial reactions in(011)Cu/Sn/Cu micro solder joints.After imersion soldering,hexagonal short rod-like Cu6Sn5 grains form on the single crystal(011)Cu side.After isothermal reflow,these short rode-like Cu6Sn5 grains at the Sn/(011)Cu interface have completely transformed into scallop-like grains which do not exhibit any preferred orientation feature.After thermomigration(with(011)Cu substrate as the cold end),the morphology of the Cu6Sn5 grains on single crystal(011)Cu has changed from the initial hexagonal short rod-like into regular prism-like.Most of the new formed Cu6Sn5 grains grow along one direction or perpendicular to each other.Meanwhile,it is found that these Cu6Sn5 grains show an extremely strong texture feature in the RD direction.