Growth And Evolution Of The Interfacial IMC In Cu/Sn/Cu Micro Joints During Isothermal Reflow

At present,advanced packaging technology is one of the effective methods to extend Moore's law.In advanced packaging,micro interconnects?height below 30?m?have been widely used.As for Cu/Sn/Cu micro interconnect,after repeated or long-term reflow,the micro joint will be mainly composed by Cu-Sn intermetallic compounds?IMCs?because of the small bonding height and fast IMC growth rate under size effect.In the present research,two kinds of substrates,the traditional electroplated Cu substrate and nanocrystalline electroplated Cu?nc-Cu?substrate,were used in micro interconnect.The interfacial IMC growth behavior based on these two Cu platings was studied separately.The main findings are as follows:During the reflow soldering process of traditional Cu/Sn?12?m?/Cu,scallop-like Cu₆Sn₅IMC was the dominant produce at both interfaces.The total thickness of the Cu₆Sn₅ layers and the consumption of the bottom Cu layer both showed a parabolic law with reflow time,and their logarithmic format followed a good linear relationship.It is also found that as the reaction time extended,the number of the Cu₆Sn₅ grains decreased,as well as,the angle between the[0001]axis of the Cu₆Sn₅ grains and the growth direction of IMC?perpendicular to the interface?also decreased overall.The grain orientation tended to gathering towards certain directions,resulting in texture structure.The anisotropic growth of the Cu₆Sn₅ grains along the<0001>direction was discussed to reveal the mechanism of orientation evolution.At the same time,the merger also exists in the evolution of Cu₆Sn₅ grains,which is initially considered to be a phenomenon of grain boundary migration.In this process,Cu₃Sn also gradually thickened,and Kirkendal holes were also generated.The new interface reaction phenomenon occurs in nc-Cu/Sn?12?m?/nc-Cu joint.At the initial stage of reflow,Cu₆Sn₅ IMC was layered instead of traditional scallop-like.Cu₆Sn₅grains near the Cu substrate are finer but the grains near the solder are larger.With the extension of the reflow time,the grains near the solder gradually changed from granular to rod-shaped and extended around.The long axis of the rod-shaped grain was parallel to the[0001]axis of the Cu₆Sn₅ grain,which related to its preferred growth.This phenomenon can be attributed to the numerous grain boundaries in nc-Cu substrate,while Cu was easy to diffuse quickly into the liquid solder.At the same time,Sn atoms are also easy to diffuse into the nc-Cu matrix,generating fine-crystalline Cu₆Sn₅.This also results in Cu being less likely to accumulate at the Cu/IMC interface,thereby inhibiting the formation of Cu₃Sn and avoiding the occurrence of Kirkendal holes and cracks.