Effects Of Temperature Gradient On The Liquid-solid Interfacial Reaction And Grain Orientation Of Micro Interconnects

As the integration level of electronic devices is continually increasing,interconnects in electronic packaging are downsizing accordingly.One inevitable phenomenon is that liquid-solid interfacial reactions during a soldering process are significantly affected by soldering conditions and atomic diffusion.Thus,thermomigration(TM)behavior of solder interconnects under temperature gradient and its effect on the interfacial reactions need to be clarified.Another inevitable phenomenon brought by miniaturization is that the number of grains in Pb-free Sn-based solder interconnects after reflowing is decreasing rapidly.A few grains even only one grain form after soldering.The performance and reliability of micro interconnects in service are facing a serious challenge caused by anisotropy properties due to the limited grain number.Herein how to achieve thousands of micro interconnects with controllable grain morphology and orientation becomes a critical issue for advanced packaging manufacturing.Therefore,in-depth studies of liquid-solid interfacial reaction behavior,technologies for controlling grain orientation and the related mechanisms in micro solder interconnects have extensive values for both theoretical research and practical applications.In this study,certain temperature gradient was generated in solder interconnects through special designed experiments.Synchrotron radiation in situ observation and electron back scattering diffraction(EBSD)technologies were used to study the TM behavior of solder interconnects under temperature gradient.The effects of temperature gradient on liquid-solid interfacial reactions and grain orientation were discussed.Promising methods for rapid fabricating fully intermetallic interconnects and controlling grain orientations in interconnects were proposed by reflowing solder interconnects under temperature gradient.The main results are summarized as follows:1.When Cu/Sn/Cu solder interconnects are reflowed under temperature gradient,Cu atoms from the hot end continually migrate to the cold end,resulting in the asymmetrical evolution of interfacial intermetallic compounds(IMCs).The growth of Cu6Sn5 at the cold end is promoted,whereas that at the hot end is inhibited and the dissolution is promoted.The hot end IMC finally maintains a critical thin layer after satisfying a dynamic equilibrium between concentration gradient and temperature gradient.A theoretical model is proposed based on Cu diffusion flux to calculate the IMC thickness at both cold and hot ends as a function of reflow time and the equilibrium IMC thickness at the hot end under temperature gradient.2.When polycrystalline Cu/Sn/Cu solder interconnects are reflowed under temperature gradient,adequate Cu atoms are supplied for the anisotropy growth of the cold end Cu6Sns grains,which shows texture features with their<0001>orientations nearly parallelling to the direction of temperature gradient.Moreover,continuous epitaxial growth of Cu6Sn5 at the cold end liquid-Sn/(111)Cu interfaces has been demonstrated.The resultant Cu6Sn5 grains show faceted prism textures with an intersecting angle of 60° and highly preferred orientation with their<1120>directions nearly paralleling to the direction of the temperature gradient.In addition,the ?-Sn grains solidified under a certain temperature gradient are observed to follow a highly preferred orientation.The mechanism has been explained in terms of the anisotropy and directional growth of ?-Sn grains.3.Serious TM-induced substrate consumption at the hot end and abnormally fast TM-enhanced IMC growth at the cold end are clearly observed in pure Sn and Sn-Ag-Cu solder interconnects.However,significant retardation of the TM-induced substrates consumption and TM-enhanced IMC growth is realized by introducing Zn into the solders.The dissolution rate of the hot end substrates and the growth rate of the cold end IMCs both decrease as the Zn content increases.The improvement in TM resistance by Zn addition is attributed to the formation of CusZn8 or Cu6(Sn,Zn)5 IMC at the hot end,the weakened diffusion of Cu atoms and the reduced driving force for IMC precipitation at the cold end.In addition,the massive spalling of interfacial Cu5Zn8 is in situ observed.The massive spalling is caused by the Zn depletion in the liquid solder,the fast dissolution of hot end Cu or Cu6Sn5-type intermetallic compound,and the poor-soluble nature of Cu5Zn8 layer.4.By reflowing solder interconnects under temperature gradient,a new transient liquid phase(TLP)bonding process is proposed for high temperature packaging applications.The evolution of the dominant IMCs depends strongly on temperature gradient.The essential cause of such dependence is attributed to the different amounts of Cu or Ni atomic fluxes being introduced into the interfacial reaction between the developed/proposed and conventional TLP bonding processes.Under the effect of temperature gradient,mass TM of Cu or Ni atoms from the hot end toward the cold end promotes the total atomic flux for interfacial reaction.As a result,the total growth of IMCs is significantly accelerated.The developed/proposed TLP bonding process consumes limited cold end Cu or Ni substrate.The mechanism for the developed/proposed TLP bonding process is discussed from the viewpoint of atomic diffusion flux.Moreover,a promising way for fabricating fully intermetallic interconnects with a regular grain morphology and a strong texture feature has been proposed by soldering single crystal(111)Cu/Sn/polycrystalline Cu interconnects under a temperature gradient.